Page 510 - Instrumentation Reference Book 3E

Light sources 493
actual power of the lamp. For example, if a xenon
arc is to be used with a spectroscopic system. the
quantity of interest is the amount of light which
can be got through the slit of the spectrometer. A
low-power lamp with a high radiance can be
focused down to give a small, intense image at
the slit and thus get a lot of light through: but if
the lamp has a low radiance, it does not matter
how powerfuul it is, it cannot be refocused to pass
the same amount of radiation through the sys-
tem. It can be easily shown in this case that the
radiance .is the only effective parameter of the
source.
If light output in the visible region only is con-
cerned, “luminance” is sometimes used instead of
“radiance.” There is a strict parallel between units i
I
and definitions of ”radiant” quantities and “lumi- 0 80 100 120
nous” quantities (BS Spec. 4727: TEC-CIE, Inter-
iiatior~cil Liglzting Vocabulciiy). The unit of light, Pcxentage of nominal voltage
the lumen, can be thought of as a unit of energy Figure 21.2 Variation with voltage of life and light output
weighted with regard to wavelength according to of a tungsten lamp (after Henderson and Marsden).
its ability to produce a visible sensation of light
(WaIsh 19158. p. 138). mous preponderance of red energy will be noted.
Tungsten lamps have a high radiance. are very
stable in light output provided the input power is
21.2.1 Incandescent lamps
stabilized, and are perfectly stable in position. The
Incandescent sources are those in which light is light output can be precisely controlled by vary-
generated by heating material electrically until it ing the input power, from zero to maximum, but
becomes white hot. Normally this material is as the filament has a large thermal mass there is
tungsten, but if only infrared radiation is wanted no possibility of deliberately modulating the light
it may be a ceramic material. In a tungsten lamp output. If a lamp is run on an a.c. supply at mains
the heating is purely resistive, and the use of frequency some modulation at twice that fre-
different filament diameters enables lamps to be quency invariably occurs and may cause trouble
made of similar power but different voltage rat- if other parts of the instrument system use mains-
ings. The higher the voltage, the finer and more frequency modulation. The modulation is less
fragile is the filament. For instrument purposes marked with low-voltage lamps which have more
small and compact filaments giving the highest massive filaments, but can only be overconie by
radiance are usually needed, and so low-voltage using either a smoothed d.c. supply or a high-
lamps are often used. For lamps used as radiation frequency power supply (10 kHz).
standards it is customary to use a solid tungsten The main drawback to tungsten lamps is the
ribbon as a filament, but these require massive limited life. The life depends on the voltage (Fig-
currents at low voltage (for example, ISA, 6V). ure 21 .?), and it is common practice in instrument
The spectral power distribution of a tungsten work to under-run lamps to get a longer life.
lamp corresponds closely to that of a Planckian Longer lamp lives are obtained with “tungsten
radiator, as shown in Figure 21.1. and the enor- halogen” lamps. These have a sinall amount of a
halogen-usually bromine or iodine-in the
envelope which retards the deterioration of the
filament. It is necessary for the wall tenperature
L of the bulb to be at least 3QO0C, and this entails
W
3 the use of a small bulb made of quartz. However,
g
._ 3300 K this allows a small amount of ultraviolet radi-
W
m - ation to escape, and with its small size and long
W
c 3000 K life, the tungsten halogen lamp is a very attractive
2500 K light source for instrument purposes.
0.3 0.8 1.3 P
21.2.1.1 Notes on Iiardling arid zise
Wavelength
Figure 21.1 Spectral powerdistribution of tungsten After lengthy use troubles can arise with lamp-
lamp. holders. usually with contact springs weakening.