Patent GB-1220814-A (GB)
Title: Radiometer apparatus
Inventor: SMITH STANLEY DESMOND
IPC: G01J1/42
Filed: 1967-03-01
Abstract: 1,220,814. Position finding. NATIONAL RESEARCH DEVELOPMENT CORP. 28 Feb., 1968 [1 March, 1967; 9 Feb., 1968], Nos. 9790/67 and 6645/68. Heading H4D. [Also in Division G1] Apparatus for measuring a physical variable such as temperature or water content of a region of an atmosphere receives radiation emitted from the region (together with other radiation arriving from other sources in that direction) and passes it through a band-pass filter to a chopper device which periodically passes it through a chamber containing a component of the atmosphere (e.g. carbon dioxide or water vapour) to a synchronous detector. The arrangement described which may be mounted on a weather satellite or may be a turbulence detector mounted on an aircraft measures the spectral emission intensity of the component at the region by using at least one chamber containing the same component as a filter which absorbs the required signal or the major part of it. The weather satellite embodiment shown has two distinct detection channels A and B receiving on different principles and from different distances. It is stated that due to self absorption by the atmosphere a signal detected at any radiated frequency originates predominately from a predetermined distance away. To obtain an adequate signal level each channel receives a band of frequencies with similar absorption characteristics and thus originating from the same zone. Channel A incorporates a single absorption chamber 2A (containing carbon dioxide which absorbs the signal at the centre of each line of carbon dioxide completely) and a detector 11A (thermistor bolometer or pyro-electric) which responds to radiators closely adjacent these centre lines. A chopper comprising a reciprocating black blade 3A alternately allows infra-red or microwave radiation arriving from the region via a mirror 1A (which may be gold plated aluminium) and background radiation (e.g. from space) arriving via a mirror 5A to enter the channel and the detector circuit which includes a transistor preamplifier, a synchronous integrator, a main amplifier and a synchronous detector responds to the difference signal. Germanium lens 6A, 7A, 8A and 12A focus the radiation on the detector. Lens 12A has a low-pass blocking filter deposited on one face and all lens have a non-reflective quarter-wave coating of zinc sulphide. The band pass filter comprises a multilayer interference filter comprising coatings on a germanium plate 10A. The radiation is directed on to the detector by a light guide 13 which is gold plated. The mirror 1A can be turned by a motor 21A to receive radiation from a reference black body 22A to receive the background radiation for periodic checks. The filter 10A may also be changed for one which does not pass the CO 2  radiation band during such checks. The equipment of this channel is suitable for detecting radiation from a region a relatively great distance away. The other channel B is more suited to near distance detection and has two chambers 2B1 and 2B2 one containing the component (CO 2 ) and the other a vacuum or the component at low pressure. The chopper comprises a reciprocating mirror 3B which passes radiation from space alternately through the two chambers and simultaneously blocks off radiation from the region under test and arriving via the mirror 1B through the chamber. It is suggested that a chopping effect could be produced by having the chambers 2B1 and 2B2 mounted for rotation as a unit. An embodiment (Fig. 10, not shown) having two channels each similar to channel A above but employing a common rotating chopper is also described. The channels operate at different pass-bands and accordingly are tuned to different ranges. The difference signal measures the temperature gradient between the two detection regions. Such a system may be mounted on an aircraft and used to detect air turbulence. A gimbal mounted unit which may be located inside a wing tip fuel tank and which can scan in vertical and horizontal directions for three-dimensional soundings is also described.