Page 511 - Instrumentation Reference Book 3E

494 Optical measurements

Screw caps are preferable to bayonet caps. The many cases the spectrum is, in effect, continuous.
envelopes of tungsten halogen lamps should not The spectrum of xenon, for example, has over
be touched by hand-doing so will leave grease on 4000 lines and extends well down into the ultra-
them whichwill burn into the quartz when hot violet region. Xenon lamps are widely used in
and ruin the surface. spectrofluorimeters and other instruments where
a very intense source of ultraviolet radiation is
required.
21.2.2 Discharge lamps
Many lamps of this kind require elaborate
Discharge lamps are those in which light is pro- starting arrangements and are particularly diffi-
duced by the passage of a current through a gas cult to restart if switched off when hot. “Igniter”
or vapor, and hence produce mostly line spectra. circuits are available, but as these involve volt-
Enclosed arcs of this kind have negative tempera- ages up to 50kV special attention should be
turekesistance characteristics, and so current given to the wiring involved. All these lamps
limiting devices are necessary. Inductors are often contain gas under high pressure even when cold,
used if the lamp is to be run on an a.c. mains and the maker’s safety instructions should be
supply. Many types of lanip are available rigidly followed.
(Henderson and Marsden 1972); some commonly Xenon arcs produce quite dangerous amounts
met with in instrument work will be mentioned here. of ultraviolet radiation-dangerous both in itself
and in the ozone which is produced in the atmos-
phere. They must never be used unshielded, and
21.2.2. I Deuterium lanp to comply with the Occupational Safety and
The radiation is generated by a low current dens- Health Administration (OSHA), interlocks
ity discharge in deuterium. Besides the visible should be arranged so that the lamp is switched
line spectrum a continuous spectrum (Figure off if the instrument case is opened. Force-ducted
21.3) is produced in the ultraviolet. The radiance ventilation should be used with the larger sizes,
is not high, but provided that the input power is unless “ozone-free” lamps are used. These are
stabilized these lamps are stable in both output lamps with envelopes which do not transmit the
and position. To obtain the necessary ultraviolet shorter ultraviolet wavelengths.
transmission either the whole envelope is made of
silica or a silica window is used. These lamps are 21.2.3 Electronic sources: light-emitting diodes
used as sources in ultraviolet spectrophotometers
and are superior to tungsten lamps for that pur- By applying currents to suitably doped semicon-
pose at wavelengths below 330 nm. ductor junctions it is possible to produce a small
amount of light. The luminance is very low but
the light output can be modulated, by modulating
the current, up to very high frequencies, and thus
the light-emitting diode (LED) is a good source
for a fiber optic communication system. LEDs
are also commonly used in display systems. The
spectral power distribution depends on the mater-
ials used in the junction. For electro-optical
communication links there is no need to keep to
the visible spectrum. and wavelengths just longer
than visible (for example, 850 nm) are often used.
There are no serious operating problems and only
200 300 400 low voltages are needed, but the light output is
Wavelength (nm) miniscule compared, say, with a tungsten lamp.
Figure 21.3 Typical spectral power distribution fora
deuterium lamp.
21.2.4 Lasers
2122.2 Coniyac*t source lamps Light from lasers differs from that from conven-
tional sources by virtue of being “coherent”
Some types of discharge lamp offer light sources whereas conventional sources produce “incoher-
of extremely high luminance. These involve dis- ent” light. In an “incoherent” beam there is no
charges of high current density in a gas at high continuous phase relationship between light at
pressure. The lamp-filling gases may be xenon, one point of the beam and any other. The energy
mercury, mercury plus iodine, or a variety of other associated with any one quantum or wave packet
“cocktails.” The light emitted is basically a line can be shown to extend over a finite length-
spectrum plus some continuous radiation, but in somewhere around 50 cm-as it travels through