Miniature Optically Immersed Thermistor Bolometer Arrays

Abstract

Immersed thermistor bolometers have been in use since 1958 as sensors for ir horizon scanners employed in attitude control of earth-orbiting vehicles.(1) These detectors usually use a single small thermistor flake optically immersed in an antireflection coated pure germanium or silicon hemisphere or hyperhemisphere.(2) Studies of the earth's atmospheric horizon from orbiting vehicles indicate that the most useful radiant power lies in the carbon dioxide spectrum near 15 micro and in the rotational water bands of atmospheric moisture in the spectrum beyond 20 micro.(3) These atmospheric constituents produce high ir optical density and hence provide small angle horizon resolution. Carbon dioxide has the additional advantage, by being uniformly distributed in the atmosphere, of providing a stable horizon. The purpose of this paper is to describe briefly two types of five-element linear arrays of thermistor flakes optically immersed in germanium and silicon lenses (Fig. 1). These detecrs were designed for an advanced horizon definition study program at NASA-Langley Research Center.(4) Germanium immersion is employed for best detectivity in the carbon dioxide spectrum from 14 micro to 16 micro and silicon for the spectrum beyond 20 micro.