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 1C 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 m 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 s 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 s.


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