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Investigation of a measurement technique to determine

Investigation of a measurement technique to determine
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Investigation of a measurement technique to determine led Investigation of a measurement technique to determine led Document Transcript

  • HSE Health & Safety ExecutiveInvestigation of a measurement technique to determine the apparent source size for light emitting diodes Prepared by National Physical Laboratory and Europtics Ltd for the Health and Safety Executive 2005 RESEARCH REPORT 345
  • HSE Health & Safety ExecutiveInvestigation of a measurement technique to determine the apparent source size for light emitting diodes Simon Hall Laura Crane David Gibbs National Physical Laboratory Hampton Road Teddington Middlesex TW11 0LW Brooke Ward Europtics Ltd Current ocular safety standards associated with the application of light emitting diodes (LED), and other intermediate sources, cite the angular subtense of the apparent source as an essential quantity for optical hazard assessment. Under these standards, the angular subtense parameter is calculated from the apparent source size of the LED package and the specified most hazardous viewing distance. However, an international standard for the measurement of the apparent source size parameter does not yet exist. This report describes the results of a study that provide rigorous practical support for a technique proposed for the measurement of apparent source size when observed from the most hazardous viewing distance. The results of this study allow, for the first time, an estimate of the potential optical hazard of LEDs and other intermediate sources, in accordance with current safety standards. This is a significant step in reducing the ambiguity that currently exists in the application of these optical safety standards. The results also verify earlier numerical modelling of an improved method for the estimation of the critical angular subtense parameter for extended sources, such as LEDs and intermediate sources. This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy. HSE BOOKS
  • © Crown copyright 2005First published 2005ISBN 0 7176 6108 3All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmitted inany form or by any means (electronic, mechanical,photocopying, recording or otherwise) without the priorwritten permission of the copyright owner.Applications for reproduction should be made in writing to:Licensing Division, Her Majestys Stationery Office,St Clements House, 2-16 Colegate, Norwich NR3 1BQor by e-mail to hmsolicensing@cabinet-office.x.gsi.gov.uk ii
  • ACKNOWLEDGEMENTSWe would like to acknowledge the software development expertise provided by OxfordFramestore Applications Ltd. iii
  • iv
  • CONTENTSExecutive Summary......................................................................................................... v ii1 Introduction .............................................................................................................. 12 Theory....................................................................................................................... 2 2.1 Angular subtense .............................................................................................. 2 2.2 Beam measurements ......................................................................................... 5 2.3 The optical system ............................................................................................ 63 Description of Apparatus.......................................................................................... 7 3.1 Initial System Design ....................................................................................... 7 3.2 8-Bit System Design......................................................................................... 7 3.3 12-Bit System Design....................................................................................... 74 Measurement Procedure ......................................................................................... 11 4.1 Preparation for measurement.......................................................................... 11 4.2 Calibration of CCD array and associated equipment ..................................... 11 4.3 LED beam width measurement ...................................................................... 11 4.4 Transform Validation Experiment.................................................................. 145 Results .................................................................................................................... 15 5.1 Initial results ................................................................................................... 15 5.2 8-Bit Transform Validation Experiment results ............................................. 17 5.3 12-Bit Transform Validation Experiment results ........................................... 20 5.4 Yellow LED - Ligitek LUY 3833/A29 .......................................................... 25 5.5 Blue LED - Nichia NSPB500 Rank WS ........................................................ 27 5.6 Green LED - Nichia NSPG500 Rank GS....................................................... 29 5.7 Red LED - Kingbright L-53SRC/E ................................................................ 31 5.8 White LED - Nichia NSPW500 Rank BS ...................................................... 33 5.9 Orange LED - Toshiba TLOH190P................................................................ 35 5.10 High Power Blue LED - Luxeon Star............................................................. 376 Uncertainty Analysis .............................................................................................. 397 Conclusions ............................................................................................................ 46 7.1 Future directions ............................................................................................. 50Appendix 1: Second moment, azimuth and principle width derivation ......................... 51Appendix 2: Design And Technical Specification For A Facility To Determine TheApparent Source Size Of Light Emitting Diodes ........................................................... 53Appendix 3: LED techinical data sheets......................................................................... 58References ...................................................................................................................... 59Glossary.......................................................................................................................... 61 v
  • vi
  • EXECUTIVE SUMMARYThe work detailed in this report was commissioned to allow the optical hazard level of lightemitting diodes (LEDs), and more laser-like intermediate sources, to be quantified. Thedramatic increase in the use of superbright LEDs for consumer, medical and industrialapplications necessitates a responsible assessment of the hazard presented by these devices.The International Electrotechnical Committee (IEC) and Commission Internationale delEclairage (CIE) cite that the angular subtense of the apparent source is an essential quantity forthe assessment of optical hazard. Under current optical hazard safety standards the angularsubtense parameter is calculated from the apparent source size and a specified most hazardousviewing distance. However an international standard for the measurement of the apparentsource size parameter does not exist.The aim of this current study is to provide rigorous practical support for a technique proposedfor the measurement of apparent source size when observed from the most hazardous viewingdistance. Development of the practical technique required the recognition of the apparatuslimitations and the development of strategies to overcome these limiting factors. Both an 8-bitand 12-bit system were tested. The 12-bit systems’ superior dynamic range and cooled arrayhighlighted the effect of stray light and noise. This demonstrated the need for a large dynamicrange in the measurement facility to measure second moment beam diameters effectively.A validation experiment suggested by the International Standards Organisation (ISO/TC 172/SC9) comprehensively verified the suitability of the technique. It is therefore proposed that theresults of this work should be used to underpin the adoption of this methodology withininternational standards for the assessment of the optical hazard potential of LEDs and otherintermediate sources.The report highlights the following: Technical specification of the critical components and the design of a facility for the measurement of apparent source size of LEDs and intermediate sources. Verification of an 8-bit and a 12-bit apparent source size measurement facility. This was achieved by computerized processing of images of spatial beam profile using a converging second moment method. High level of agreement between the propagation parameters derived through the 8-bit and 12-bit methods using the IR LED. This was an unexpectedly good correlation between results, considering the dynamic range limitations of the 8-bit camera. Evaluation of the astigmatic state of the beam by analysis of the change of azimuth as the beam propagates. This was carried out by azimuth determination of the beam by the comparison of the second moment widths in perpendicular axes. Measurement of a selection of 8 LEDs with differing peak emission wavelengths, construction and beam propagation characteristics. Visualisation of real beam propagation using a montage of beam images and spatial profiles related to the propagation envelope for one of the LEDs. Effective demonstration that the point in the beam envelope where a sharp image of the electronic structure of the LED is obtained does not necessarily correspond to the beam waist or location of the apparent source. vii
  • Identification of general astigmatism (as opposed to simple astigmatism) in the output beam from one of the LEDs. Populated angular subtense contour plot with results from this work. This plot enables the easy estimation of the angular subtense of real LEDs and intermediate sources from the measured beam propagation characteristics. Verification of the technique using a test suggested by the International Standards Organisation (ISO/TC 172/SC 9) identifying that this method can be applied successfully to the analysis of beam propagation parameters and hence the apparent source size determination for stigmatic and simple astigmatic beams from LEDs. Development of this technique would allow the assessment of generally astigmatic beams in line with ISO 11146-2 ‘Lasers and laser-related equipment. Test methods for laser beam widths, divergence angle and beam propagation ratio. Part 2: General astigmatic beams’.The results of this study allow, for the first time, the effective characterisation of the opticalhazard of LEDs and other intermediate sources, in accordance with the IEC and CIE standards.This is a significant step in reducing the ambiguity that currently exists in the applicationof these optical safety standards. The results also verify earlier numerical modelling of animproved method for the estimation of the critical angular subtense parameter for extendedsources, such as LEDs and intermediate sources. viii
  • 1 INTRODUCTIONThe assessment of the optical hazard associated with beams from sources of light intermediatein quality between a laser and light emitting diodes (LED)1 has been a challenging problem forthe international standards community for a large number of years.This report has been produced to contribute to the international debate regarding the opticalhazard due to LEDs. The current requirements for the classification of LEDs follows IEC 608253 and requires a measurement of “apparent source size and its location”. The CIE publicationCIE S 009/E:2002 “Photobiological Safety of Lamps and Lamp Systems” cites apparent sourcesize as part of the methodology to calculate angular subtense and hence Retinal Hazard.However a procedure for establishing apparent source size and location is not described.The apparent source size of an LED is a critical parameter used in the assessment of the ocularviewing hazard of these devices under ISO 60825-1 ‘Safety of laser products. Equipmentclassification, requirements and user’s guide’. Under the committee draft IEC 60825-13‘Measurements for the classification of laser products’ a proposed measurement method isdescribed to determine the apparent source size of LEDs. The validity of this method has beenquestioned at a national and international level and continues to be debated within the variousstandards bodies such as IEC, ISO and CIE. Specifically, the applicability of propagationmodels to low divergence beams from LEDs has been challenged.Previously the validity of these models has not been demonstrated through physicalmeasurement of LED devices. This project aimed to resolve this situation through theconstruction of a suitable measurement facility and by performing an assessment of a range ofcommercially available LED sources. 1
  • 2 THEORYFigure 1 is a schematic diagram of the proposed measurement method for the determination ofthe apparent source size and beam characteristics of LEDs. A CCD diode array camera systemis placed on a movable carriage in front of the LED source. The relay lens of the camera systemallows the CCD to capture a spatial intensity profile of the beam at a particular plane. The beamwidth is then calculated using a modified second moment technique. It is necessary to ensurethat enough of the beam power has been captured to allow an accurate determination of thebeam width. To address this problem a self-converging width measurement technique is used toestimate the beam width at each measurement plane and represent the true value to anacceptable level of uncertainty. This measurement is repeated at a number of locations along thetest beam axis until sufficient data points have been obtained to allow the fitting of a maximumlikelihood hyperbola using a least squares fitting technique. The coefficients of the fittedhyperbola allow the derivation of the beam propagation parameters of the source. A’ B’ u v LED CCD do A B AA’ – plane of beam waist BB’ – plane of transformed beam waist u - distance from beam waist to lens v – distance from lens to transformed beam waist do – beam waist diameter Figure 1 Proposed methodology to determine apparent source size of LEDsIf the beam waist is not accessible for direct measurement then using an aberration-freefocussing system, or transform lens can create an artificial waist. This may be necessary if, forexample, the beam waist is formed within the LED package or there is insufficient space toperform the required number of measurements either side of the waist. The position anddiameter of this artificial waist can then be used, along with the known properties of thetransform lens, to calculate the location and size of the original beam waist. The equations usedto calculate the location and size of the original beam waist using this procedure are given inSection 6 as part of the uncertainty derivation process.2.1 ANGULAR SUBTENSEThe angular subtense of an apparent source of radiation in the 400 nm to 1400 nm wavelengthrange is required by current laser safety standards 3 to permit calculation of the relaxation factorC6, for thermal retinal damage from extended sources. It is the ratio of the angular subtense ofthe source in question to that of a source that would form the realistic minimum spot size on theretina (1.5 mrad). Classification or assessment of the thermal hazard from a source requires thatboth the angular subtense (see Figure 2) and location of an extended source be known beforethere can be a relaxation of the maximum permissible exposure (MPE). The location of thesource is required so that the angular subtense can be calculated for viewing this from the 2
  • minimum conceivable eye accommodation distance of 100 mm (in IEC standards) 3. It should benoted that this latter assumption may not describe the full range of potential hazards. It ispossible that some large divergence sources, when held closer than 100 mm from the eye, mightproduce a significant thermal hazard in a blurred retinal spot even though the eye cannotachieve a sharp focus. Optical Image of Source Optical Source Angular Eye Subtense Figure 2 Classical representation of Angular SubtenseIt is a simple matter to measure the physical size of the chip of a LED that has a Lambertianradiation pattern but it is more difficult to know or measure the location or size of the apparentsource with low divergence beams from a LED. Such beams can have a near planar wavefront,which would imply that the apparent source is located at infinity with an unknown angularsubtense. However, recent advances in the characterization of optical beams, both coherent andincoherent, enable prediction of their propagation envelopes 2,16,17. It is now possible to assessthe intrabeam-viewing hazard by using known beam characteristics to estimate the angularsubtense of an extended source that would present the greatest hazard to a retina 3.The level of the thermal hazard to the retina is defined here as the power or energy permillimeter of beam diameter falling on the retina 19. The process of calculation of the size of thebeam formed on a retina and the fraction of incident power passing through the pupil has beenperformed for a wide range of feasible conditions. The calculations assume that the beam has adivergence of less than 30° and has a power density profile that produces the greatest peakirradiance on the retina (i.e. a Gaussian profile).Measurements of the enclosed power envelope of beams from lasers have confirmed that theypropagate with a hyperbolic profile, the constants of which are modified when passing through alens. The new constants can be used to estimate the location of the waist of the new hyperbolaand its Gaussian beam diameter as a function of propagation distance. In this way it is possibleto determine the spot size on the retina formed by a beam after passing through the lens of theeye. d01 – beam waist diameter of input beam L1 – waist to lens distance Zr1 – Rayleigh length of input beam fe – focal length of lens L2 – lens to transformed waist distance Zr1 – Rayleigh length of output beam d02 – beam waist diameter of output beam dr – beam diameter on retina Lr –transformed waist to retina distance Figure 3 Calculation of spot size (dr) on the retina of the eye. 3
  • For a given set of beam propagation constants (waist diameter and divergence say) it is possibleto predict the hazard level (P/d) at the retina. The hazard level results from calculations of thefraction of beam power that passes through the 7mm iris of the eye as a function of both thestrength of the eye lens (assumed to vary anywhere between 14.5 mm and 17 mm) and thedistance of the incident beam waist from the eye. The maximum hazard occurs when the eyeaccommodates itself at the most hazardous viewing distance. In the interests of simplicity, IEC60825-1 assumes that this most hazardous viewing distance is 100 mm but this is not alwaysfound to be the case.Previous calculations (numerically verified by workers in Austria and the UK) haveconcentrated on determining the spot size on the retina at the most hazardous viewing conditionas a function of the two beam propagation parameters, beam waist diameter and far-fielddivergence. Knowing the spot size at the retina and by assuming the eye to be 17 mm "long",the artifact of the angular subtense of the apparent source has been estimated over the mostrelevant range of incident beam parameters. The values of angular subtense can be displayed ascontours in the two-dimensional map of waist diameter and divergence. Further calculationsbased on the measured values of these parameters will also reveal the location of the apparentsource. If the Rayleigh length of the beam is significantly less than 50 mm then the source canbe assumed to coincide with the measured beam waist location.While some rather extreme conditions have been assumed when modeling the beam (e.g. aGaussian beam profile), the procedure for estimating angular subtense from beam parametermeasurements is thought to offer an unambiguous and non-subjective result. While theprocedure may over-estimate the hazard level it can permit a greater relaxation of the MPE levelthan simply assuming that C6=1.A CCmap showing the range of angular subtense values as contours against the Beam waistwidth and the beam divergence was produced from these calculations (Fig 4)2. Contour of equal angular subtense in mrad Beam waist diameter and divergence of LED Angular subtense, , of LED Figure 4 Theoretical plot of beam waist diameter (width) vs. beam divergence showing contours of angular subtense 4
  • The contours show equal values of angular subtense in milliradian. To use the contour plot, thewaist diameter and the divergence of the LED beam are measured. The results are plotted on thegraph and the value of the angular subtense, , is then read from the contour just below themeasured point.The objective of this investigation was to demonstrate that it is possible to determine thepropagation characteristics of the beam produced by a LED. This information could then beused to estimate the size of the image formed on the retina and from this the angular subtense ofthe apparent source at the eye at a given distance. These results then enable the population of atheoretical contour map of the computed angular subtense as a function of the measured beamcharacteristics of LEDs. The angular subtense for all beam types can then be determined bymeasuring the beam waist diameter and the divergence.2.2 BEAM MEASUREMENTSMeasurement of the optical constants of the propagation envelope of a beam has been thesubject of considerable research over the last ten years. A consequence of this work is theevolution of ISO standards for the measurement of the diameter and divergence of a beam. ISO11146:1999. “Test methods for laser beam parameters: beam widths, divergence angle andbeam propagation factor” 6 is the current draft standard being reviewed by ISO. The proceduresand techniques that are described here for the determination of the diameter and location of theapparent source of a beam are based on the principles underlying the ISO standards7 forstigmatic and simple astigmatic beams. The proposed methods are applicable to beams whosefull divergence angle is less that 30°. Relaxation of the laser safety criteria should not be appliedto a beam displaying general astigmatism.There are a number of methods available for measurement of the diameter of a beam as well asits far-field divergence. The basic principles for those methods have been established in an ISOstandard. They are applicable to laser beams with a relatively small beam propagation ratio, M2.Recent research has demonstrated that adequate steps have to be taken to counter the effects ofnoise and offset errors when measuring the transverse irradiance distribution of a beam. Whenthese steps are taken, the propagation behaviour of incoherent broadband beams as well as high-quality laser beams can be predicted reproducibly with considerable precision.To accurately measure the second moment beam diameter both the number of pixels and thelevel of digitisation of the signal received on each pixel has to be considered. For beams with arapidly changing beam diameter the number of bits in the digitisation process becomes morecritical. Noise on the image acquired by the camera both from electrical and optical sourcesmust be removed by setting a discrimination level. This effectively reduces the dynamic rangeof the camera and this favours cameras with an inherently large dynamic range due to a largernumber of bits available on the digitisation electronics.The methods leading to estimates of the diameter of a beam use a procedure known as theConverging Second Moment diameter or width measurement (CSM). The schematic of thismethod is shown in Figure 5. These methods are being defined in the revision of ISO 11146 thatis currently in preparation. 5
  • Figure 5 Schematic of converging second moment iterationThe preferred method for measuring all the propagation characteristics of a beam is to performCSM diameter measurements at a number of locations either side of the beam waist. Thecalculation of second moment width is described in Appendix 1.2.3 THE OPTICAL SYSTEMThe beam measurement process consists of using a CCD sensor to image the irradiance profileat a minimum of ten measurement locations either side of the beam waist. The proposed opticalsystem contains variable magnifying optics that are designed to facilitate imaging the transverseirradiance profiles to occupy approximately one quarter of the sensor screen height. Othercomponents are included in the system to attenuate the beam power to avoid sensor saturationand to provide spatial calibration of the pixel array of the sensor. 6
  • 3 DESCRIPTION OF APPARATUS3.1 INITIAL SYSTEM DESIGNAn initial specification of the 12-bit measurement system was written and can been found inAppendix 2. This specification details the required elements to measure apparent source size ofLEDs.3.2 8-BIT SYSTEM DESIGNBoth an 8-bit and 12-bit camera systems were used for measurement. The final system designfor the 8-bit system was identical to that described in Section 3.3, except for the camera andzoom lens. The details of the 8-bit camera are given in Section 3.2.1. The details of theassociated zoom lens are presented in Section 3.2.2.3.2.1 8-Bit Camera SystemThe 8-bit system consisted of a analogue CCD interline transfer camera connected to an 8 bitframe grabber card. A Leica Monozoom optic was used to adjust the size of the image of thepropagating beam from the test LED. An 8-bit system would imply a digitised dynamic range of28 =256 bits. This takes no account of noise or camera processing. The dynamic range in thesemeasurements was assumed at the start to be one of the greatest limiting factors of themeasurement. This assumption was later shown to be true by adjusting discrimination levels andplotting the effect against the measured second moment values for identical camera frames.3.2.2 Leica Monozoom 7The camera zoom lens used for the 8-bit system was a 1:7 par-focal microscope zoom. Duringzooming the focus could be maintained, whilst providing a wide field of view and a longworking distance. The zoom did not include an integral iris and the zoom setting could not belocked. The latter meant that special care was required to ensure that the zoom was notdisturbed during measurements, otherwise the dimensional calibration would be lost. Theshortcomings of this zoom prompted the acquisition of a higher specification zoom system toform part of the 12-bit set-up.3.3 12-BIT SYSTEM DESIGN3.3.1 Electrical MeasurementsThe LED sources were operated at a constant current using a power supply stabilised to 0.02%.Setting a constant voltage is also possible, although this is more likely to be affected bydifferences in contact potential. To measure the current to the LED, a standard resistor wasplaced in series with the power supply and the LED source. The potential across the standardresistor was measured using a calibrated digital voltmeter. Using this value, the current to theLED was calculated and recorded. This ensured that the same electrical conditions were usedfor each LED measurement. 7
  • 3.3.2 Transform Lens Following a survey of commercially available products, it was identified that a single large diameter achromatic lens of sufficient power and quality for the measurements was not available. Two high quality achromats were combined to provide an equivalent effect. The large diameter was required to provide effective coupling of the LED output to the camera input to reduce vignetting. Optics of large diameter also allows the inner portion of the lens to be used which introduces less aberration to the measurement process. It is critical that the geometry of the lens is known accurately, so that the lens transformation properties can be calculated (see Figure 6 and Figure 7).Figure 6 Schematic diagram of achromat showing critical measurements needed to allow the lens transformation properties to be calculated (all dimensions in mm). A description of the parameters used can be found in the glossary. The distance between the two lenses and the distance from the LED required calculation to ensure that the beam would not overfill the aperture of the camera zoom. In addition, the transformed waist diameter must not be too small as to cause measurement problems due to the camera resolution. Additionally, the Raleigh length (distance for the beam diameter to increase by 2) should be long enough to allow accurate distance measurement to be carried out. High quality lens mounts with yaw and tilt adjustments were purchased to allow uniaxial alignment of the measurement system. Figure 7 Scale drawing of the two transform achromats showing some of the calculated measurement distances, definitions of parameters are given in section 6 (all dimensions in mm) 8
  • 3.3.3 Optical RailA 2-metre cast iron optical rail was used as the primary bench for the mounting of the opticalcomponents. A second, machined, aluminium rail was used to mount the LED and the achromatlenses. This secondary rail was mounted on a roller carriage on the primary bench. All carriagesand benches carried vernier scales to ensure accurate measurement readings.3.3.4 LED MountA stable LED clamp which could in turn be mounted on a 3 axis gimbal mount with height andtransverse adjustment was required. A commercially available solution was unavailable so aclamp was designed and produced by the NPL workshop. This was mounted on a high stabilitygoniometric mount with height and transverse adjustment provided by two other stages. Thisprovided a low vibration mount with high resolution and repeatable displacement.3.3.5 Beam AttenuationNeutral Density (ND) optical filters were used to attenuate the light input to the camera. Criticalattributes were spectral neutrality and spatial uniformity. Tests were made on NPL’s primaryZygo Fizeau interferometer to inform the purchase of a high quality set of filters with lowwavefront aberration. The filters were placed in a mount that allowed stacking of filters with anadjustment for variable tilt to reduce inter-reflection. The proposed initial system did not includea scatter screen and thus the use of an iris with the zoom system would have caused vignetting.Later adoption of the scatter screen allowed this option.3.3.6 Rotating diffuserIt was found to be necessary to use a rotating frosted scatter screen to allow visualisation of thebeam profile at the focal point of the zoom system. Measurements made without this systemcaused vignetting problems. The rotating diffuser had the added advantage that it allowed theuse of the integral iris in the zoom lens to attenuate the LED light. Several measurements weremade to ensure that the real beam diameter was not greater than the diameter obtained by theuse of the screen.3.3.7 GraticuleA photoetched transparent graticule with traceable calibration was used to calibrate the imagingsystem (zoom in combination with the CCD array and analysis software).3.3.8 12-Bit Camera SystemThe 12-bit system utilized a superior zoom system that had a larger input optic and a greatermagnification range. The CCD detector used produced a digital 12 bit output and incorporated atwo stage peltier cooler to both reduce the temperature of the array and the level of noiseacquired. The dynamic range of the system was 212=4096 bits, the software used a“discriminator level” which allowed the baseline for detection to be raised above the ambientnoise level. The zoom system incorporated an iris which allowed the light levels to be reducedwithin the range afforded by the detector integration time adjustment.Calculations were performed to ensure that the LED and lens(es) were located so that the beamcould converge to a waist and re-expand within the travel range of the optical bench system. Atthe same time the anticipated diameter of the beam at the transforming lens(es) was examined to 9
  • ensure that the beam size was not large enough to introduce significant aberration or vignettingeffects.3.3.9 Leica Z16 ZoomTwo Leica zoom microscope systems were assessed, the Z6 and Z16 models. The Z16 wasfound to have a greater focal range and would allow the measurement of a greater range of LEDtypes. The Z16 is an apochromatic zoom system with central beam path. A planapochromatic0.5X objective was used and the zoom range was 0.57× – 9.2×. This high quality optic has asimilar field of view to the Monozoom 7 but has significantly lower aberrations. The inbuilt iriscan also be employed to attenuate light levels when used in conjunction with a scatter screen.The lockable zoom setting allows calibration at a particular fixed zoom level.3.3.10 Final 12-bit System DesignFigure 8 shows the final components used in the 12-bit system for the measurement of angularsubtense. Achromats Rotating ND Filter and CCD camera LED diffuser Filter holder Planofocal Zoom Lens Secondary optical bench Primary optical bench Figure 8 Final optical arrangement of the 12-bit system for the measurement of apparent source size 10
  • 4 MEASUREMENT PROCEDURE4.1 PREPARATION FOR MEASUREMENTThe optical arrangement for the measurement of apparent source size is detailed in theschematic diagram, Figure 8. Prior to measurement the components must be carefully aligned toensure that the LED beam is parallel to the optical axis of the primary and secondary bench.The following points detail the steps required to align the components used with the optical axisof the bench. a) Establish a reference He-Ne beam parallel to the optical bench by the use of at least two movable irises or apertures. b) Align the centre of the Zoom lens with the HeNe beam and use the imaging software to ensure that the suitably attenuated beam is in the centre of the CCD array field of view. c) Introduce the transform achromats one at a time and centre them on the beam. Ensuring that the emerging beam is still creating a centred image in the camera. d) Place the viewing screen at the focal point of the zoom lens by utilizing a reference grid that can be resolved by the imaging software and can be coincident with the frosted side of the screen. e) A microscope, focused on the optical axis of the system, is used to locate and record the positions of the components along the optical bench. This facility is used to set accurately the appropriate distances between the LED and the achromat(s).4.2 CALIBRATION OF CCD ARRAY AND ASSOCIATED EQUIPMENT a) A reference grid or graticule is inserted in place of the frosted screen with the reference grid plane coincident with the plane of the frosting, as determined using a telescope. A CCD frame of the reference grid is recorded and analysed by the software to derive the calibration factor (pixels/mm) to be used to convert subsequent beam pixel measurements into linear dimensions. b) All equipment used to measure the electrical characteristics of the LED were calibrated and traceable to national standards, as is essential for such a system.4.3 LED BEAM WIDTH MEASUREMENTOnce the system is aligned and the calibration procedures performed, the following steps arerequired to predict the position of the beam waist from the vertex of LED. a) A combination of ND filters and the iris of the Zoom lens are used to attenuate the beam irradiance so that the full dynamic range of the CCD system is used. This is done by locating the position of maximum irradiance, then placing filters in the beam path so that the signal is just about saturating the CCD pixels. As readings are taken either side 11
  • of the maximum, the iris of the zoom lens and the exposure time of the camera can be adjusted to maintain the signal level at the full dynamic range of camera. The Zoom is also set so that approximately a quarter of the CCD field of view appears to be filled by the largest diameter that is to be measured;b) The image acquisition software is used to capture at least 10 equidistant beam images either side of the beam waist. Each image has an associated image of background optical noise captured at the same time by blanking out the LED with a black felted beam stop;c) The background frame is subtracted from the beam image frame before the digital width analysis process is performed;d) The corrected image is processed using the convergent second moment (CSM) method to limit the dimensions of the CCD window that is subsequently analysed and hence reduce noise contribution to the second moment evaluation. The CSM values of the beam in the laboratory (CCD array) vertical and horizontal axes are calculated. A cross- moment of the beam distribution in the converged window is used to calculate the azimuth of the principal axes of potentially non-circular distributions. This figure enables the calculation of the dimensions of the beam along its principal axes. The ratio of the principal dimensions (ellipticity), the azimuth angle of the principal axes relative to the laboratory axes; and the calibrated linear magnitude of the principal dimensions are recorded. The convergence of the 2nd moment calculations can be seen in Figure 9 The program then outputs the final 2nd moment measurements in the X and Y axes; 33.71 Figure 9 CSM software illustrating the calculation of the second moment values.e) A least-squares (maximum probability) process is used to discover the best fitting hyperbolic envelope to the propagating beam in each of its principal planes. The coefficients of the hyperbolas are processed to reveal: the locations of the beam waists relative to the vertex of the LED; the transverse dimensions of the waists; the values of the Rayleigh Lengths of the beam along their principal planes; and the far-field divergences in those planes; 12
  • f) If the beam is found to be astigmatic (i.e. the ellipticity of the beam is found to be greater than 1.15 or less than 0.83) and there is a monotonic variation in the azimuth of the principal planes of the propagating beam (twist) then the beam is deemed to suffer from general astigmatism and no further investigation or relief of the thermal hazard factor C6 can be justified without a more detailed analysis procedure; g) If the beam is identified as stigmatic or simple astigmatic the determined values of the beam waist widths and far-field divergences can be placed on the angular subtense contour map (Figure 4) and the contour below the lowest uncertainty ellipse can be used to identify the angular subtense to be used to determine the appropriate value of the thermal hazard relaxation factor C6.; h) If the Rayleigh Length in the least divergent principal plane is less than 50 mm then the location of the apparent source can be regarded as the location of the beam waist in that plane. If the Rayleigh Length is greater than 50 mm then the possible error in hazard assessment can be greater than 5% and the location of the centre of curvature of the wavefront arriving at the most hazardous viewing distance should be used to identify location of the apparent source.Figure 10 shows the required optical elements of the system to measure the angular subtense ofan LED. Transform achromats LED in Rotating diffuser goniometric mount Cooled CCD Camera Zoom lens ND filters Enclosure to reduce scattered light Figure 10 Experimental apparatus 13
  • 4.4 TRANSFORM VALIDATION EXPERIMENTTo validate the suitability of the proposed measurement method for determining the beampropagation parameters a Transform Validation Experiment can be undertaken. This techniqueuses the beam propagation parameters to predict the size of a beam waist produced when aknown lens is inserted into the beam. This prediction is then verified by using the measurementtechnique to measure the true diameter of the new beam waist with the lens inserted. The aim isto achieve 10% (1 sigma) agreement between the predicted value and the measured diameter ofthe beam waist. The transform validation experiment is shown in Figure 10 and schematically inFigure 11. The points below detail the steps required in the transform validation method. Measure the transformed waist and estimate original waist Use the estimated waist to predict new waist formed by inserted lens Measure and estimate new waist for comparison with step 2. If the estimate and the prediction agree sufficiently well the validation is complete MeasureLED CCDStep 1 Estimate PredictLEDStep 2 Compare MeasureLED CCDStep 3 Figure 11: The three steps of the primary ISO Transform Validation Experiment 6The results of the Transform Validation Experiment are presented in Section 5.2. 14
  • 5 RESULTS5.1 INITIAL RESULTSThe ideal methodology to measure the LED beam would be through direct imaging. Somedifficulties were encountered due to vignetting of the beam by the zoom lens. This effect can beseen in the asymmetry of the hyperbolic plot produced from the second moment analysis, Figure12. IR LED measurement showing Vignetting effect 5.000 4.500 2 y = 0.0003x - 0.0142x + 3.4009 2 R = 0.98272nd moment Beam radius 4.000 Horizontal width (mm) Vertical width (mm) Poly. (Vertical width (mm)) Poly. (Horizontal width (mm)) 3.500 2 y = 0.0003x - 0.0146x + 3.297 2 R = 0.9854 3.000 2.500 -60.0 -40.0 -20.0 0.0 20.0 40.0 60.0 80.0 100.0 Distance from Beam Waist Figure 12 Skewed fit of second moment values obtained showing vignetting effect of zoom apertureNoise effects from the intereflections between the filters used to attenuate the light from theLED were found to be a particular problem. The differences in measured second momentdiameter caused by different filter combinations can be seen on Figure 13. The stray light noiselevels on the camera were very high and a discriminator level of 50 was required to produce theanalysis.For the final measurements the procedure was adapted to only utilise the minimum number offilter elements by manually finding the camera position that resulted in the greatest localirradiance. The integration time of the camera and/or the iris in the zoom lens were then reducedas much as possible to reduce the signal output from the camera pixels to a point where a NDfilter would reduce the signal levels to just below saturation. This was to ensure that the greatestdynamic range of measurement was employed. 15
  • IRED through 2 lens transform - X-axis (Aug 26 Disc 50) 20.0 18.0 Converged 2M beam width (mm) LSq Fit Hyperbola 4 filter set A 16.0 2 filter set B 1 filter C 14.0 1 filter D 4 filter set E 3 filter set E 12.0 2 filter set F 10.0 8.0 280 285 290 295 300 305 310 Distance past vertex (mm) Figure 13 IRED Led measurements demonstrating filter effectsInitial measurement work concentrated upon the confirmation of earlier work using an OsramIR LED 2. Details of this LED can be found in Appendix 3. Early evaluations were pursued witha 50 mm focal length singlet lens to examine the field of view required for the experiment. 16
  • 5.2 8-BIT TRANSFORM VALIDATION EXPERIMENT RESULTS5.2.1 Direct Measurements of IR LEDThe 8-bit system using the Cohu CCD camera and Leica monzoom 7 lens was used for theinitial work to confirm the method for measurement of angular subtense. Figure 14 details therequired apparatus to measure the beam waist of an LED directly. Measurements of the beamwidth were made using the procedure in Section 4.3. Rotating ND Filter and CCD camera diffuser Filter holder LED Planofocal Zoom Lens Secondary optical bench Primary optical bench Figure 14 Optical set-up for direct apparent source size measurement SFH 400 IRED (950 nm) @ 50 mA Raw Beam (5 Aug) Converged 2M beam 6.5 width (mm) 5.5 4.5 3.5 2.5 0 5 10 15 20 25 Distance past vertex (mm) Figure 15 Plot of converged beam width for IRED LEDThe results, shown in Figure 15 and Table 1, indicate that the IR LED produces a beam with awaist external to the LED. The M2 value is very large but the beam still fits the hyperbolic 17
  • envelope well. The existence of an external beam waist allows the effective measurement of thedirect propagation envelope. Table 1 Calculated beam parameters for Osram IRED LED Parameter Value Units Waist position from LED vertex zo 5.61 mm Waist diameter Wo 3.38 mm Rayleigh distance Zr 9.76 mm Divergence 347 mrad M2 9695.2.2 Two Lens transform of IR LED Achromats Rotating ND Filter and CCD camera LED diffuser Filter holder Planofocal Zoom Lens Secondary optical bench Primary optical bench Figure 16 Optical set-up for apparent source size measurement using two achromat lensesThe second step in the validation of the measurement method is to predict the beam waistdiameter. This requires the use of a transform lens in the optical arrangement (see Figure 16).Two achromats were used to produce a beam waist that would not overfill the field of view ofthe CCD. The lenses also ensured that the Rayleigh Length was sufficiently long to provide anappropriate number of measurement planes. Figure 17 shows the results of these measurementswith a hyperbolic fit to the data points. 18
  • IRED 2 Lens Transform X-axis 7.5 7.0 6.5 CSM width (mm) 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 220 230 240 250 260 270 Distance past vertex (mm) Figure 17 Hyperbolic fit to data from IR LED through two achromat lensesA summary of the beam characteristics for the Osram LED is presented in Table 2. Thecomplete summary of the Transform Validation Experiment is located in Tables 4 and 5 ofSection 5.3 as it was thought more appropriate to put them in the context of the 12-bit systemand hence allow comparison. Table 2 Summary of beam characteristics for IRED LED using 8-bit camera system SFH 400 IRED using 8-bit Camera Transformed Beam Beam property Goodness of fit Waist position from LED vertex Zo 254.1 mm 0.01 Waist diameter Wo 3.12 mm 0.10 Rayleigh distance Zr 8.6 mm 0.27 Divergence 361 mrad 16 M2 950 67 Original Beam from Inverse Transform Beam property Uncertainty Waist position from LED vertex Zo 4.9 mm 3.0 Waist diameter Wo 3.47 mm 0.22 Rayleigh distance Zr 10.7 mm 0.5 Divergence 324 mrad 26 M2 950 129 Uncertainties Used Focal length etc. 0.5 % Datum positions 0.1 mm Width measurements 0.025 mm Measurement locations 0.15 mm 19
  • 5.3 12-BIT TRANSFORM VALIDATION EXPERIMENT RESULTSThe 12-bit PCO Sensicam camera with the Leica Z16 zoom lens was used for furthermeasurements of apparent source size on the Osram IRED LED and a range of visible LEDs.The details of all LEDs measured can be seen in Appendix 3.5.3.1 Low Level NoiseIt was found that the discriminator level used to eliminate low level noise caused a much greatereffect upon measured beam width than might be expected. The intensity of the imaged LED wasalways set as close as possible to the saturation point of the CCD to produce the greatestdynamic range possible. With the 12-bit system, the maximal number of bits of dynamic rangewould be 4096. Figure 18 shows the effect upon the second moment beam size due to change ofdiscrimination level. Hereafter the discrimination setting was kept at 5 bits. IRED raw beam width vs Discriminator Level 48 Pos 1 Pos 3 Second moment (X-axis) Pos 6 43 Pos 6 Pos 10 38 Pos 11 Pos 12 Pos 13 33 Pos 14 Pos 15 28 0 5 10 15 20 Discriminator Level Figure 18 Variation in discriminator level with beam width5.3.2 Transform Validation Experiment results from IRED LEDThe measurements performed using the 8-bit system (described in section 5.2) were repeated todemonstrate the differences between the two systems and to validate the procedure formeasurement of angular subtense. The beam width of the LED was measured directly using themeasurement arrangement shown in Figure 14.The resulting data is plotted in Figure 19.The next step in the Transform Validation Experiment was to measure the beam of the LED viatwo achromat lenses. Several measurements were made and a hyperbolic fit was made to theresulting data. The fit is shown in Figure 20. 20
  • IRED Raw - X-axis (SEP 30) 7.0Converged 2M beam width (mm) - 2% uncertainty 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 5 10 15 20 25 30 Distance past vertex (mm) Figure 19 Plot of converged beam width for IR LED IRED 2 Lens Transform X-axis 7.5 7.0 6.5 CSM width (mm) 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 220 230 240 250 260 270 Distance past vertex (mm) Figure 20 Hyperbolic fit to data through two achromats from IR LED 21
  • Table 3 Summary of beam characteristics for IRED LED using 12-bit camera system SFH 400 IRED Transformed Beam Beam property Goodness of fit Waist position from LED vertex Zo 252.8 mm 0.02 Waist diameter Wo 2.73 mm 0.09 Rayleigh distance Zr 10.7 mm 0.35 Divergence 255 mrad 12 M2 588 43 Original Beam from Inverse Transform Beam property Uncertainty Waist position from LED vertex Zo 4.7 mm 2.7 Waist diameter Wo 3.00 mm 0.19 Rayleigh distance Zr 12.9 mm 0.6 Divergence 232 mrad 18 M2 588 78 Uncertainties Used Focal length etc. 0.5 % Datum positions 0.1 mm Width measurements 0.025 mm Measurement locations. 0.15 mmBeam propagation parameters calculated from the hyperbola equation. These parameterswere then back propagated by calculation through the lens using the known lensparameters allowing calculation of the LED emitted beam properties. The beamparameters are presented in Table 3.These can then be compared with the previous measurements of the LED direct beam.Tables 4 and 5 give a complete summary of the Transform Validation Experimentresults for both the 8-bit and 12-bit camera systems. 22
  • Table 4 Validation results for IRED LED - X axisLED SFH 400 IRED (950 nm) X – Axis12-bit camera + Z16 lensProperty Direct UC Trans. UC Inverse UC Difference Agreement % beam % beam % transformed % (ITB-RB) (Difference/RB (DB) beam (ITB) x 100)Waistlocation 4.28 0.04 252.8 0.03 4.7 9.7 -0.42 -9.8(mm fromvertex)Waistdiamter 3.06 0.01 2.73 0.14 3 0.58 0.06 2.0(mm)Rayleighlength 12.13 0.03 10.7 0.56 13 1.8 -0.87 7.2(mm)Divergence 253 1 255 19 232 55 21 8.3(mrad)8-bit camera system + monozoom 7 lensWaistlocation 5.6 254.1 4.9 0.7 12.5(mm fromvertex)Waistdiamter 3.38 3.12 3.47 -0.09 2.7(mm)Rayleighlength 9.76 8.6 10.7 -0.94 9.6(mm)Divergence 346 361 324 22 6.4(mrad)Note: UC = Uncertainty (%) 23
  • Table 5 Validation results for IRED LED - Y axis LED SFH 400 IRED (950 nm) Y - AXIS 12-bit camera + Z16 lens Property Direct UC Trans. UC Inverse UC Difference Agreement % beam % beam % transformed % (ITB-RB) (Difference/RB (DB) beam (ITB) x 100) Waist location 4 0.03 252.8 0.03 4.5 9.9 -0.5 -12.5 (mm from vertex) Waist diamter 3.19 0.007 2.77 0.16 3.05 0.61 0.14 4.4 (mm) Rayleigh length 11.93 0.03 10.3 0.58 12.5 1.8 -0.57 -4.8 (mm) Divergence 267 1 270 21 245 60 22 8.2 (mrad) 8-bit camera system + monozoom 7 lens Waist location 5.66 253.9 4.45 1.21 21.4 (mm from vertex) Waist diameter 3.34 3.09 3.45 -0.11 3.3 (mm) Rayleigh length 10.01 8.36 10.43 -0.42 4.2 (mm) Divergence 333 370 331 2 0.6 (mrad) 24
  • 5.4 YELLOW LED - LIGITEK LUY 3833/A29The measurements of the yellow LED were performed using an arrangement of twoachromats. Images of the beam were taken and the beam width calculated. Themeasurement points and the resultant hyperbolic fit are plotted in Figure 21. Thehyperbolic fits the measured data well. It is always difficult to make an initial estimateof the position of the waist from the LED vertex. Ideally iterative measurements wouldallow the spread of data to be symmetric around the beam waist position. Yellow LED 2 Lens Transform X-axis 7.5 7.0 6.5 CSM width (mm) 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 220 230 240 250 260 270 Distance past vertex (mm) Figure 21 Hyperbolic it to data from a yellow LED through two achromats 25
  • Table 6 Summary of beam characteristics for Yellow LED using 12-bit camera system Yellow LED - Ligitek LUY 3833/A29 Transformed Beam Beam property Goodness of fit Waist position from LED vertex Zo 242.4 mm 0.02 Waist diameter Wo 3.81 mm 0.05 Rayleigh distance Zr 16.7 mm 0.20 Divergence 228 mrad 4 M2 735 20 Original Beam from Inverse Transform Beam property Uncertainty Waist position from LED vertex Zo -3.9 mm 2.8 Waist diameter Wo 4.67 mm 0.19 Rayleigh distance Zr 25.0 mm 0.7 Divergence 186 mrad 9 M2 735 64 Uncertainties Used Focal length etc. 0.50% Datum positions 0.1 mm Width measurements 0.025 mm Measurement locations 0.15 mmTable 6 presents a summary of the beam characteristics for the yellow LED. From the resultsthe waist position of the inverse transformed beam (the direct beam) can be seen to be inside theLED chip. This provides an interesting contrast to the IR LED. It should also be noted that thedivergence of this LED is significantly less than the other "display" LEDs examined in thisstudy. 26
  • 5.5 BLUE LED - NICHIA NSPB500 RANK WSMeasurements of the 2-lens transformed beam from the blue LED were made at positions eitherside of the beam waist and the results are shown in Figure 22.The departure of data from the smooth fitted curve, shown in Figure 22, was thought to be dueto filter changes creating intereflections and problems for the CSM beam width measurement. Blue LED 2 Lens Transform X-axis 5.4 5.2 CSM width (mm) 5.0 4.8 4.6 4.4 4.2 4.0 235 240 245 250 255 260 Distance past vertex (mm) Figure 22 Hyperbolic fit to data from Blue LED through two achromats 27
  • Table 7 Summary of beam characteristics for Blue LED using 12-bit camera system Blue LED - Nichia NSPB500 Rank WS Transformed Beam Beam property Goodness of fit Waist position from LED vertex Zo 249.3 mm 0.03 Waist diameter Wo 4.05 mm 0.17 Rayleigh distance Zr 11.8 mm 0.51 Divergence 343 mrad 21 M2 1171 112 Original Beam from Inverse Transform Beam property Uncertainty Waist position from LED vertex Zo 1.4 mm 2.7 Waist diameter Wo 4.47 mm 0.30 Rayleigh distance Zr 14.4 mm 0.8 Divergence 310 mrad 27 M2 1171 170 Uncertainties Used Focal length etc. 0.50% Datum positions 0.1 mm Width measurements 0.025 mm Measurement locations 0.15 mmWith these results an externally located beam waist location outside the Blue LED package canbe seen from the positive waist position in the inverse transform section of Table 7. This issimilar to the Osram SFH 400 IRED, but the waist is not as conveniently far away from theLED vertex which facilitated the direct beam measurement in section 5.2. 28
  • 5.6 GREEN LED - NICHIA NSPG500 RANK GSMeasurements of the beam width were made at positions either side of the beam waist and theresults are shown in Figure 23. Green LED 2 Lens Transform X-axis 5.4 5.2 CSM width (mm) 5.0 4.8 4.6 4.4 4.2 4.0 235 240 245 250 255 260 Distance past vertex (mm) Figure 23 Hyperbolic fit to data from Green LED through two achromatsFigure 23 shows a smaller data divergence from the fitted curve. These deviations can bedisregarded because the rest of the data fits so well. 29
  • Table 8 Summary of beam characteristics for Green LED using 12-bit camera system Green LED - Nichia NSPG500 Rank GS Transformed Beam Beam property Goodness of fit Waist position from LED vertex Zo 248.6 mm 0.03 Waist diameter Wo 4.12 mm 0.15 Rayleigh distance Zr 12.7 mm 0.47 Divergence 325 mrad 17 M2 1129 93 Original Beam from Inverse Transform Beam property Uncertainty Waist position from LED vertex Zo 0.5 mm 3.0 Waist diameter Wo 4.82 mm 0.29 Rayleigh distance Zr 17.4 mm 0.8 Divergence 278 mrad 22 M2 1129 148 Uncertainties Used Focal length etc. 0.5 % Datum positions 0.1 mm Width measurements 0.025 mm Measurement locations 0.15 mmThe results in Table 8 summarise the beam characteristics for the green LED. It shows anexternal waist from the LED. The measurement procedure requires at least 10 measurements ofthe beam width either side of the waist position, therefore the waist would not be positioned farenough away from the LED vertex to be easily measured as a direct beam. 30
  • 5.7 RED LED - KINGBRIGHT L-53SRC/EMeasurements of the beam width were made at positions either side of the beam waist and theresults are shown in Figure 24. Red LED 2 Lens Transform X-axis 5.4 5.2 CSM width (mm) 5.0 4.8 4.6 4.4 4.2 4.0 235 240 245 250 255 260 Distance past vertex (mm) Figure 24 Hyperbolic fit to data from Red LED through two achromats 31
  • Table 9 Summary of beam characteristics for Red LED using 12-bit camera system Red LED - Kingbright L-53SRC/E Transformed Beam Beam property Goodness of fit Waist position from LED vertex Zo 244.0 mm 0.03 Waist diameter Wo 4.14 mm 0.13 Rayleigh distance Zr 13.8 mm 0.44 Divergence 301 mrad 13 M2 1052 75 Original Beam from Inverse Transform Beam property Uncertainty Waist position from LED vertex Zo -3.8 mm 3.0 Waist diameter Wo 4.94 mm 0.28 Rayleigh distance Zr 19.6 mm 0.9 Divergence 252 mrad 18 M2 1052 126 Uncertainties Used Focal length etc. 0.5 % Datum positions 0.1 mm Width measurements 0.025 mm Measurement locations 0.15 mmA summary of the beam characteristics for the red LED is presented in Table 9. As seen withthe yellow and blue LEDs in sections 5.4 and 5.5, a beam waist location inside the red LEDpackage can be seen from the negative waist position in the inverse transform section of Table9. 32
  • 5.8 WHITE LED - NICHIA NSPW500 RANK BSMeasurements of the beam width were made at positions either side of the beam waist and theresults are shown in Figure 25. White LED 2 Lens Transform X-axis 5.0 4.8 4.6 CSM width (mm) 4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 235 240 245 250 255 260 Distance past vertex (mm) Figure 25 Hyperbolic fit to data from White LED through two achromats 33
  • Table 10 Summary of beam characteristics for White LED using 12-bit camera system White - Nichia NSPW500 Rank BS Transformed Beam Beam property Goodness of fit Waist position from LED vertex Zo 245.9 mm 0.03 Waist diameter Wo 3.54 mm 0.16 Rayleigh distance Zr 11.6 mm 0.52 Divergence 305 mrad 19 M2 911 90 Original Beam from Inverse Transform Beam property Uncertainty Waist position from LED vertex Zo -3.6 mm 3.5 Waist diameter Wo 4.35 mm 0.29 Rayleigh distance Zr 17.5 mm 0.9 Divergence 248 mrad 21 M2 911 131 Uncertainties Used Focal length etc. 0.5 % Datum positions 0.1 mm Width measurements 0.025 mm Measurement locations. 0.15 mmA summary of the beam characteristics for the white LED is presented in Table 10. 34
  • 5.9 ORANGE LED - TOSHIBA TLOH190P Orange LED One Lens Transform. X-axis 13. 12. 11. CSM width (mm) 10. 9.0 8.0 7.0 6.0 180 200 220 240 260 280 Distance past vertex (mm) Figure 26 Orange LED one achromat transformThe orange LED only used one achromat to perform the beam transformation to give anappropriate image to analyse on the 12-bit camera.The orange LED data did not fit well to a hyperbola. This is clearly demonstrated in Figure 32.Further study of the measurement data indicated that the propagating beam was astigmatic andhence would not fit to the beam propagation model. Astigmatic beams could be treated using themethodology described in ISO 11146-2 7 but this is beyond the scope of this study.The astigmatic nature of the beam can be discovered from the steady change of azimuth angle asthe beam propagates, see Figure 33. The apparent sudden jump of the angle is due to the beamwidths in the X and Y direction reaching the same value at that point in the Z direction. Thisindicates a nearly circular beam and makes the azimuth angle indeterminate. As described in theISO standard 11146-2 the insertion at this point of a cylindrical lens at the right azimuth anglemay remove the astigmatism. 35
  • Astigmatism in beam from Orange LED 20.00 15.00 10.00 WoX WoY 5.00 Elipticity x 10 Azimuth (degrees) 0.00 170 190 210 230 250 270 290 310 -5.00 -10.00 Distance past Vertex (mm)Figure 27 Plot demonstrating the astigmatism of the orange LED and hence the lack of fit to a hyperbola. 36
  • 5.10 HIGH POWER BLUE LED - LUXEON STAR Figure 28 Photo of Luxeon Star LED with Fraen 10° lensThe Luxeon Star LED, including the Fraen 10° lens associated with the LED, is shown inFigure 29. Details for the Luxeon Star are given in Appendix 3. Luxeon V-Star Batwing LED (Royal blue) + Fraen 10° Lens. (x-axis) 13.0 12.0 CSM width (mm) 11.0 10.0 9.0 8.0 7.0 340 350 360 370 380 390 400 410 Distance past vertex (mm)Figure 29 Hyperbolic fit to data from high power royal blue LED through one achromat 37
  • The values for the beam width of the Luxeon Star LED are plotted in Figure 30. The angularsubtense of this device far exceeds max (100 mrad) and hence falls outside the region where thecoefficient C6 (IEC 60825-1) value depends upon angular subtense. It should be noted that thisLED carries a Class 2 warning label. 38
  • 6 UNCERTAINTY ANALYSIS 18With reference to the ISO Guide to Uncertainty in Measurement the uncertainties areseparated into: Type A, those uncertainties evaluated by statistical methods and Type B, those evaluated by other methods.The equations used to derive the beam propagation parameter are partially differentiated withrespect to all the measured quantities to produce contributions to the uncertainty budget.A simplified summary of the Type B uncertainty budget is shown in the Table 11. Theuncertainties quoted are the reduced values (coverage factor k=1). Table 11 List of Type B uncertainty values Source of Uncertainty Value Focal lengths 0.5 % Datum positions 0.1 mm Width measurements 0.025 mm Measurement locations 0.15 mmThe second moment width measurements were fitted to a hyperbolic curve and the curvecoefficients were then used to derive the beam propagation parameters. The uncertainty of thismeasurement was therefore derived by the partial differentiation of the equations defining thepropagation parameters. This was checked using a step-wise uncertainty analysis, whichproduced close agreement with the original method (partial differentiation is a more rigorousmethod).Correlation has not been considered in this analysis but the uncertainties were combined usingsum of squares to give the most conservative estimate of uncertainty.The uncertainty derivations had to consider three measurement configurations a) No Transform lenses used (LED has an external waist) b) One Transform lens used c) Two Transform lenses usedThe tables below give examples of each configuration. The Type B uncertainties listed in Table11 are added in quadrature to provide the uncertainty values for component positions. 39
  • 6.1.1 No Transform Lens UsedThis example is for the 12-bit direct beam measurement of Osram IR LED. Table 12 details theuncertainty components. The uncertainty references are at the end of the Section 6. Table 12 Uncertainties for measurement of Osram IR LED Waist position zo 4.28 mm +/- 0.04 Waist diameter Wo 3.06 mm +/- 0.0067 Rayleigh distance Zr 12.13 mm +/- 0.0287 Divergence 252 mrad 0.81 M2 639 +/- 3.17The uncertainties for the raw beam were calculated by knowledge of the Type B uncertainty inthe measurement of distance modified by the local gradient of the hyperbola. The resultingcovariances were then added in quadrature to obtain estimates for the 1 standard deviation level.6.1.2 One Lens datasheetThis example is for the 12-bit direct beam measurement of high power Luxeon Star LED OsramIR LED (optical arrangement and dimensions are shown in Figure 31). Table 13 details theuncertainty components for this measurement. Figure 30 Diagrams illustrating the required dimensions for the Luxeon star LED 40
  • Table 13 Uncertainties for measurement of Luxeon Star LED Distance 1 Uncertainty Symbol Formulae (mm) Uncertainty calculation code LED Vertex to lens datum L4 253.00 0.21 RMSDatum of Lens 1 to 1st principle L8 19.7 0.16 M/F plane Separation of principle planes L9 5.3 0.11 M/F Effective focal length of lens fe 76.2 0.38 M/F Location of measured beam B Zo1 Z0 370.08 0.04 Ref 1 waist 2 CDistance of measured waist from X1 X 1 Zo1 L4 L8 L9 fe 15.88 0.48 RMS focal plane Rayleigh Length of measured 1 1 2 Zr1 Zr A C B 24.51 0.50 Ref 2 beam C 4 B2 Waist width of measured beam Wo1 Wo A 7.56 0.16 Ref 3 4CFar-field divergence of measured Wo 1 10 3 308.3 8.97 Ref 4 beam Zr f2 Transform Parameter G1 G 2 2 6.81 0.24 Ref 5 ( X1 Zr ) Distance of output waist from focal plane Y1 Y1 G X 1 108.1 5.0 Ref 6 Location of output beam waist Zo2 Zo2 L4 L8 Y1 fe 88.4 5.0 Ref 7 wrt vertex of LEDWaist width of LED output beam Wo2 Wo 2 Wo1 G1 19.7 0.5 Ref 8Rayleigh Length of LED output Zr2 Zr2 G Z r1 166.9 6.8 Ref 9 beam Far-field divergence of LED Wo 2 10 3 118.2 18.3 Ref 10 output beam Zr 41
  • 6.1.3 Uncertainty Budget For Beam Waist (two lens transform)Figure 32 illustrates some of the critical measurements made for the Ligitek LUY 3833/A29Yellow LED evaluation. Figure 31 Critical measurements for two lens transformationTable 14 Uncertainties for measurement of Yellow LED Uncertainty Distance 1 Symbol Formulae calculation (mm) Uncertainty codeLED Vertex to lens 2 L5 161.8 0.14 RMS datum Datum of Lens 2 to L10 13.23 0.15 M/F 1st principle plane Separation of principle planes of L11 4.9 0.10 M/F Lens 2Effective focal length fe2 100 0.50 M/F of Lens 2LED Vertex to lens 1 L4 81.60 0.14 RMS datum Datum of Lens 1 to L8 19.7 0.16 M/F 1st principle plane Separation of L9 5.3 0.11 M/F principle planesEffective focal length fe1 76.2 0.38 M/F of lensLocation of measured B Zo1 Z0 242.43 0.02 Ref 1 beam waist 2 CDistance of measured waist from input X1 X1 Zo1 L4 L8 L9 fe -37.50 0.55 RMSfocal plane of Lens 2 Rayleigh Length of 1 1 2 Zr1 Zr A C B 16.70 0.20 Ref 2 measured beam C 4 42
  • Waist width of B2 Wo1 Wo A 3.81 0.05 Ref 3 measured beam 4CFar-field divergence Wo of measured beam 1 10 3 228.2 3.85 Ref 4 Zr 2 f2Transform Parameter G1 G1 5.93 0.16 Ref 5 ( X 12 Z r21 )Distance of waist ofintermediate beam Y1 Y1 G X1 -222.5 6.8 Ref 6from focal plane of Lens 2 Location ofintermediate beam Zo2 Z o2 L5 L10 Y1 f e2 297.5 6.8 Ref 7waist wrt vertex of LED Waist width of Wo2 Wo 2 Wo1 G1 9.3 0.2 Ref 8intermediate beamRayleigh Length of Zr2 Zr2 G Z r1 99.1 2.9 Ref 9intermediate beamFar-field divergence Wo 93.7 0.6 Ref 10of intermediate beam 2 103 Zr Distance between intermediate waist X 2 Zo2 L4 L8 L9 f e1 114.7 X2 6.8 RMSand input focal plane of Lens 1 2 f1Transform Parameter G2 G2 2 0.2526 0.018 Ref 5 (X2 Z r22 ) Distance of waist of input beam from Y2 Y2 G2 X2 28.98 2.7 Ref 6focal plane of Lens 1 Location of input beam waist wrt Zo3 Z 03 L4 L8 Y2 f1 -3.88 2.8 Ref 7 vertex of LEDWaist width of LED output beam Wo3 Wo 3 Wo 2 G2 4.67 0.2 Ref 8Rayleigh Length of Zr3 Z r3 G2 Z r 3 25.03 0.73 Ref 9LED output beamFar-field divergence Wo3 3 3 103 186.41 9.36 Ref 10of LED output beam Zr 3 43
  • 6.1.4 Uncertainty ReferencesThe fitting used for the measurement data was : (W W )2 A a (B b) z (C c) z 2Example of one lens analysis as fitted to the Luxeon blue LED propagation data. Where A = 13074 B= -70.35 C= 0.0950 and a = 0.972 b= 0.00518 c = 6.911E-06Example of two lens analysis as fitted to the Yellow LED transformed output beam Where A = 3075 B= -25.25 C= 0.0521 sl = 0.1 and a = 0.143 b= 0.00117 c= 2.395E-06These are the partially differentiated equations for the derived parameters and therefore providethe measurement uncertainty of the named parameters. 2 1 2 c BReference 1: Zo b 2 C C 2 2 1 bB c B2Reference 2: Zr a2 A 2CZr 2C C 2C 2 1 bB cB2Reference 3: Wo a2 2Wo 2C 4C 2 2 103 2 WoReference 4: Wo Zr Zr Zr 2 2 2 2 2 Gi 2 Gi 2 GiReference 5: f Gi Xi Xi f Z ri Z ri Gi Gi2 X i Gi G Gi Gi2 Z ri where 2 , 2 i and 2 2 Xi f2 f f Z ri f 2 2Reference 6: Yi Gi Xi Xi Gi 2 2 2Reference 7: Zo(i 1) 2 l Yi f2or1 2 Woi Gi 2Reference 8: Wo ( i 1) Woi Gi 2 Gi 44
  • 2 2Reference 9: Zr ( i 1) Gi Zri zri Gi 2Reference 10: 103 2 Woi i Woi Zri Zri Zri 45
  • 7 CONCLUSIONSThe results from this investigation show that this method can be applied successfully to theanalysis of beam propagation parameters 17 and hence the apparent source size determination forstigmatic and simple astigmatic beams from LEDs. It is also capable of identifying beams thatsuffer from general astigmatism and which are not currently eligible for relaxation ofclassification limits using this simplified form of analysis. The good agreement obtained for theISO Transform Validation Experiment for both the 8-bit and 12-bit systems shows that thetechnique is highly robust. This level of agreement was unexpected, due to the increased noiselevels inherent in the 8-bit system as a result of the smaller dynamic range and the un-cooledanalogue camera.Assuming a maximum permissible inaccuracy is 10% at a measuring distance of 100 mm, thegeometric approximation can only be used with beams whose Rayleigh Length is less thanapproximately 50 mm. This limit effectively marks the boundary above which diffractioneffects become noticeable and classical ray-tracing optics cannot be used. The measured LED’sRayleigh Lengths were all less than 50 mm which allows us to consider the apparent source sizeand its location, to be the same as the direct beam waist.The detection of general astigmatism in the beam from the orange LED shows that the in builtchecks of the techniques applicability work well.Figure 33 shows the high level of agreement between the propagation parameters derivedthrough the 8-bit and 12-bit methods using the IR LED. It also includes the parameters derivedthrough the Transform Validation Experiment. Figure 34 shows the measurement results fromeach of the LED’s plotted on the contour plot for angular subtense as a function of the measuredbeam characteristics of LEDs. The size of the ellipse indicates the level of measurementuncertainty for each LED.The montage, presented in Figure 32, clearly depicts how the beam images, profiles and resultscorrespond. The beam image mapped onto the beam propagation envelope with the appropriate2D spatial intensity profiles is a helpful visualization of the evolution of the beam as it travelsthrough space. The important aspects to note are that the point in the beam propagation wherethe LED chip structure is in focus does not correspond to the position of the beam waist. This isan important result because it has been the practice of some safety assessors to use the positionof sharp focus to estimate the apparent source size. In this situation this methodology wouldresult in an estimate of the apparent source size that was greater that the real value. This wouldproduce a lower value of the potential hazard of the LED than actuality. 46
  • Figure 32 Montage of the spatial beam profiles which make up the propagation envelope of the LED 47
  • Com parison of Beam Param eter Measurem ents - Y-axis 14 12 10millimeters or d/rad 8 12-bit raw beam 12-bit t ransf ormed 8-bit raw beam 6 8-bit transformed 4 2 0 Zo Wo Zr Theta Comparison of Beam Parameter Measurements - X-axis 16 14 12 millimeters or d/rad 10 12-bit raw beam 12-bit transformed 8 8-bit raw beam 8-bit transformed 6 4 2 0 Zo Wo Zr Theta Figure 33 Comparison of 8-bit and 12-bit camera results 48
  • Figure 34 Plot showing contours of angular subtense, including results for the measured LEDs 49
  • 7.1 FUTURE DIRECTIONSThis project has allowed the identification of many areas where the technique can be improvedwith further work. An improvement of the length measuring system would result in a reductionof the uncertainty of the validation process and would produce greater accuracy in thedetermination of beam waist, divergence and thus retinal hazard. An optically encoded servomotor slide would reduce the distance measurement uncertainty by an order of magnitude. Thereplacement of a manual vernier slide with an electrically driven version would remove the needto manually read measurement position. This would allow better exclusion of background lightby the use of a local light tight enclosure coated with diffusing black paint. The lack ofextraneous light would improve the measurement dynamic range and reduce the probability ofproblems caused by optical artefacts on the CCD images.A custom produced achromat with a larger diameter and shorter focal length would serve toreduce the cumulative uncertainty. The reduction in the number of optical surfaces throughwhich the light propagates would serve to reduce aberration of the beam wavefront and theproduction of scattered light. A custom manufactured graticule would allow calibration of thewhole field of view at higher magnification zoom settings.An ideal development of this project would be to determine the real spot size produced by agiven source by producing an “artificial” eye or eye analogue. Apparent source size was createdas an artifice to allow the comparative measurement of the effect of viewing sources larger thana “point” source yet smaller than the 100 mrad subtense advocated in the IEC safety standard.This would then inform the debate about the effect of problematic beam profiles on the retinaand thus would allow a thermal diffusion model to be produced with finite element analysis.Figure 35: Diagram showing the lens of an eye transforming a LED beam. The size of the spot on the retina is not measurable with current techniques.The correct treatment of generally astigmatic beams from LEDs and other intermediate sourceswould require the use of the mathematical method outlined in “ISO 11146-2 7 Lasers and laser-related equipment. Test methods for laser beam widths, divergence angle and beam propagationratio. Part 2: General astigmatic beams”. The existing software could be adapted and themeasurement technique amended to provide the required technique 8. 50
  • APPENDIX 1: SECOND MOMENT, AZIMUTH AND PRINCIPLE WIDTH DERIVATIONThe reduced second order moments can be determined by a measurement of the energy densitydistribution over a limited area or window: y2 x2 2 y1 x1 ( x x )2 I ( x, y, z) x ( z) y2 x2 x2 y1 x1 I ( x, y, z) y2 x2 2 y1 x1 ( y y )2 I ( x, y, z) y ( z) y2 x2 y2 y1 x1 I ( x, y, z)where the summations are carried out over a rectangle parallel to the x- and y-axes and: 3 3 x1 x d x x2 x d x 2 2 and 3 3 y1 y d y y2 y d y 2 2The concept of second moment measurements is extended to include the “mixed moments” ofthe spatial and divergence properties of the beam. For example, the spatial mixed moment is: y2 x2 y1 x1 (x x )( y y ) I ( x, y, z ) 2 xy ( z) y2 x2 xy y1 x1 I ( x, y, z )The three spatial moments describe the lateral extent of the power density distribution of thebeam in the reference plane. The directions of minimum and maximum extent are calledprincipal axes which are always orthogonal to each other. Any power density distribution ischaracterized by the extents along its principal axes and the orientation of the principal axes.The beam width along the direction of that principal axis, which is closer to the x-axis of thelaboratory system, is given by: ½ ½ 2 2 2 2 2 2 d x z 2 2 x y x y 4 xyand the beam width along the direction of that principal axis, which is closer to the y-axis by: ½ ½ 2 2 2 2 2 2 d y z 2 2 x y x y 4 xy 51
  • x2 y2 2 2 where sgn x y x2 y2Finally, the azimuthal angle between the principal axis that is closer to the X-axis and the X-axis is : 2 xy ½ arctan 2 x y2 52
  • APPENDIX 2: DESIGN AND TECHNICAL SPECIFICATION FOR A FACILITY TO DETERMINE THE APPARENT SOURCE SIZE OF LIGHT EMITTING DIODESIntroductionThe angular subtense of an apparent source of radiation in the 400 nm to 1400 nm wavelengthrange is required by current laser safety standards to permit calculation of the relaxation factorC6, for extended sources. It is the ratio of the angular subtense of the source in question to thatof a source that would form the realistic minimum spot size on the retina (1.5 mrad).Classification or assessment of the thermal hazard from a source requires that both the angularsubtense and location of an extended source be known before there can be a relaxation of themaximum permissible exposure (MPE) of the eye. The location of a source is required so thatthe angular subtense can be calculated for viewing from the minimum conceivable eyeaccommodation distance of 100 mm.It is a simple matter to measure the physical size of the chip of a LED that has a Lambertianradiation pattern but it is more difficult to know or measure the location or size of the apparentsource of collimated beams from an LED. Such beams can have a nearly plane wavefront whichwould imply that the apparent source is located at infinity with an unknown angular extent.However, recent advances in the characterization of optical beams, both coherent andincoherent, enable prediction of their propagation envelopes. It is now possible to assess theintrabeam viewing hazard by using known beam characteristics to estimate the angular subtenseof an extended source that would present the greatest hazard to a retina.Measurement of the optical constants of the propagation envelope of a beam have been thesubject of considerable research over the last ten years. A consequence of this work is theevolution of ISO standards1 for the measurement of the diameter and divergence of a beam. Theprocedures and techniques that are proposed here for the determination of the diameter andlocation of the apparent source of a beam2 are based on the principles underlying the ISOstandards for simple astigmatic beams. Should a beam display general astigmatism (twist) norelaxation of the laser safety criteria should be given.Beam measurementsThere are a number of methods available for measurement of the diameter of a beam as well asits far-field divergence. The basic principles for those methods have been established by the ISOstandard. They are applicable to laser beams with a relatively small beam propagation ratio, M2.Recent research has demonstrated that adequate steps have to be taken to counter the effects ofnoise and offset errors when measuring the transverse irradiance distribution of a beam. Whenthese steps are taken, the propagation behaviour of incoherent broadband beams as well as high- 1 ISO 11 146:1999. “Test methods for laser beam parameters: beam widths, divergence angle and beam propagation factor”. 2 The measurements proposed in this document are applicable to beams whose full divergence angle is less that 30°. 53
  • quality laser beams can be predicted reproducibly with considerable precision. The methodsleading to estimates of the diameter of a beam use a procedure known as the Converging SecondMoment diameter or width measurement (CSM). Those methods are being defined in therevision of ISO 11146 that is currently in preparation.The preferred method for measuring all the propagation characteristics of a beam is to performCSM diameter measurements at a number of locations either side of the beam waist.The Optical SystemThe beam measurement process consists of using a CCD sensor to image the irradiance profileat about ten locations, ideally either side of the beam waist. The proposed optical systemcontains variable magnifying optics that are designed to facilitate imaging the transverseirradiance profiles so that they occupy approximately one quarter of the sensor screen height.Other components are included in the system to attenuate the beam power and avoid sensorsaturation and to provide spatial calibration of the pixel array of the sensor.A schematic diagram of the proposed system is displayed later in this section.The main base bench contains two sub-systems, the LED Bench and the Imaging Bench.The LED BenchThe LED bench is capable of axial movement so as to transport the emerging beam relative tothe imaging bench. The movement can be achieved with manual or automatic means. Themovement distance relative to a datum should be determined with an uncertainty less than 0.1mm. The bench is to be 400 mm long and fitted with two or three component holders that arecapable of smooth and fine adjustment about a number of specified axes.Unit 1 - The first carriage is a holder for a range of LEDs. The LED should be firmly held in acantilevered stand and restrained from any movement that might arise from movement of itspower supply leads. An arrangement consisting of a split clamping plate with a suitable range ofcollets that will accommodate the diameters of the selected LEDs. This unit should be capableof adjustment along the transverse x and y axes, the axial z axis and rotation about the Ox andOy axes. The LEDs will probably all have a cylindrical form with discrete diameters of 3 mm, 5mm, 8 mm or 10 mm.Unit 2 - The next carriage has a cantilevered lens holder that is capable of placing a lens withits vertex touching the output face or vertex of the lens of the LED. The mount should hold thelens normal to the optical axis of the system and be capable of smooth adjustment along thetransverse x and y axes and the axial z axis. i) The lens should not need to have an aperture greater than f/2. This lens is designed to transform beams whose waist is virtual and behind the LED into one with an accessible waist in the measurement region of the system. The design of the lens should be based on a multi-element composition aimed at minimising aberrations. It should also be broadband A/R coated to minimise reflections at the wavelength range that is to be studied. ii) An examination of the capability of a +50 mm focal length transform lens on the range of beam parameters under consideration is given in the Annex. The results suggest that this lens will produce transformed beams whose parameters are all within the measurement capability of the proposed system. 54
  • A second carriage, similar to Unit 2 should be provided so that a second transform lens can bemounted on the LED bench for the “validation” trials.The Imaging BenchA static imaging bench is to be provided with three main carriages. These are for an attenuatorwheel, a calibration graticule and the CCD camera fitted with a “zoom” microscope lens system.Unit 3 - This is the main beam attenuation device. It consists of an indexable wheel containing,say, eight neutral density filters. The apertures in the filter holder should be sufficient toaccommodate at least 95% of the beam power. The ND filters should exhibit a high degree ofuniformity across 95% of their full aperture and, ideally, should be antireflection coated for thewavelength range that is to be studied.The filter wheel holder should be capable of adjustment along both transverse x and y axes. Itshould also be capable of rotation about the Oy axis so that reflections can be diverted from thebeam path.Unit 4 - This carriage contains a holder for a calibration graticule. It should be capable of fineadjustment along all the x, y and z axes. The location of the graticule along the z axis should becapable of placement with a separation D1 from the objective of the microscope between 30mm and 300 mm. The graticule should be capable of being adjusted by rotation about an Oz axisthrough the centre of its aperture. The design of the graticule should enable viewing with theCCD array to present contrast and sharpness sufficient to enable use of the provided calibrationsoftware with an adequately low level of uncertainty. Trials will be performed with a number ofgraticules to permit analysis and select a suitable design.The function of the graticule is to enable calibration of the transverse dimensions and linearityof the CCD sensor array. The graticule is not required during actual beam measurementsalthough it could be advantageous to use the beam to illuminate the graticule at its ownwavelength.The graticule has to be removed from the beam path during measurements but it should becapable of being relocated in its original position in the object plane of the microscope withoutrequiring readjustment.Units 5 and 6 - A sturdy carriage is required to hold firmly a zoom microscope and CCDcamera with a high degree of stability. The function of the zoom microscope is to enlarge theimage of the transverse irradiance distribution in the object plane and focus it on the CCD arrayof Unit 6. The zoom function enables adjustment of the magnification so that, ideally, the imagefills approximately one quarter of the array. i) The Microscope - The zoom microscope is required to have a field of view that will be adjustable between 0.25 mm and 50 mm. A microscope that can satisfy this requirement is the Leica MonoZoom77 Video Microscope System. When fitted with either x0.25 or x2 objectives in combination no amplifier lens or a x3 amplifier, it may just cover the required field size range when used in conjunction with an appropriately sized CCD array sensor. In addition, an aperture diaphragm (P/N 007/023) can be incorporated into the system to provide continuously variable attenuation of the transmitted power. This is a very attractive facility since it will enable power adjustment between the steps available with the ND filters. 55
  • A MonoZoom 7 instrument has been hired to enable assessment of its optical performance and the magnitude of residual aberrations. Should the performance be inadequate an alternative set of components could be assembled from the Leica Z16 APO system. ii) The Camera - It is thought that a CCD sensor system with a 12-bit dynamic sensitivity is required to provide sufficient resolution and noise control for beam profile analysis. Furthermore, a pixel number in excess of 1 million is estimated to be required to give sufficient resolution for spot diameter measurements. However, these aspects are to be studied in one of the principle topics of this programme. One further aspect influencing the selection of a camera is the physical size of the CCD array. The height dimension of a 4:3 video array is the dimension that will control the field of view seen through the microscope. The dimensions of an array are not always given in a camera specification. It could be referred to as a b” screen where it will have a height of 6.6 mm and width of 8.8 mm, giving an 11 mm diagonal. Alternative array sizes can be quoted as ½” and a”. These dimensions must be obtained before all the lens accessories for the microscope system can be selected.Additional ComponentsThree more optical aspects of the system need to be considered and provided. These are a“beam stop” and background illumination shields. The need for these components arises fromthe requirement to eliminate background optical noise from the CSM width estimates. When thebeam passes through optical components, residual multiple internal reflections and other sourcesof stray light will combine with the main beam and tend to enlarge estimates of beam widths.The first precaution is to place the whole system in a light-tight enclosure so that light from theenvironment does not reach the CCD array. The next stage is to attenuate any light scattered orreflected out of the main beam by placing an absorbing shield with an aperture at some distancedown the beam path. The aperture should limit beam divergences above 30°.The final precaution is to make a record of any residual light or optical imperfections byrecording a frame of the average background field and subtract it from the beam profile. Thiscan be done by inserting a physical beam stop into the path of the beam so that the only lightthat passes is from the sources of extraneous noise and recording this as the background. Theexact nature and location of this beam stop must be the subject of further discussion. 56
  • Figure 36 Proposed optical apparatus for measurement of apparent source size 57
  • APPENDIX 3: LED TECHINICAL DATA SHEETSVISIBLE LEDsRed: Kingbright L-53SRC/Ehttp://www.us.kingbright.com/data/spec/W1503SRC-D.pdfYellow: Ligitek LUY 3833/A29http://www.ligitek.com/2-2.htmGreen: Nichia NSPG500 Rank GShttp://www.nichia.co.jp/specification/led_lamp/NSPG500S.pdfBlue: Nichia NSPB500 Rank WShttp://www.nichia.co.jp/specification/led_lamp/NSPB500S.pdfWhite: Nichia NSPW500 Rank BShttp://www.nichia.co.jp/specification/led_lamp/NSPL500S.pdfIR LEDIR: Osram SFH 400http://www.osram.convergy.de/scripts/product_family.asp?CLSOID=10024&FAMILYOID=20412HIGH BRIGHTNESS LEDSOrange: Toshiba TLOH190Phttp://www.semicon.toshiba.co.jp/td/ja/Opto/Visible_LED/20030620_TLOH190P(F)_datasheet.pdfBlue: Luxeon Starhttp://www.lumileds.com/pdfs/DS23.pdfTable 15 Summary of optical and electrical characteristics of LEDsLED Size Colour Peak Typical Typ Max Bandwidth Divergence,model (mm) (nm) Lumious current Fwd (nm) ½ Intensity (mA) voltage (mcd) (V)Kingbright 5 Red 660 1500 20 2.5 20 15L-53SRC/ELigitek 5 Yellow 595 2700 20 2.8 - 12LUY3833/A29Nichia 5 Green 520 11600 30 4.0 40 -NSPG500Rank GSNichia 5 Blue 470 3460 30 4.0 30 -NSPB500Rank WSNichia 5 White 595 6400 30 4.0 N/a -NSPW500Rank BSOsram 5 Ired 950 - 300 5 55 6SFH 400Toshiba 10 Orange 612 20000 50 4 10 6TLOH190PLuxeon - Blue 470 100000 700 5 25 10Star 58
  • REFERENCES1 “ICNIRP statement on light-emitting diodes (LED’s) and laser diodes: implications for Hazard assessment”, Health Physics June 2000, Volume 78, Number 6 (http://www.icnirp.de/documents/led.pdf) 2 1. Ward B.A. “Measurement of Laser and LED Beams for prediction of Angular Subtense ILSC 2003 conference Jacksonville, FL, USA3 BS EN 60825-1:1994, Incorporating Amendment 1,2 and 3, Safety of laser products. Equipment classification, requirements and user’s guide4 ISO 11146:1999 Lasers and laser-related equipment -- Test methods for laser beam parameters -- Beam widths, divergence angle and beam propagation factor5 BS EN ISO 11554:2003 Optics and optical instruments. Lasers and laser-related equipment. Test methods for laser beam power, energy and temporal characteristics6 01/714513 DC ISO/CD 11146-1. Lasers and laser-related equipment. Test methods for laser beam widths, divergence angle and beam propagation factor. Part 1: Stigmatic and simple astigmatic beams7 01/714514 DC ISO/CD 11146-2. Lasers and laser-related equipment. Test methods for laser beam widths, divergence angle and beam propagation ratio. Part 2: General astigmatic beams8 ISO/PDTR 11146-3. Lasers and laser-related equipment. Test methods for laser beam widths, divergence angle and beam propagation ratio. Part 3: Alternative test methods and geometrical laser beam classification and propagation (BSI draft 01/714515 DC)9 ISO 13694:2000 Optics and optical instruments – Lasers and laser-related equipment – Test methods for laser beam power (energy) density distribution10 IEC 60825-13. Ed.1. Safety of laser products. Part 13: Measurements for classification of laser products (BSI draft 03/307798 DC)11 IEC TR 60825-14 ed. 1. Safety of laser products. Part 14. A users guide (BSI Draft 02/206661 DC)12 Henderson R and Schulmeister K “Laser Safety” Bristol, IOP,200413 ISO 11145:2001, Optics and optical instruments . Lasers and laser-related equipment . Vocabulary and symbols. 59
  • 14 ISO 13694, Optics and optical instruments . Lasers and laser-related equipment . Test methods for laser beam power (energy) density distributions15 IEC 61040:1990, Power and energy measuring detectors. Instruments and equipment for laser radiation.16 Amarande S, Giesen A Hügel H “Propagation analysis of self-convergent beam width and characterization of hard-edge diffracted beams” APPLIED OPTICS Vol. 39, No. 22, 1 August 200017 Siegman A.E “ Defining the Effective radius of Curvature for a Nonideal Optical Beam” IEE Journal of Quantum Electronics Vol 27 No 5 May 199118 ISO Guide to the expression of uncertainty in measurement 1995, ISBN 92-67- 10188-919 Wood. RM “Laser-Induced Damage of Optical Materials” Bristol IOP 2003 60
  • GLOSSARYGamma A numerical value, or the degree of contrast in a television picture, which is the exponent of that power law which is used to approximate the value of the magnitude of the output signal as a function of the input signal over the region of interest.Interline Transfer A technology of CCD design, where rows of pixels are output from the camera. The sensors active pixel area and storage register are both contained within the active image area. This differs from "frame transfer" cameras that move all active pixels to a storage register outside of the active area.Vignetting In an optical system, the gradual reduction of image illuminance as the off-axis angle increases, resulting from limitations of the clear apertures of elements within the system. This is called vignetting and is shown in Figure 37. Figure 37 Illustration of vignetting effectStigmatism Property of a beam having circular power density distributions in any plane under free propagation and showing power density distributions after propagation through a cylindrical lens all having the same or orthogonal orientation as that lensSimple astigmatism Property of a non-stigmatic beam whose azimuth shows a constant orientation under free propagation, and retains its original orientation after passing through a cylindrical optical element whose axis is parallel to one of the principal axes NB: The principal axes of a power density distribution corresponding to a beam with simple astigmatism are called the principal axes of that beam. 61
  • Generalised Rayleigh length (ZR,g) Distance along the beam axis from the generalized beam waist where the generalized beam diameter is a factor of 2 larger than the generalized beam waist diameter.EFL (Effective Focal length) The effective focal length (EFL) or equivalent focal length (denoted f in Figure 38) is the distance from the focal points of the lens (F and F" in the Figure) to the respective principal points (H or H"). The EFL determines magnification and hence the image size. The term f appears frequently in the lens formulas and tables of standard lenses. Unfortunately, the principal points are usually inside the lens, so that it is an inconvenient measurement for precisely positioning a lens or determining mechanical clearances. Consequently, most lenses specifications include measurements made from the focal planes to the surfaces (verticies) of the optic (e.g., the front focal length ff, and the back focal length fb). Figure 38 Illustration of optical path through a lensBack Focal Length The Back focal length fb is the distance from the secondary vertex (A2) to the rear focal point (F"), as illustrated in Figure 38.Front Focal Length The front focal length ff is the distance from the front focal point (F) to the primary vertex (A1), as illustrated in Figure 38. 62
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