Patent GB-940963-A (GB)
Title: Colour pyrometer
IPC: G01J5/60
Filed: 1961-08-25
Abstract: 940,963. Electric analogue computers. M. C. PFISTER. Aug. 25, 1961, No. 30659/61. Heading G4G. [Also in Division G1] A radiation pyrometer for measuring the colour temperature of an object comprises a detector head for viewing the object and providing two output signals significant of the intensity of radiation emanating from the body at two different wavelengths, a computer for determining from the ratio of the two output signals of the detector head the colour temperature of the object, an adjustable radiation attenuator disposed between the position of the object and the detector head, and a control stage responsive to one of the output signals of the detector head to adjust the attenuation produced by the radiation attenuator so as to maintain the maximum intensity of radiation falling on the detectors sensibly constant. General. The detector head includes a polygonal mirror 3, Figs 1 and 2, comprised of four symmetrically disposed concave mirrors, which is rotated to deflect the radiation from the source (not shown) on to a plane mirror 4 from which it is reflected to a first photoconductive cell 10. An opposing two of the faces of the mirror 3 are aligned with filters F1, each passing infra-red radiation in a waveband centred about 2. 8Á, and the other two are aligned with filters F2 each passing a band centred about 1. 5Á, so that pulsating signals are produced by the cell 10 whose amplitudes depend alternately on the source intensity in each of these two wavebands. These signals are supplied to a pre-amplifier 15, Fig. 3, and thence to a measuring and computer circuit (see below) to form the quotient of the signals, and also to a control circuit (see below) which retroactively controls an adjustable attenuator 8, Fig. 1, in the form of a variable aperture located in front of mirror 4, so as to maintain constant, in terms of the maximum amplitude signals from the cell 16, the maximum radiation intensity incident upon the cell. Limit switches 50 and 51 actuated by the attenuator are arranged to disable the power supplies to the measuring and computer circuit if either the maximum or minimum aperture is reached. Rotated with the mirror 3 and angularly disposed between its faces are four plane mirrors 5 to direct the incident radiation on to a second photocell 11 alternately with the incidence upon the measuring circuit photocell 10. The field of view of the second cell 11 and each mirror 5 is less than, and overlapped at both ends by, that of the first cell 10 and each face of the mirror 3. By this means, if the output of the cell 10 falls below a threshold value, thus indicating that the source is not completely within the field of view of the measuring circuit photo-cell 10, a relay R5 controlled via an amplifier 55 is released and the power supply to the measuring circuit disabled. Control circuit for retroactive adjustment of attenuator 8. This comprises thyratrons 18 and 19 to the grid of each of which is supplied positivegoing signals corresponding to the intensities in the passbands of the filters F1 and F2. The anodes are supplied in antiphase to each other with A.C. from a transformer 20 so that thyratron 18 can conduct only in synchronism with a signal corresponding to the F1 filter whereas thyratron 19 can conduct only in synchronism with a signal corresponding to the F2 filter. A capacitor 22 connected to both grids and both cathodes is thus charged to the peak voltage of the greater of the two signals. This voltage, together with a reference voltage from a rectifier 24, is supplied to a null detector 25 whose output controls a motor 9 which adjusts the attenuator 8. Measuring and computer circuit. This comprises thyratrons 26 and 27 which ,in similar fashion to those in the control circuit, are supplied with positive-going signals corresponding to the intensities in the pass bands of the filters F1 and F2. The anode voltages of the two thyratrons are varied in antiphase by a transformer 28 so that each can conduct only in synchronism with a respective one of these two signals. A capacitor 35 connected between the grid and cathode of thyratron 26 is thereby charged to the peak voltage of one of the signals (and it is arranged by means described later that this is the greater of the two), whilst another capacitor 36 connected similarly to thyratron 27 is charged to the peak voltage of the other signal. This latter voltage is supplied, together with the fraction of that across capacitor 35 appearing at the tapping of a potentiometer 39 connected in parallel with it, is supplied to a difference detecting circuit 41 which rotates a servomotor M coupled to both the tapping point and an indicator 40 to indicate the colour temperature in terms of the ratio of the two signals. To ensure that the capacitor 26 is charged to the larger voltage of the two signals, a reversing circuit U controls the phase of the supply to the anode supply transformer 28. This circuit comprises further thyratrons 45 and 46, to the grids of which are supplied the two positive going signals together with a fixed negative bias voltage. This is always below the tube cut-off voltage by an amount less than the constant amplitude at which the larger signal is maintained by the control circuit action. The anodes of the thyratrons are supplied with A.C. in antiphase and thus only that which has a positive anode voltage when its grid is raised above the striking voltage by the larger of the two signals conducts. This conduction energizes one or other of relays R 1  and R 2 , respectively connected in the thyratron anode circuits, and this actuates a switching circuit, Fig. 4 (not shown) to control the phase of the supply to transformer 28 supplying the thyratrons 26 and 27 of the measuring circuit. In the event of both relays failing to operate or both relays operating, corresponding respectively to excessively strong or excessively weak signals, the power supply to the measuring and computer circuit is disabled so as to prevent the production of a false measurement. Specifications 938,262 and 940,964 are referred to.