Characterization of Light Emitting Diodes(LEDs) and Compact Fluorescent Lamps(CFLs) by UV-Visible Spectrophotometry C. Mar...
IntroductionABSTRACT: The acceptance and commercial utilization of Compact Fluorescent Lamps(CFLs) and, more recently, Lig...
 Compact Fluorescent Lighting technology is the current main alternative  to incandescent lighting. Compact Fluorescent L...
 Light Emitting Diodes (LEDs) are a viable alternative to CFL technology and  manufacturers are beginning to offer LED op...
Figure 2: Typical LED Architecture   Figure 3: White LED ArchitectureFigure 4: UV-Vis EmissionSpectrum of a White LED
 This poster will demonstrate the use of the Shimadzu UV-1800 scanning  spectrophotometer to characterize and measure CFL...
LED BandwidthsFigure 6 shows the spectral emission of a red LED emitting at 634 nm and a red diodelaser emitting at 653 nm...
LEDs can be designed to emit a specific narrow-band wavelength or dominant color. Thewavelength that an LED emits is relat...
Figure 7: Spectral acquisitions of commercial LEDs to include colors of:UV361, UV375, UV400, Blue, Teal, Aqua, Green, Yell...
Reported Measured          Center                                                Measured Voltage versus  LED Wavelength  ...
CIE Chromaticity ValuesWith the variety of CFL coating phosphors available, CFL technologyoffers many color options that m...
Two important abilities for successful manufacturing, quality control, and comparison ofLEDs is the measurement of dominan...
Figure 11: Calculated CIE x,y chromaticity and dominantwavelength values for the LED series.                       Figure ...
LED Equilibrium MeasurementsLEDs reach an equilibrium operating temperature only after a period of time. Theequilibrium te...
Figure 13: Equilibrium scans for the Blue LED         Figure 14: Equilibrium scans for the Red LED.    LED    Initial WL F...
LED Temperature MeasurementsUnlike the minor spectral changes observed in LED equilibrium temperaturemeasurements above, L...
Figure 15: Spectral emission of a yellow LED with increasing temperatures (24, 42, 51, 69, and 87 Deg C).
LED Output vs Temperature                        602                                                                      ...
The calculated CIE x,y chromaticity coordinates for these temperature-dependentspectral scans, Figure 17, shows the center...
LED Pulsed SupplyIn some applications LEDs are not driven by a constant DC voltage but rather by apulsed square wave with ...
Figure 18: Spectral emission curves for a yellow LED pulsed at frequencies (clockwisefrom top left) 100 Hz, 500 Hz, 1 KHz,...
Yellow LED pulsed at 500 Hz                     90                     80Percent Duty Cycle                     70       y...
Figure 21: Spectral emission curves for a yellowLED for various duty cycles (clockwise from topleft) 70%, 50%, and 30%
SummaryAs the move to replace incandescent lamps with the more energy efficient CFL andLED technologies continues, researc...
References1. Davidson, Paul, 16 December 2007, “It’s Lights Out for Traditional Light Bulbs.”http://www.usatoday.com/money...
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Characterization of Light Emitting Diodes and Compact Fluorescent Lamps by UV-Vis Spectrophotometry

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The acceptance and commercial utilization of Compact Fluorescent Lamps (CFLs) and, more recently, Light Emitting Diodes (LEDs) have grown significantly in the past five years. This change was fueled in part by the United States Congress 2007 legislative mandate to eliminate sales, and phase out the use, of conventional incandescent lighting beginning in 2012. This mandate has ultimately increased research into CFL and LED technologies. This poster demonstrates the use of a typical laboratory UV-Vis spectrophotometer, with no modification to the bench or software, to measure and characterize CFL and LED lamps. Using a customized accessory, spectral characteristics such as peak wavelength (λp), Full Width at Half Maximum (FWHM), centroid wavelength (λc), dominant wavelength, color, and color purity were readily determined for a variety of commercial CFLs and LEDs.

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Characterization of Light Emitting Diodes and Compact Fluorescent Lamps by UV-Vis Spectrophotometry

  1. 1. Characterization of Light Emitting Diodes(LEDs) and Compact Fluorescent Lamps(CFLs) by UV-Visible Spectrophotometry C. Mark Talbott, PhD, Robert H. Clifford, PhD Shimadzu Scientific Instruments, Columbia, MD, USA, 800-477- 1227, www.ssi.shimadzu.com
  2. 2. IntroductionABSTRACT: The acceptance and commercial utilization of Compact Fluorescent Lamps(CFLs) and, more recently, Light Emitting Diodes (LEDs) have grown significantly in thepast five years. This change was fueled in part by the United States Congress 2007legislative mandate to eliminate sales, and phase out the use, of conventionalincandescent lighting beginning in 20121. This mandate has ultimately increasedresearch into CFL and LED technologies. This poster demonstrates the use of a typicallaboratory UV-Vis spectrophotometer, with no modification to the bench or software, tomeasure and characterize CFL and LED lamps. Using a customized accessory, spectralcharacteristics such as peak wavelength (λp), Full Width at Half Maximum(FWHM), centroid wavelength (λc), dominant wavelength, color, and color purity werereadily determined for a variety of commercial CFLs and LEDs.
  3. 3.  Compact Fluorescent Lighting technology is the current main alternative to incandescent lighting. Compact Fluorescent Lamps (CFL) reportedly consume 75% less energy than incandescent counterparts.2 CFL lamps do contain a small quantity of mercury which may offer a potential future safety risk both for homes and in landfills. A typical UV-Visible spectrum from a CFL is shown in Figure 1. Figure 1: UV-Vis emission spectra of a typical compact fluorescent lamp (CFL) (violet) and a Mercury lamp (green).
  4. 4.  Light Emitting Diodes (LEDs) are a viable alternative to CFL technology and manufacturers are beginning to offer LED options as alternatives to CFL lighting. LEDs are semi-conductor devices with a p-n junction. When a forward voltage is biased across the p-n halves of the junction, photons of light are emitted. Because of the large refractive indices of the junction materials the photons tend to leave the LED die at narrow angles between the junction. Typical LED design technology places the LED die on an anode aluminum reflector. The cathode is bonded to the top of the die with a gold wire and the complete package is encapsulated in a polymer that has appropriate optical properties. The end of the plastic encapsulation can be shaped to form a lens, adding directionality to the LED’s output3. A typical yellow LED design is shown in Figure 2. The reflector, gold wire, and LED die can readily be seen. In addition, the LED was biased with a low forward voltage to demonstrate that the photon emission is from the area between the p-n junction. Another LED design that is typical for white LEDs is to imbed a blue LED die in a luminescent gel (Figure 3). When the blue LED die is forward biased, the blue photons stimulate the gel to fluoresce, providing broad spectrum white light. A UV- Vis scan of such an LED (Figure 4) clearly shows the 455 nm blue excitation peak and the broader white spectrum.
  5. 5. Figure 2: Typical LED Architecture Figure 3: White LED ArchitectureFigure 4: UV-Vis EmissionSpectrum of a White LED
  6. 6.  This poster will demonstrate the use of the Shimadzu UV-1800 scanning spectrophotometer to characterize and measure CFL and LED properties. A mercury lamp calibration post is offered as an optional accessory for the UV-1800 spectrophotometer. An arm was attached to this post that allowed vertical height adjustment, Figure 5. To the arm was attached a clamping assembly for LEDs and a mirror to direct light from CFLs housed outside the lamp compartment into the entrance slit of the monochromator. No modifications or changes were made to the spectrophotometer or its lamps to perform the tests. In addition, Shimadzu’s UVProbe software and UVPC Color Analysis software provided the functionality required to acquire all spectra collected and presented in this poster. Figure 5: UV-1800 Spectrophotometer with modified mercury lamp calibration post including the LED clamp and mirror.
  7. 7. LED BandwidthsFigure 6 shows the spectral emission of a red LED emitting at 634 nm and a red diodelaser emitting at 653 nm. The bandwidth of the LED (16 nm FWHM) is not as narrow asthat of the laser (1.5 nm FWHM). However, even with the large bandwidths LEDs areconsidered to be monochromatic sources. This can be evidenced by examining theposition of the CIE x,y color coordinates for the individual LEDs. Figure 7 shows thespectral emission of a series of LEDs. Accurate spectral acquisition and measurement ofLEDs require a spectrophotometer with a monochromator capable of resolving thesenarrow bandwidth sources. The UV-1800 spectrophotometer’s 1nm bandwidth is wellwithin the resolution requirement for accurate LED spectral acquisitions. Figure 6: UV-Vis spectral emission scans of a red LED at 634 nm and a red diode laser at 653 nm.
  8. 8. LEDs can be designed to emit a specific narrow-band wavelength or dominant color. Thewavelength that an LED emits is related to the bandgap energy of the semiconductormaterials used in manufacturing the p-n junction. The equation, = 1.24 / eV, relates theLED emission wavelength to the bandgap energy for a specific LED. For visible LEDs,this limits the bandgap energy to between 3.10 to 1.55 eV. An array of LED colors canbe readily purchased from the commercial market. Emission spectra for these LEDswere acquired with the UV-1800 using a constant current power source to power theLEDs under test so that the constant forward current (If) was limited to 19.18 milliamps.The forward voltage (Vf) was allowed to vary as needed by the LED. The spectralacquisitions were acquired using a fast scan speed and a fixed sampling interval of 0.5.UVProbe software was operated in the energy mode and the silicon photodiode gainwas set at 3.Figure 7 shows the spectral acquisitions for each of these commercial LEDs exhibiting agiven dominant color. Table 1 shows reported and measured values for each LED.Characteristic LED information obtained from these scans were: Peak Wavelength - Wavelength at the maximum spectral band energy Center Wavelength - Wavelength at the center of the Full Width Half Max boundary Forward Voltage (Vf) - Voltage forward biased across the LED Power - Measured voltage times current
  9. 9. Figure 7: Spectral acquisitions of commercial LEDs to include colors of:UV361, UV375, UV400, Blue, Teal, Aqua, Green, Yellow, Orange, Red, Deep Red, and IR.
  10. 10. Reported Measured Center Measured Voltage versus LED Wavelength Volts mWatts Wavelength BandwidthUV361 361.000 4.54 87.1 362.915 13.13 Center WavelengthUV375 375.000 3.422 65.6 374.45 9.84 700 y = -112.79x + 829.57 Center Wavelength (nm) R² = 0.8511UV400 400.000 3.364 64.5 391.455 10.95 600Blue 470.000 3.037 58.2 460.48 18.86Teal 490.000 3.059 58.7 490.23 25.42 500Aqua 505.000 3.41 65.4 505.61 28.78 400Green 525.000 2.818 54.0 518.41 27.22 300Yellow 590.000 1.972 37.8 593.435 13.77Orange 610.000 1.965 37.7 607.595 14.95 200Red 630.000 1.919 36.8 633.245 16.37 1.5 2.5 3.5 4.5 5.5DeepRed 660.000 1.831 35.1 653.075 21.27 Measured forward voltage (volts)Table 1: Measured and calculated values for the Spectralacquisitions of commercial LEDs to include colors of: Figure 8: Measured LED voltage versus measuredUV361, UV375, UV400, Blue, Teal, Aqua, Green, Yellow, Orang center wavelength.e, Red, and Deep Red. Figure 8 demonstrates the linear relationship expected between the center wavelength (color) observed and the forward voltage. The spectral scans and measurements demonstrate the value of being able to monitor this relationship when comparing LEDs, in design, and in manufacture.
  11. 11. CIE Chromaticity ValuesWith the variety of CFL coating phosphors available, CFL technologyoffers many color options that may be more suitable for variousenvironments and work spaces. The ability to measure and quantify colorvalues is paramount to the successful design and testing of new CFLphosphors. Figures 9 and 10 show in tabular and graphical format, thecalculated x,y chromaticity values for spectra acquired from variouscommercial compact fluorescent lamps.Figure 9: Calculated CIE x,y chromaticity values forvarious commercial compact fluorescent lamps. Figure 10: Plot of the calculated commercial CFL on the x,y chromaticity coordinates system.
  12. 12. Two important abilities for successful manufacturing, quality control, and comparison ofLEDs is the measurement of dominant wavelength and purity. The dominant wavelengthis defined4 as the point on the International Commission on Illumination (CIE) 1931coordinates that is intersected by a line that is drawn from a theoretical illuminant “E”which has (CIE) coordinates located at the center (x=1/3, y=1/3) through the x,ycoordinate values calculated from the LED spectra. Purity is the ratio of the distancefrom the illuminant “E” chromaticity coordinates to the LED calculated coordinates overthe distance from the illuminant “E” chromaticity coordinates to the coordinates of thedominant wavelength. Illuminant “D65” was used in the Color Analysis software as it hasCIE x,y coordinates of (0.31271,0.32902) and most closely matches those of illuminate“E”.Figure 11 shows the calculated CIE chromaticity and dominant wavelength values for theLED series tested. Figure 12 shows the plot of those values on the CIE chromaticitycoordinate system. The plot shows that the LEDs on the red end of the spectrum lie onthe border of the chromaticity plot and by definition would be expected to have a highpurity value. LED colors starting with green, however, begin to fall away from the borderand lie more internal to the coordinate system. As such, the dominant wavelengths forthese higher energy LEDs will be shifted from the measured center wavelength. This canbe readily seen by comparing the calculated dominant wavelengths in Figure 11 with thedominant wavelengths reported in Table 1 and contrasting those to the measured centerwavelengths in Table 1.
  13. 13. Figure 11: Calculated CIE x,y chromaticity and dominantwavelength values for the LED series. Figure 12: Plot of the calculated LED chromaticity coordinates.
  14. 14. LED Equilibrium MeasurementsLEDs reach an equilibrium operating temperature only after a period of time. Theequilibrium temperature is a function of the ambient temperature and the ability of theLED to lose heat through the package leads or attached heat sink. The ability to monitoran LED’s output to steady-state operation is important to evaluate spectral outputchanges and to assure that measurements taken coincide with normal operatingconditions. The UV-1800 spectrophotometer was used to acquire spectra of selectedLEDs from initial startup to steady-state operation. UVProbe software was set to acquirea series of repeat spectra with the wavelength range centered around the emission peakof the LED. Delay time between acquisitions was set to zero. In addition, forward biasvoltage across the LEDs was also measured during this period with a current limit set to19.18 milliamps. The peak pick function of UVProbe was used to determine thedominant wavelengths of the spectral scans. Time between scans was on the order ofthree seconds.Equilibrium spectra for blue and red LEDs are shown in Figures 13 and 14. Table 2 givesthe measured dominant wavelength and forward voltage for the initial and final scan foreach LED. As steady-state operation was achieved, the red LED exhibited more changein dominant wavelength, intensity and voltage than did the blue LED. However, thecalculated CIE x.y chromaticity values show no change between initial and final scansfor both LEDs, Table 3. This data demonstrates that the Spectrophotometer was capableof recording LED output variances that would not be visually observable.
  15. 15. Figure 13: Equilibrium scans for the Blue LED Figure 14: Equilibrium scans for the Red LED. LED Initial WL Final WL Initial Vf Final Vf Blue 460.5 460.5 3.054 3.021 Red 651.0 652.0 1.844 1.834 Table 2: Dominant Wavelengths and Voltages Table 3: CIE chromaticity values
  16. 16. LED Temperature MeasurementsUnlike the minor spectral changes observed in LED equilibrium temperaturemeasurements above, LEDs can exhibit significant spectral emission with largerchanges in temperature. The spectral output temperature dependence of a yellow LEDwas measured using the UV-1800 spectrophotometer. Temperatures were measured atthe LED with a thermocouple. Temperatures at the LED were altered using a heat gunon various settings and distances from the LED.Figure 15 shows that significant changes in the LED’s spectral emission curves wereobserved with increasing temperatures. Temperatures tested were 24, 42, 51, 69, and87 degrees C. Also observed in Figure 15 was not only a change in the intensity of thedominant spectral emission, but the appearance of a lower energy peak that wasapproximately 12 nm higher in wavelength and became more prominent as thetemperature was increased.Figure 16 shows numerically the temperature dependence of the dominant wavelengthand overall peak area (integrated from 530 to 630 nm). The graph shows an increase indominant wavelength of approximately 0.14 nm/C. The corresponding decrease inoverall band area would represent a decrease in LED output intensity with increasingtemperature. The measured forward bias voltage across the diode did not change withincreasing temperature and remained constant at 1.057 volts.
  17. 17. Figure 15: Spectral emission of a yellow LED with increasing temperatures (24, 42, 51, 69, and 87 Deg C).
  18. 18. LED Output vs Temperature 602 8 y = 0.1438x + 587.91 R² = 0.9917 7 600 y = -0.0612x + 8.5964 Peak Maximum (nm) Band Area (Energy^2) 6 R² = 0.9809 598 5 596 4 3 594 Peak 2 592 1 590 0 0 20 40 60 80 100 Temperature Deg CFigure 16: Graph showing the temperature dependence of the dominant peak wavelength(red) and overall peak area (blue).
  19. 19. The calculated CIE x,y chromaticity coordinates for these temperature-dependentspectral scans, Figure 17, shows the center wavelength changing as the temperatureincreases. The change in temperature would result in a visibly noticeable change in LEDcolor. Figure 17: CIE plot showing the change in x,y coordinates of the yellow LED as the temperature was increased.
  20. 20. LED Pulsed SupplyIn some applications LEDs are not driven by a constant DC voltage but rather by apulsed square wave with variable duty cycle. Frequencies of 100 Hz to 1 kiloHertz aretypical5. LEDs operated at these frequencies would show no visible flutter to the humaneye. By using a pulsed power supply, a smaller duty cycle can be used which allows forcurrent pulses that would normally be above the upper operating limit for the LED. In thisway higher output can be achieved from a given LED with power conservation.The spectrophotometer was used to characterize a yellow LED driven by a pulsedvoltage source with duty cycle control. Pulse frequencies tested were 100 HZ, 500 Hz, 1kHz, and 3.3 kHz. Pulsed duty cycles tested were 30%, 50% and 78%.Figure 18 shows the spectral acquisitions from this testing. For all frequencies, as theduty cycle was reduced, the emission energy decreased linearly. Figure 19 shows thisfor the 500 Hz tests. Although there was a noticeable power decrease with the reducedduty cycle, Figure 20 demonstrates that there was no observable change in thecalculated CIE x,y chromaticity coordinates or dominant wavelength.Figure 21 shows the spectra graphed in groups of duty cycle. The graphs show for anygiven duty cycle, there was very little change in spectra emission power and/or dominantwavelength.
  21. 21. Figure 18: Spectral emission curves for a yellow LED pulsed at frequencies (clockwisefrom top left) 100 Hz, 500 Hz, 1 KHz, and 3.3 KHz. In each graph the spectra representduty cycles of 34 %, 50% and 70%.
  22. 22. Yellow LED pulsed at 500 Hz 90 80Percent Duty Cycle 70 y = 11.659x + 1.7436 60 R² = 1 50 40 30 20 10 0 2 3 4 5 6 7 Band Area (energy^2) Figure 19: Spectral emission curves for a yellow LED pulsed at 500 Hz. Figure 20: Calculated x,y chromaticity coordinates for the 500 Hz pulsed LED spectra
  23. 23. Figure 21: Spectral emission curves for a yellowLED for various duty cycles (clockwise from topleft) 70%, 50%, and 30%
  24. 24. SummaryAs the move to replace incandescent lamps with the more energy efficient CFL andLED technologies continues, research will continue to be focused on thedevelopment of future LED systems. Much of this research will be conducted inacademic and private laboratories that may not have access to the specializedequipment that is normally used to characterize LED systems.This poster has demonstrated that the Shimadzu scanning UV-1800spectrophotometer with no modifications to the bench or software, and only a minormodification to a readily available mercury lamp maintenance stand, can readily beused to measure and evaluate both CFL and LED characteristics that are ofsignificant importance to their design, comparison, and manufacture.Acknowledgements:Suja Sukumaran, Ph.D, Jeff Head, MSc, Shimadzu Scientific Instrument, Maryland, USA
  25. 25. References1. Davidson, Paul, 16 December 2007, “It’s Lights Out for Traditional Light Bulbs.”http://www.usatoday.com/money/industries/energy/environment/2007-12-16-light-bulbs_N.htm, Accessed 06 March 2012.2. US Environmental Protection Agency, ”Light Bulbs for Consumers”http://www.energystar.gov/index.cfm?fuseaction=find_a_product.showProductGroup&pgw_code=LB,Accessed 06 March 2012.3. Instrument Systems Optische Messtechnik, “Instrument Systems and LEDs: Total MeasurementSolutions.”http://www.instrumentsystems.de/fileadmin/editors/downloads/Products/LED_brochure_e.pdf, Accessed 20, July 2011.4. Instrument Systems Optische Messtechnik, “Handbook of LED Metrology.”http://www.instrumentsystems.com/fileadmin/editors/downloads/Products/LED_Handbook_e.pdf, Accessed, 13 August 2011.5. Labsphere, “The Radiometry of Light Emitting Diodes.”, http://labsphere.com/uploads/technical-guides/The%20Radiometry%20of%20Light%20Emitting%20Diodes%20-%20LEDs.pdf, Accessed, 20July 2011.

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