Apparatus for measuring temperatures

Claim 1

Apparatus for measuring the temperature of a substrate including a body having a reflective surface for defining in cooperation with said substrate a confined space which acts as a black body when the body and the substrate are at the same temperature, radiant energy detector means having a single stationary radiation detector for alternately sensing the radiant energy incident from a point on the reflective surface and a point on said substrate within said confined space, a quartz prism mounted on a tuning fork, said tuning fork being so positioned with respect to the paths along which the radiant energies incident from said points on said reflective surface and said substrate pass that upon activation of the tuning fork the prism is alternately outside both paths so as to permit the radiant energy incident from one of said points to fall on said radiation detector and across both paths so as to permit the radiant energy incident from the other of said points to fall on said radiation detector, said radiant energy detector means providing an output measurably responsive to the difference in the radiant energies incident from those two points, means for heating the body, and means for controlling the heating means in response to the output of the radiant energy detector to adjust the temperature of the body to the temperature of the substrate.