An optical pyrometer is an instrument to measure high temperatures. It works on the principle of comparing the radiation emitted by the body whose temperature is to be measured with that of an...

An optical pyrometer is an instrument to measure high temperatures. It works on the principle of comparing the radiation emitted by the body whose temperature is to be measured with that of an incandescent fi lament. The instrument is calibrated in such a way that it measures the temperatures which a black body would have if it were emitting radiation equal to that of the body under investigation.

A pyrometer records the temperature of a body as 1600 K with a redlight filter (= 0.65 mm). Find the time temperature of the body if its emissivity
 at 0.65 mm is 0.7.

Hints: If T is the temperature of the body at wavelength
 and emissivity
, we have
. Neglecting unity in the denominator of Planck’s equation (7.10).