1.5 source of artificial light

1. Introduction
2. The tungsten-filament lamp
3. Tungsten–halogen lamps
4. Xenon lamps and gas discharge tubes
5. Fluorescent lamps and tubes
6. Laser light sources and LEDs

COMPILED BY TANVEER AHMED 4

 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).

 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).


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.


 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 lamp
Some light sources show only minor deviations from
Planckian distribution:
of these, the tungsten-filament lamp is a prime example.


 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 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.


 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.

 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.


An electric current can be made to pass
through
xenon gas
by using a high-voltage pulse to cause
ionisation.


 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.


 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.


 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.


 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

 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.


 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.


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.


 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


 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 sources are increasingly being used
in optical measuring equipment,
certain types of spectrometers
and monitoring equipment of many different types.


 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.


 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


 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.


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.

 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.


 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 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)

 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.

 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).


 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.


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 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


 Table 1.3 shows the materials used to make
LEDs to produce light of different colours.


 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).


 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.


 Figure 1.18 shows diagrammatically the
construction of a semiconductor
 junction laser.


There are two aspects of artificial light sources
that are of particular interest to colour
scientists:
1. Lamp efficacy
2. Colour-rendering properties


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


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.


The wavelength-dependent factor that
converts radiant energy measures
to luminous or photometric measures
is known as the Vλ function. It varies with wavelength across
the 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.

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 dispersed
1. in non-visible emission
2. and some by heat loss
3. and other inefficiencies.


 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).


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.


 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.


1.5 source of artificial light

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1.5 source of artificial light