1.5 source of artificial light

COMPILED BY TANVEER AHMED 1

SOURCES OF ARTIFICIAL LIGHT



SOURCES OF ARTIFICIAL LIGHT


Sources of light TABLE OF CONTENT

1. Introduction2. The tungsten-filament lamp3. Tungsten–halogen lamps4. Xenon lamps and gas discharge

tubes5. Fluorescent lamps and tubes6. Laser light sources and LEDs


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).


Introduction incandescence,

Common incandescent sources range from

1. the sun, 2. through tungsten 3. 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).


Other methodsof producing light, probably in decreasing order of importance, are:

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 laser

3. (c) cathodoluminescent sources based on phosphors, as used in television and VDU screens

4. (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.


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.


The tungsten-filament lampSome light sources show only minor deviations from

Planckian distribution: of these, the tungsten-filament lamp is a prime example.


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.



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.



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.


Tungsten–halogen lampsTungsten filaments can be heated to higher temperatures with longer lamp lifetimes if

some halogen (iodine or bromine vapour) is present in the bulb.


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.


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.


Xenon lamps and

gas discharge

tubesAn electric current can be made to

pass through xenon gas

by using a high-voltage pulse to cause ionisation.


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.


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.


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.


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


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.



When mercury arcs with clear

quartz or silica envelopes are used,

protection is also required from generated ozone.



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).



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.


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.



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.



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.


Fluorescent lamps and tubesThe 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 electricaldischarge is produced.


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.



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


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.


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 .



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).



Laser light sources

Laser sources are increasingly being used in optical measuring equipment, certain types of spectrometers

and monitoring equipment of many different types.



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.


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


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.


tubes

The ubiquitous fluorescent tube consists of a long glass vessel

containing mercury vapourat low pressure sealed at each end

with metal electrodes between which an electrical

discharge is produced.


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.


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.


Figure 1.15 A schematic illustration of the steps leading to laser action: (a) the Boltzmannpopulation of states, with more atoms in the ground state;

(b) when the initial state absorbs, thepopulations are inverted (the atoms are pumped to the excited state);

(c) a cascade of radiationthen occurs, as one emitted photon stimulates another atom to emit, and so on: the radiation is coherent (phases in step)


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 directionas the stimulating light,

and it will be coherent with it, i.e. all the crests and troughsoccur exactly in step, as indicated in Figure 1.15.


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).


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 the ▪ extremely rapid chemical ▪ And physical processes that take place

immediately after light is absorbed.


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.


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


Materials used to make LEDs Table 1.3 shows the materials used to make

LEDs to produce light of different colours.


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).


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.


diode laser

Figure 1.18 shows diagrammatically the construction of a semiconductor

junction laser.


PROPERTIES OF ARTIFICIAL LIGHT

SOURCESThere are two aspects of artificial light sources that are of particular interest to colourscientists:1. Lamp efficacy2. Colour-rendering properties


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


Lamp efficacyThe 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.


Lamp efficacy

The wavelength-dependent factor that converts radiant energy measures to luminous or photometric measures is known as the Vλ function. It varies with wavelength acrossthe visible spectrum (Figure 1.19).


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 the visible spectrum, i.e. 380–770 nm.


Lamp efficacy


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 Vλ = 0.76, with

a maximum efficacy near 150 lm W–1.

Some energy is dispersed1. in non-visible emission 2. and some by heat loss 3. and other inefficiencies.

Lamp efficacy


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).


Colour-rendering propertiesthe 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.


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.

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1.5 source of artificial light