Patent GB-979619-A (GB)
Title: Combined spectrophotometer and radiometer
IPC: G01J3/32
Filed: 1962-10-08
Abstract: 979,619. Spectrophotometer. PERKIN-ELMER CORPORATION. Oct. 8, 1962 [Oct. 16, 1961], No. 38033/62. Heading G1A. [Also in Division G2] General. A combined spectrophotometer and radiometer comprises a spectrophotometer section, a radiometer section, means for collecting radiation from an object and means for directing substantially all of the collected radiation sequentially to the spectrophotometer section and the radiometer section. The apparatus includes also a reference source of radiation and the means for directing the collected radiation sequentially to the two sections is arranged so that when one section is receiving collected radiation the other is receiving reference radiation. The apparatus further includes a second source of reference radiation and this radiation is directed sequentially with the collected and other reference radiation to only the radiometer section. The apparatus is intended for measurements on a distant object, such as the exhaust plume of a missile, although no details are given of the wavelength range over which measurements are made or the character of the radiation detectors other than that they produce electrical outputs and that a slow spectral scanning rate is necessary in the spectrophotometer section if a thermistor is used. The apparatus is arranged to form two images from the distant object, one being the so-called "target" and immediately surrounding "background" and the other an adjacent but wholly separate "background" area. Separate detectors respond to the two images in the radiometer section, whilst the two images are presented to the spectrophotometer section alternately at high speed. In circuits for processing the detector signals the "background" signal is subtracted from the "target" signal to produce information due to the "target" alone. In the processing circuits, intermediate magnetic tape recording is employed and the spectrophotometer output is displayed by oscilloscope. Optical details, Fig. 1. Radiation to be measured is received by an optical system formed of concave reflector 10 and convex reflector 16 and is brought to a focus at a pair of apertures 18 forming the entrance of the spectrophotometer section, the optical path including a rotary timeshare chopper 30, a black-body radiator 20, through which the radiation passes by way of a frusto-conical aperture 20, and a rotary reticle chopper 40 which functions to occult the two entry apertures 18 alternately at high speed. One aperture is located in the image plane so as to coincide with the "target" and its immediate surround whilst the other is located so as to coincide with a "background" area. Chopper 30 has a 60 degrees blade 32 which is reflective on both faces and a 15 degrees blade which is reflective on face 34 but is blackened on the other face 35. When each blade comes into the radiation path the incoming radiation is cut off from the entrance to the spectrophotometer and in its place there passes reflected radiation originating from body 20. When blade 32 is in the radiation path the incoming radiation is reflected by way of mirror 48 to the radiometer section 50. This comprises an image reducing optical system, comprised of a plane reflector 52 and an ellipsoidal concave reflector 54, and a pair of closely adjacent detectors with associated amplifiers 59, 60. One detector is located in the image plane to coincide with the "target" and immediate surround whilst the other is located so as to coincide with a "background" area. When the 15 degrees blade is in the radiation path the radiometer receives radiation from surface 35 and when neither blade of chopper 30 is in the path, the radiometer receives radiation from black body 20. The function of chopper 30 is thus (1) to direct the incoming radiation in sequence to the spectrophotometer and radiometer sections, (2) to substitute reference radiation from body 20 when the incoming radiation is interrupted and (3) to divert a brief pulse of radiation from surface 35 to the radiometer section only. The blades of the chopper 30 pass through a heat sink 37 whereby surface 35 is maintained at ambient temperature, whilst body 20 incorporate a heating element and a temperature sensor connected in a control system and arranged to maintain the body 12C. above ambient temperature. In the output signals from the radiometer detectors the difference in level due to body 20 and blade surface 35 serves as a reference for equalizing the signals in the two detector channels (see below). In the spectrophotometer, radiation passing through the entry apertures 18 is directed by way of plane reflector 66 and collimating reflector 68 through a conventional dispersing prism 70. The radiation is then reflected back by an arrangement which functions in a manner equivalent to a vertical plane reflector rocking about a vertical axis and which comprises (1) a plane mirror 72 carried on the end of a rotating shaft which is not perpendicular to the plane of the mirror and (2) a roof-angle mirror assembly 76, 78 with the reflecting surface at right-angles and with the intersection 77 of the surfaces horizontal. Reflector 68 is tilted with respect to the optical axis of the system whereby the radiation reflected back impinges on a reflector 82 is directed through an exit slit 84 to a detector 90 incorporating an optical system 92, 94 similar to system 52, 54. As reflector 72 nutates, the dispersed radiation is swept back and forth across slit 84 thus presenting the detector 90 with each part of the spectrum in succession. The mirror may be nutated at a speed between 2.5-15 times a second to produce a corresponding number of back and forth spectrum scans (the lower speeds being used when detector 90 is of the slow response type, e.g. a thermistor). Chopper 40 normally operates at 7350 times per second so that detector 90 produces a large number of signal samples of the "target" and "background" radiation during each spectral sweep. Chopper 30 operates at 2/163 cycles per second so that each spectral sweep will be interrupted (when radiation passes to the radiometer) at least once. The rotation rates of mirror 72 and chopper 30 are arranged however not to be integrally related so that after a number of spectral sweeps the whole of the spectrum will have been analysed. Chopper 30, chopper 40 and mirror 72 are driven from a common synchronous A.C. motor via gear trains, Fig. 3 (not shown), which include variable ratio transmissions in the drives to chopper 40 and mirror 72. Additionally the drive to chopper 30 includes a clutch controlled by an electromagnet connected in circuit with cam-operated switches on the chopper shaft. By this means the chopper may be stopped in preselected angular positions. In one position the chopper is stopped so that neither of the blades is in the radiation path. The apparatus may then function as only a spectrophotometer. In another position the chopper is stopped with the 60 degrees blade in the radiation path so that the spectrophotometer receives radiation from body 20 continuously. This position is employed for calibration purposes, the temperature control system of body 20 being arranged to bring the body to an elevated temperature, e.g. 500K. The spectrophotometer section includes a shutter, Fig. 3a (not shown), actuated by an electromagnet and arranged to block that one of entry apertures which sees the background radiation. The shutter is brought into use on large and near targets where no "background" correction is necessary. Signal processing details, Figs. 4 and 5 (not shown). The signals from the two radiometer detectors are first amplified and then "clamped" to establish the signals due to black body 20 at a zero level. Thereafter the signals are amplitude modulated on separate carriers and recorded on one track of a four-track magnetic tape recorder. When played back the two carriers are separated and demodulated to derive the original signals. These are then applied to a circuit in which the reference pulses due to the 15 degrees chopper blade are gated out, compared in amplitude and employed to adjust the amplitude of the "background" signal until the pulse amplitudes are equal. This automatically compensates for any differences in the gains of the two signal channels. The final "target" and "background"signal are subtracted to obtain a signal representing the intensity of radiation due to the target only. Correctly timed pulses to control the clamping and gating operations are derived from a photoelectric detector 140, Fig. 1, associated with chopper 30, the signals from the detector being recorded on another track of the tape. The signal from the spectrophotometer detector 90, Fig. 1, is passed through an amplifier which is subjected to automatic gain control. The gain control system includes a sampling stage which is keyed once for back and forth sweep through the spectrum, the system functioning to integrate the signal level during one sweep and control the amplification in accordance with the integrated value during the next succeeding one. A pulse to time the keying operation is provided by a switch operated from the shaft carrying mirror 72. A locally generated reference pulse of predetermined amplitude is also added to the signal during the interval when the incoming radiation is interrupted by the 60 degrees blade of chopper 30. The signal is recorded on a track of the magnetic tape recorder. When played back, the signal is applied to a synchronous demodulator which functions to separate the "target" and "background" signals into separate channels. For this purpose the demodulator is provided with reference signal at 7350 c/s which is derived from a photoelectric detector 130 associated with chopper 40 and recorded on a track of the recorder. In order to equalize the channel gains for two signals, the reference