The detection of thermal radiation using linear expansion

Abstract

A thermal radiation detector has been made which depends on the expansion of a thin metallic strip a few millimetres long. Expansions of the order of 10 -12 to 10 -10 cm are converted into rotations of 10 -10 to 10 -8 rad by means of a mechanical system using a galvano- meter-type suspension strip as a flexure pivot. The rotations are measured with an optical lever and photoelectric amplifying system. Constantan 0.1u thick is employed as the expansion material, and the receiving area is about 1 mm 2 . The device has been developed primarily to see whether the thermal fluctuation limit of sensitivity could be more closely approached than has been possible with conventional detectors. E. W. Elcock and C. W. McCombie have considered the limit for the expansion detector, and have shown that it arises from the radiation dam ping of oscillations in the expansion strip, and that this gives the standard result for a receiver of a given area. Observations on several of the best of the expansion devices show that they come within a factor of between two and three of the best possible sensitivity for a receiving area of 1 mm 2 , achieving a noise equivalent signal of less than 1.5 x 10 -11 W for a bandwidth of 1 c/s.