Narrow-band Radiation Pyrometers
About narrow-band radiation pyrometers
Narrow-band radiation pyrometers are highly stable instruments which
suffer a drift in accuracy which is typically only 1°C in 10 years.
They are also less sensitive to emissivity changes than other forms of
radiation pyrometer. They use photodetectors of either the
photoconductive or photovoltaic form whose performance is unaffected
by carbon dioxide or water vapor in the path between the target object
and the instrument.
A photoconductive detector exhibits a
change in resistance as the incident radiation level changes whereas a
photovoltaic cell exhibits an induced voltage across its terminals
which is also a function of the incident radiation level. All
photodetectors are preferentially sensitive to a particular narrow
band of wavelengths in the range 0.5-1.2 µm and all have a form of
output which varies in a highly non-linear fashion with temperature;
thus a microcomputer inside the instrument is highly desirable.
Four commonly used materials for
photodetectors are cadmium sulphide, lead sulphide, indium antimonide
and lead tin telluride. Each of these is sensitive to a different band
of wavelengths and therefore all find application in measuring the
particular temperature ranges corresponding to each of these bands.
The output from a narrow-band radiation pyrometer is normally chopped
into an a.c. signal in the same manner as used in the chopped
broad-band pyrometer. This simplifies the amplification of the output
signal which is necessary to achieve an acceptable measurement
resolution. The typical time constant of a photon detector is only 5
µs which allows high chopping frequencies up to 20 kHz. This gives
such instruments an additional advantage in being able to measure fast
transients in temperature as short as 10 µs.
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Temperature Measurements
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