In the brightness type of pyrometer, the radiation from the heated surface being examined is collected by a lens and focused onto a thermistor or thermocouple. The net radiation from a heated ideal blackbody follows the Stefan-Boltzmann Law, which states that the total radiation from a black body is given by
สมการ 14.11
where
W = total radiation expressed in watts/cm2
T = absolute temperature of the hot body, K
o = Stefan-Boltzmann constant, 5.672 x 10 -12 w cm -2 deg -4
The problem with applying this equation is that it is difficult to find an ideal blackbody in an industrial environment (e.g.,red objects and blue objects at the same temperature do not emit the same amount of radiation). Emissivity is defined as the radiation efficiency of an actual surface in comparison with that of an ideal blackbody radiator. Consequently, to take such emissivity differences,to take such emissivity differences into consideration, Plank's equation for graybodies is used.