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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