1.5 source of artificial light

1. SOURCES OF ARTIFICIAL LIGHT 1 COMPILED BY TANVEER AHMED

2. 2 COMPILED BY TANVEER AHMED

3. SOURCES OF ARTIFICIAL LIGHT 3 COMPILED BY TANVEER AHMED

4. Sources of light TABLE OF CONTENT Introduction The tungsten-filament lamp Tungsten–halogen lamps Xenon lamps and gas discharge tubes Fluorescent lamps and tubes Laser light sources and LEDs 4 COMPILED BY TANVEER AHMED

5. Introduction incandescence, 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). 5 COMPILED BY TANVEER AHMED

6. Introduction incandescence, Common incandescent sources range from the sun, through tungsten 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 ). 6 COMPILED BY TANVEER AHMED

7. Other methods of producing light, probably in decreasing order of importance, are: (a) electrical discharges through gases (e.g. sodium and xenon arcs ) (b) photo luminescent sources such as the fluorescent tube, long-lived phosphorescent materials and certain types of laser (c) cathodoluminescent sources based on phosphors, as used in television and VDU screens (d) electroluminescent sources based on certain semiconductor solids and phosphors, as in light-emitting diodes (LEDs ) (e) chem iluminescent sources as used in light sticks. 7 COMPILED BY TANVEER AHMED

8. Introduction 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. 8 COMPILED BY TANVEER AHMED

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. 9 COMPILED BY TANVEER AHMED

10. The tungsten-filament lamp 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. 10 COMPILED BY TANVEER AHMED

11. The tungsten-filament lamp 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. 11 COMPILED BY TANVEER AHMED

12. The tungsten-filament lamp 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. 12 COMPILED BY TANVEER AHMED

13. Tungsten–halogen lamps Tungsten filaments can be heated to higher temperatures with longer lamp lifetimes if some halogen (iodine or bromine vapour ) is present in the bulb. 13 COMPILED BY TANVEER AHMED

14. Tungsten–halogen lamps 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. 14 COMPILED BY TANVEER AHMED

15. Tungsten–halogen lamps 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 . 15 COMPILED BY TANVEER AHMED

16. Xenon lamps and gas discharge tubes An electric current can be made to pass through xenon gas by using a high-voltage pulse to cause ionisation . 16 COMPILED BY TANVEER AHMED

17. Xenon lamps and gas discharge tubes 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. 17 COMPILED BY TANVEER AHMED

18. Xenon lamps and gas discharge tubes 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. 18 COMPILED BY TANVEER AHMED

19. Xenon lamps and gas discharge tubes 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. 19 COMPILED BY TANVEER AHMED

20. Gas discharge tubes 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 20 COMPILED BY TANVEER AHMED

21. The mercury GAS discharge lamp 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. 21 COMPILED BY TANVEER AHMED

22. The mercury GAS discharge lamp When mercury arcs with clear quartz or silica envelopes are used, protection is also required from generated ozone. 22 COMPILED BY TANVEER AHMED

23. The mercury GAS discharge lamp 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). 23 COMPILED BY TANVEER AHMED

24. The mercury GAS discharge lamp 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 . 24 COMPILED BY TANVEER AHMED

25. The sodium- vapour GAS discharge lamp 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 . 25 COMPILED BY TANVEER AHMED

26. The sodium- vapour GAS discharge lamp 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. 26 COMPILED BY TANVEER AHMED

27. The sodium- vapour GAS discharge lamp 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 . 27 COMPILED BY TANVEER AHMED

28. Fluorescent lamps and tubes 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. 28 COMPILED BY TANVEER AHMED

29. Fluorescent lamps and tubes 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. 29 COMPILED BY TANVEER AHMED

30. Fluorescent lamps and tubes 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 30 COMPILED BY TANVEER AHMED

31. Fluorescent lamps and tubes 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. 31 COMPILED BY TANVEER AHMED

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 . 32 COMPILED BY TANVEER AHMED

33. Fluorescent lamps and tubes 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). 33 COMPILED BY TANVEER AHMED

34. 34 COMPILED BY TANVEER AHMED

35. Laser light sources Laser sources are increasingly being used in optical measuring equipment, certain types of spectrometers and monitoring equipment of many different types. 35 COMPILED BY TANVEER AHMED

36. 36 COMPILED BY TANVEER AHMED

37. Laser light sources 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. 37 COMPILED BY TANVEER AHMED

38. Laser light sources 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 38 COMPILED BY TANVEER AHMED

39. Laser light sources 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. 39 COMPILED BY TANVEER AHMED

40. tubes 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. 40 COMPILED BY TANVEER AHMED

41. tubes 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. 41 COMPILED BY TANVEER AHMED

42. Laser light sources 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. 42 COMPILED BY TANVEER AHMED

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) 43 COMPILED BY TANVEER AHMED

44. Laser light sources 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 characteristics of laser light that it is emitted in precisely the same direction as the stimulating light, and it will be coherent with it, i.e. all the crests and troughs occur exactly in step, as indicated in Figure 1.15. 44 COMPILED BY TANVEER AHMED

45. Laser light sources 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). 45 COMPILED BY TANVEER AHMED

46. Laser light sources 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 th e extremely rapid chemical And physical processes that take place immediately after light is absorbed. 46 COMPILED BY TANVEER AHMED

47. LEDs 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. 47 COMPILED BY TANVEER AHMED

48. The mechanism LED 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 48 COMPILED BY TANVEER AHMED

49. Materials used to make LEDs Table 1.3 shows the materials used to make LEDs to produce light of different colours . 49 COMPILED BY TANVEER AHMED

50. LEDs are manufactured 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 ). 50 COMPILED BY TANVEER AHMED

51. diode laser 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. 51 COMPILED BY TANVEER AHMED

52. diode laser Figure 1.18 shows diagrammatically the construction of a semiconductor junction laser. 52 COMPILED BY TANVEER AHMED

53. PROPERTIES OF ARTIFICIAL LIGHT SOURCES There are two aspects of artificial light sources that are of particular interest to colour scientists: Lamp efficacy Colour -rendering properties 53 COMPILED BY TANVEER AHMED

54. Lamp efficacy 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 54 COMPILED BY TANVEER AHMED

55. Lamp efficacy The human eye is stimulated more strongly by light of some wavelength 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 the same radiant flux appear quite dim by comparison. 55 COMPILED BY TANVEER AHMED

56. Lamp efficacy The wavelength-dependent factor that converts radiant energy measures to luminous or photometric measures is known as the V λ function. It varies with wavelength across the visible spectrum (Figure 1.19). 56 COMPILED BY TANVEER AHMED

57. where Km = luminous efficacy of radiation at 555 nm (about 683 lm W–1), at which wavelength the V λ function has a maximum value of 1.000. The limits of the integral in Eqn 1.7 are effectively those of the visible spectrum, i.e. 380–770 nm. 57 COMPILED BY TANVEER AHMED

58. Lamp efficacy 58 COMPILED BY TANVEER AHMED

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 at 589 nm where V λ = 0.76, with a maximum efficacy near 150 lm W–1. Some energy is dispersed in non-visible emission and some by heat loss and other inefficiencies. Lamp efficacy 59 COMPILED BY TANVEER AHMED

60. Lamp efficacy 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). 60 COMPILED BY TANVEER AHMED

61. Colour -rendering properties 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. 61 COMPILED BY TANVEER AHMED

62. Colour -rendering properties 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. 62 COMPILED BY TANVEER AHMED