GB950427A - Infrared radiometer with background elimination - Google Patents

Abstract

950,427. Radiation detectors. BARNES ENGINEERING CO. Aug. 7,1962 [Oct. 5, 1961], No. 30186/62. Heading G1A. In an arrangement for detecting radiation from a target in the presence of background radiation, that portion of the latter which occurs in the same spectral region as the target radiation is balanced out in a differential arrangement involving two detectors selectively responsive to the spectral region of the target radiation and to a different spectral region of the background radiation respectively. The invention is particularly concerned with the detection of targets radiating in the infra-red region (e.g. rocket exhausts) in the presence of sunlit backgrounds. In one embodiment (Figs. 1 and 2) the detection arrangement comprises two photoconductive (lead sulphide) detectors 6, 7 together with respective filters 4, 5 mounted in a frame 8 which may be moved in the direction of the arrow by means of a micrometer screw 9. Filter 4 passes a (short wave) band in the visible spectrum (see also Fig. 3) and filter 5, which may be of germanium, has a cut-off at about 1.8 Á and so passes radiation in the infrared region. In operation, the device is initially balanced by pointing it at the sky or at a sunlit background and adjusting screw 9 until the relative proportions of the incoming radiation (imaged by optical systems 1,2,3 on the plane of the detectors) which fall on the detectors is such as to result in zero output,the detectors being differentially connected electrically. Fig. 2 shows the balance adjustment with a typical background radiation in which 10 to 15% of the latter falls on detector 6 and the remainder on detector 7, the respective filters 4 and 5 not being shown. After initial balance, which may have to be repeated whenever the background conditions change, the device is then effective in response to radiation in the infra-red region to produce a resultant electrical output. In a second embodiment (Fig. 4) the two detectors 11, 12, are arranged back-to-back, the former, 11, which may be made of silicon, gallium arsenide or other material which responds to (short wave) radiation in the visible spectrum and transmits long wave (infra-red) radiation, being mounted on a substrate 13,of fused silica or other material transparent to the near or intermediate infra-red (e.g. lead sulphide or indium antimonide), on the other side of which the second detector 12 is mounted, the latter, which responds to the near or intermediate infra-red being of lead sulphide or indium antimonide. The two detectors are differentially connected electrically and for balancing out background radiation a germanium filter plate 14 which passes infra-red but cuts-off radiation in the visible spectrum completely is moved across the field of view until zero output is obtained. Specification 898,205 and U.S.A. Specification 2,961,545 are referred to.