The electrical-substitution radiometer shown schematically determines the optical (radiant) power of a beam by measuring the electrical power required to heat the receiver to the same temperature. With a beam, such as a laser of optical power P _opt, incident on the receiver, its temperature, T_s, increases above that of the chamber walls held at a uniform temperature, T_sur = 77 K. With the optical beam blocked, the heater on the backside of the receiver is energized and the electrical power, P _e1ec, required to reach the same value of T_s is measured. The purpose of your analysis is to determine the relationship between the electrical and optical power, considering heat transfer processes experienced by the receiver.

Consider a radiometer with a 15-mm-diameter receiver having a blackened surface with an emissivity of 0.95 and an absorptivity of 0.98 for the optical beam. When operating in the optical mode, conduction heat losses from the backside of the receiver are negligible. In the electrical mode, the loss amounts to 5% of the electrical power. What is the optical power of a beam when the indicated electrical power is 20.64 m W? What is the corresponding receiver temperature?

Liquid nitrogen Chamber walls. T Pect Laser beam Heater, Peiec Insulation Receiver, T,