Radiation Pyrometers
About the radiation pyrometers
All the alternative forms of radiation
pyrometer have an optical system which is similar to that in the
optical pyrometer and focuses the energy emitted from the measured
body. They differ, however, by omitting the filament and eyepiece and
using instead an energy detector in the same focal plane as the
eyepiece was. The radiation detector is either a thermal detector,
which measures the temperature rise in a black body at the focal point
of the optical system, or a photon detector. Thermal detectors respond
equally to all wavelengths in the frequency spectrum whereas photon
detectors respond selectively to a particular band within the full
spectrum.
Thermopiles, resistance thermometers and thermistors are all used as
thermal detectors in different versions of these instruments. These
typically have time constants of several milliseconds, because of the
time taken for the black body to heat up and the temperature measuring
instrument to respond to the temperature change.
Photon detectors are usually of the photoconductive or photovoltaic
type. Both of these types respond very much faster to temperature
changes than thermal detectors because they involve atomic processes,
and typical measurement time constants are a few microseconds.
The size of objects measured by a radiation pyrometer is limited by
the optical resolution, which is defined as the ratio of target size
to distance. A ratio of 1:300 is regarded as good, and this would
allow temperature measurement of a 1 mm sized object at a range of 300
mm. With large distance/target size ratios, accurate aiming and
focusing of the pyrometer at the target are essential. It is now
common to find 'through the lens' viewing provided in pyrometers,
using a principle similar to SLR camera technology, as focusing the
instrument for visible light automatically focuses it for infrared
light.
Various forms of electrical output are
available from the radiation detector: these are functions of the
incident energy on the detector and are therefore functions of the
temperature of the measured body. Whilst this therefore makes such
instruments of use in automatic control systems, their accuracy is
often inferior to optical pyrometers. This reduced accuracy arises
first because a radiation pyrometer is sensitive to a wider band of
frequencies than the optical instrument and the relationship between
emitted energy and temperature is less well defined. Secondly, the
magnitude of energy emitted at low temperatures gets very small,
increasing the difficulty of accurate measurement.
The forms of radiation pyrometer differ mainly in the technique used
to measure the emitted radiation. They also differ in the range of
energy wavelengths which each is designed to measure and hence in the
temperature range measured. One further difference is the material
used to construct the energy-focusing lens. Outside the visible part
of the spectrum, glass becomes almost opaque to infrared wavelengths,
and other lens materials such as arsenic trisulphide are used.
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Temperature Measurements
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