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1.5 source of artificial light


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1.5 source of artificial light

  4. 4. 1. Introduction2. The tungsten-filament lamp3. Tungsten–halogen lamps4. Xenon lamps and gas discharge tubes5. Fluorescent lamps and tubes6. Laser light sources and LEDs COMPILED BY TANVEER AHMED 4
  5. 5.  Artificial light is produced in many ways. The most important method (and historically the earliest) is to heat or burn matter so that the constituent atoms or molecules of the source are excited to such an extent  that they vibrate and collide vigorously,  causing them to be constantly activated  and as a result to emit radiation over  the UV,  visible  and near-IR regions of the electromagnetic spectrum (similar to the Planckian or black body radiator). This phenomenon, referred to as incandescence, produces a continuous spectrum over quite a wide range of wavelengths (dependent mainly on the temperature of the source). COMPILED BY TANVEER AHMED 5
  6. 6.  Common incandescent sources range from1. the sun,2. through tungsten3. and tungsten–halogen sources ▪ to burning gas mantles, wood, coal or other types of fires and candles (the last mentioned have colour temperatures in the region of 1800 K). COMPILED BY TANVEER AHMED 6
  7. 7. 1. (a) electrical discharges through gases (e.g. sodium and xenon arcs)2. (b) photo luminescent sources such as the fluorescent tube, long-lived phosphorescent materials and certain types of laser3. (c) cathodoluminescent sources based on phosphors, as used in television and VDU screens4. (d) electroluminescent sources based on certain semiconductor solids and phosphors, as in light-emitting diodes (LEDs)5. (e) chem iluminescent sources as used in light sticks. COMPILED BY TANVEER AHMED 7
  8. 8.  Many of these other sources emit over selected regions of the electromagnetic spectrum ▪ giving line and band spectra, ▪ and these may be inherently coloured as a consequence of selected emission in the visible region. For example, the sodium-vapour lamp is orange-yellow due to a concentration of emission around 589.3 nm (the sodium D line), although an almost equally intense band of radiation is emitted near 800 nm in the near-IR. COMPILED BY TANVEER AHMED 8
  9. 9. The tungsten-filament lamp Some light sources show only minor deviations from Planckian distribution:of these, the tungsten-filament lamp is a prime example. COMPILED BY TANVEER AHMED 9
  10. 10.  The radiation is derived from  the heating effect of  passing an electric current  through the filament  while it is held inside a bulb which either contains an inert gas or is evacuated or at a low pressure to keep oxidation of the filament to a minimum. COMPILED BY TANVEER AHMED 10
  11. 11.  The character of the emitted radiation (and therefore the colour temperature) is controlled to a large extent by the filament thickness (resistance) and the applied voltage. For a given filament, increasing the voltage increases the light output but decreases the lamp lifetime.COMPILED BY TANVEER AHMED 11
  12. 12.  In practice tungsten lamps are produced with a variety of colour temperatures, ranging from  the common light bulb at 2800 K  to the photographic flood at 3400 K  (which has quite a short lifetime). Temperatures must be kept well below 3680 K, which is the melting point of tungsten. COMPILED BY TANVEER AHMED 12
  13. 13. Tungsten–halogen lamps Tungsten filaments can be heated to higher temperatures with longer lamp lifetimes ifsome halogen (iodine or bromine vapour) is present in the bulb. COMPILED BY TANVEER AHMED 13
  14. 14.  When tungsten evaporates from the lamp filament of an ordinary light bulb  it forms a dark deposit on the glass envelope. In the presence of halogen gas, however, it reacts to form  a gaseous tungsten halide, which then migrates back to the hot filament. At the hot filament the halide decomposes, depositing  some tungsten back on to the filament  and releasing halogen back into the bulb atmosphere,  where it is available to continue the cycle. COMPILED BY TANVEER AHMED 14
  15. 15.  With the envelope constructed from  fused silica or quartz,  tungsten–halogen lamps  can be made very compact with higher gas pressures. They can then be run at higher temperatures (up to 3300 K) with higher efficacy (lumens per watt). Such lamps are commonly used in  slide and overhead projectors  and in visible-region spectrometers  and other optical instruments,  and in a low-voltage version in car headlamps. Mains voltage lamps are used for  floodlighting  and in studio lighting in the film and television industry. COMPILED BY TANVEER AHMED 15
  16. 16. An electric current can be made to pass through xenon gasby using a high-voltage pulse to cause ionisation. COMPILED BY TANVEER AHMED 16
  17. 17.  Both pulsed xenon flash tubes and continuously operated lamps operating  at high gas pressures (up to 10 atm) are available,  the latter giving almost continuous emission over the UV and visible region. COMPILED BY TANVEER AHMED 17
  18. 18.  Largely because of its spectral distribution, which when suitably filtered resembles  that of average daylight (Figure 1.10), the high- pressure xenon arc has become very  important for applications in colour technology.COMPILED BY TANVEER AHMED 18
  19. 19.  It is now an international standard source for light-fastness testing, and is increasingly being used  as a daylight simulator for colorimetry,  and in spectroscopic instrumentation  (flash xenon tubes in diode array spectrometers), as well as in general scientific work involving  photo biological and  photochemical studies  and in cinematography. COMPILED BY TANVEER AHMED 19
  20. 20.  Electrical discharges through  gases  at low pressure  generally produce line spectra. These emissions arise when the electrically excited atoms jump between  quantised energy levels of the atom The mercury discharge lamp was one of the earliest commercially important sources of this type COMPILED BY TANVEER AHMED 20
  21. 21.  its blue-green colour being due to ▪ line emissions at ▪ 405, ▪ 436, ▪ 546 ▪ and 577 nm. There is a high-intensity 366 nm line emission in the UV, which makes it  necessary for the user of an unfiltered mercury lamp to wear protective UV-absorbing goggles. COMPILED BY TANVEER AHMED 21
  22. 22.  When mercury arcs with clear  quartz or silica envelopes are used,  protection is also required from generated ozone.COMPILED BY TANVEER AHMED 22
  23. 23.  The intensity and width (wavelength ranges) of the line emissions depend to a large extent on the size of  the applied current  and the vapour pressure  within the tube. By adding metal halides to the mercury vapour,  extra lines are produced in the spectrum  and the source effectively becomes a white light source (HMI lamp). COMPILED BY TANVEER AHMED 23
  24. 24.  Mercury light sources are used extensively in the  surface coating industry (UV curing),  in the microelectronics industry (photolithography),  as the basic element in fluorescent lamps and tubes  as an aid to assessment of fluorescent materials  in colour-matching light booths  and, to a limited extent, for assessing the stability of coloured materials to UV irradiation. The metal halide lamps are used  in floodlighting applications, while the special HMI lamp was developed as a supplement to daylight in outdoor television productions. COMPILED BY TANVEER AHMED 24
  25. 25.  Another well-known light source of this type is the sodium-vapour lamp which, in its high-pressure form, was developed in the 1960s particularly for street lighting and floodlighting applications. COMPILED BY TANVEER AHMED 25
  26. 26.  The spectral emission lines in this case are considerably broadened,  with the gas pressures being sufficiently high to produce a significant absorption at the D line wavelength (589.3 nm).  A typical SPD curve for a high- pressure  sodium lamp is shown in Figure 1.12.COMPILED BY TANVEER AHMED 26
  27. 27.  The main value of the sodium- vapour lamp lies in its relatively high efficacy  (100–150 lm W–1).  Cited refractive index values for liquids and transparent materials  are usually based on measurements using the D line radiation from a low-pressure sodium lamp.COMPILED BY TANVEER AHMED 27
  28. 28. The ubiquitous fluorescent tube consists of a long glass vessel containing mercury vapour at low pressure sealed at each end with metal electrodes between which an electrical discharge is produced. COMPILED BY TANVEER AHMED 28
  29. 29.  The inside of the tube is  coated with phosphors  that are excited by the high-energy UV lines from the mercury spectrum  (mainly 254, 313 and 366 nm lines), which by photoluminescence (or a mixture of fluorescence an phosphorescence)  are converted to radiation above 400 nm. COMPILED BY TANVEER AHMED 29
  30. 30.  The spectrum that is produced is dependent on  the type of phosphor mixture used;  thus the lamps vary from the red deficient  ‘cool white’ lamp, which uses halophosphate phosphors,  to the broad-band type in which  long-wavelength phosphors are incorporated to enhance the colour rendering properties COMPILED BY TANVEER AHMED 30
  31. 31.  A third type, known as the three-band fluorescent or prime colour lamp, uses narrow-line phosphors to give emissions at approximately  435 nm (blue), 545 nm  (green) and 610 nm (red)  and an overall white light colour of surprisingly good colour rendering properties. The characteristics of these lamps have been extensively studied by Thornton and they have been marketed  as Ultralume (Westinghouse) in the USA  and TL84 (Philips) in the UK. COMPILED BY TANVEER AHMED 31
  32. 32.  The characteristics of the three types of fluorescent tubes are compared in Figure 1.13. The first two lamps show prominent line emissions at the mercury wavelengths of 404, 436, 546 and 577 nm. The much higher efficacy of the three-band fluorescent (TL84) lamps over other types has resulted in their use in store lighting, but this has aggravated the incidence of colour mismatches (metamerism) caused by changing illuminants . COMPILED BY TANVEER AHMED 32
  33. 33.  High-pressure mercury lamps have also been designed  with red-emitting phosphors  coated on the inside of the lamp envelope to improve colour rendering;  these include the MBF and MBTF lamps. The latter have a tungsten-filament ballast which raises  the background emission in the higher-wavelength regions (Figure 1.14).COMPILED BY TANVEER AHMED 33
  35. 35. Laser sources are increasingly being used in optical measuring equipment, certain types of spectrometersand monitoring equipment of many different types. COMPILED BY TANVEER AHMED 35
  37. 37.  The red-emitting  He–Ne gas laser  was one of the earliest lasers developed, but it is the red-emitting diode laser  which has become familiar in its application to barcode reading devices in supermarkets and elsewhere. Yet another type emits in the IR region, and is widely  used in compact disc (CD) players. COMPILED BY TANVEER AHMED 37
  38. 38.  The term ‘laser’ is an acronym for the process in which ▪ light ▪ amplification occurs by ▪ stimulated ▪ emission of ▪ radiation. In order to explain laser action we have to appreciate some of the aspects of atomic and molecular excitation COMPILED BY TANVEER AHMED 38
  39. 39.  In the gas discharge tubes mentioned in section 1.5.4,  light emissions arise from  electrical excitation of electrons  from their normal ground state  to a series of excited states and ions, and it is the subsequent loss of energy from these excited states which results in spontaneous emission at specific wavelengths according to the Planck relation given in Eqn 1.5. COMPILED BY TANVEER AHMED 39
  40. 40. The ubiquitous fluorescent tube consists of a long glass vessel containing mercury vapour at low pressure sealed at each end with metal electrodes between which an electrical discharge is produced. COMPILED BY TANVEER AHMED 40
  41. 41.  The inside of the tube is  coated with phosphors  that are excited by the high-energy UV lines from the mercury spectrum  (mainly 254, 313 and 366 nm lines), which by photoluminescence (or a mixture of fluorescence an phosphorescence)  are converted to radiation above 400 nm. COMPILED BY TANVEER AHMED 41
  42. 42.  In a laser means are provided to hold  a large number of atoms or molecules  in their meta-stable excited states, usually by careful optical design in which the radiation is ▪ reflected many times between accurately parallel end mirrors. The system shown in Figure 1.15 is said to exist with ‘an inverted population’ allowing stimulated rather than spontaneous emission. COMPILED BY TANVEER AHMED 42
  43. 43. Figure 1.15 A schematic illustration of the steps leading to laser action: (a) the Boltzmann population of states, with more atoms in the ground state; (b) when the initial state absorbs, the populations are inverted (the atoms are pumped to the excited state); (c) a cascade of radiation then occurs, as one emitted photon stimulates another atom to emit, and so on: the radiation is coherent (phases in step)COMPILED BY TANVEER AHMED 43
  44. 44.  Thus if a quantum of light of  exactly the same wavelength  as the spontaneous emission interacts with the excited state  before spontaneous emission has occurred,  then stimulated emission can occur immediately (Figure 1.16).It is one of the characteristicsof laser light that it is emitted in preciselythe same directionas the stimulating light,and it will be coherent with it,i.e. all the crests and troughsoccur exactly in step, asindicated in Figure 1.15. COMPILED BY TANVEER AHMED 44
  45. 45.  Because of the optical design of the  laser cavity  and the consequent coherence of laser light,  it is emitted in a highly directional manner and can be focused on to very  small areas giving  a high irradiance capability. The use of Brewster angle windows in the discharge tube section of a gas laser also results in the emitted radiation being highly polarised (Figure 1.17). COMPILED BY TANVEER AHMED 45
  46. 46.  Certain types of laser can also be operated to give  Highpower short-lived light pulses, nowadays reaching down to femtosecond  (1 fs = 1 x 10 –15 s) timescales, which can be used to study the ▪ extremely rapid chemical ▪ And physical processes that take place immediately after light is absorbed. COMPILED BY TANVEER AHMED 46
  47. 47. Semiconductor materials are used in the manufacture of light-emitting diodes (LEDs) and in diode lasers,the wavelength of emission being determined by the chemical composition of the semiconductor materials. COMPILED BY TANVEER AHMED 47
  48. 48.  The mechanism of light production in the LED arises from the phenomenon of  electro-luminescence, where the electrical excitation between the ▪ conduction band in the n-type semiconductor ▪ and the valence band in the p-type material ▪ results in an energy gap and hence light emission by electron hole recombination across the p–n semiconductor junction COMPILED BY TANVEER AHMED 48
  49. 49.  Table 1.3 shows the materials used to make LEDs to produce light of different colours. COMPILED BY TANVEER AHMED 49
  50. 50.  The commonest LEDs are manufactured from  gallium combined  with arsenic  And phosphorus  in different ratios to give variation in colour and wavelength of the emitted light. For example, with an As : P ratio of ▪ 60 : 40 a red emission (690 nm) is produced, ▪ a ratio of 40 : 60 gives orange (610 nm) ▪ and a ratio of 14 : 86 gives yellow (580 nm). COMPILED BY TANVEER AHMED 50
  51. 51.  Similar materials can be used to form a diode laser, ▪ where the end faces of the semiconductor ▪ double layer are polished to give the necessary multi- reflection; These materials have a high ▪ refractive index, ▪ so readily produce the required ▪ internal reflections at their surfaces. COMPILED BY TANVEER AHMED 51
  52. 52.  Figure 1.18 shows diagrammatically the construction of a semiconductor junction laser. COMPILED BY TANVEER AHMED 52
  53. 53. There are two aspects of artificial light sourcesthat are of particular interest to colourscientists:1. Lamp efficacy2. Colour-rendering properties COMPILED BY TANVEER AHMED 53
  54. 54. the luminous efficacy of the lamp in lumens per watt (lm W–1),which is a measure of the amount ofradiation emittedfor a given input ofelectrical power, weighted by the ease by which that radiation is detected by the human observer COMPILED BY TANVEER AHMED 54
  55. 55. The human eye is stimulated morestrongly by light of somewavelength regions of the visible spectrum than by others;thus yellow-green light at 555 nm is the most readily seen, while blue and red light of thesame radiant flux appear quite dim by comparison. COMPILED BY TANVEER AHMED 55
  56. 56. The wavelength-dependent factor thatconverts radiant energy measuresto luminous or photometric measuresis known as the Vλ function. It varies with wavelength acrossthe visible spectrum (Figure 1.19). COMPILED BY TANVEER AHMED 56
  57. 57. where Km = luminous efficacy of radiation at 555 nm(about 683 lm W–1),at which wavelength the Vλ function has a maximum value of 1.000.The limits of the integral in Eqn 1.7 are effectively those of thevisible spectrum, i.e. 380–770 nm. COMPILED BY TANVEER AHMED 57
  59. 59. A lamp emitting radiation only at 555 nm would have this maximum efficacy of 683 lm W–1.The nearest practical approach, however, is the sodium lamp emitting at589 nm where Vλ = 0.76, with a maximum efficacy near 150 lm W–1.Some energy is dispersed1. in non-visible emission2. and some by heat loss3. and other inefficiencies. COMPILED BY TANVEER AHMED 59
  60. 60.  Figure 1.19 also includes the V/λ curve, effective at scotopic or low light levels (under twilight conditions, for instance); this curve has a maximum at 510 nm  and is relatively higher in the  blue  but becomes effectively zero above 630 nm  (many red objects appear black under these conditions). COMPILED BY TANVEER AHMED 60
  61. 61. the colour-rendering characteristics of the lamp, which is a measure of how good the lamp is at developing the accepted ‘true’ hues of a set of colour standards. COMPILED BY TANVEER AHMED 61
  62. 62.  A traditional red letter box or red bus illuminated by  sodium-vapour street lighting  appears a dullish brown; similarly, the human face takes on a sickly greenish hue when  viewed in the light from a vandalised fluorescent street lamp  (where the phosphor-coated glass envelope has been removed  and the light is from the unmodified mercury spectrum). Both these lamps would be recognised as having ▪ poor colour-rendering properties. COMPILED BY TANVEER AHMED 62