Patent GB-786196-A (GB)
Title: Improvements relating to apparatus for the measurement of radiation
IPC: G01T1/185
Filed: 1954-07-29
Abstract: 786,196. Radiation measurements. CENTRAL ELECTRICITY AUTHORITY, HASLER, E. F., and SPURR, G. July 29, 1955 [July 29, 1954], No. 22202/54. Class 40 (3). An instrument for measuring the intensity of ultra-violet radiation or radiation of longer wavelength comprises an electronic blocking oscillator with a photo-cell containing a solid material responsive to such radiation connected in its grid or equivalent return path in such a way that the leak resistance or the potential of a point to which the leak is returned is varied by the radiation to vary the discharge rate of the grid or equivalent condenser and hence the pulsing rate, which is thus a measure of intensity. In the Fig. 1 embodiment the photo-cell 9 forms the grid leak of a cathode coupled Hartley blocking oscillator provided with a stabilized H.T. supply, the rate at which the oscillator pulses depending on the resistance and hence the illumination of the photo-cell. Valve 1 and cell 9 are mounted on polytetrafluorethylene bases to keep stray leakage resistance to a minimum and plastic film condensers may be used for this purpose. The pulses from the oscillator cathode are fed to a binary divider stage operating as follows: The positive going half of a first pulse is fed to the grid 27 of double triode 1 via rectifiers 20, contacts 22, 24 of relay OR and the CR. delay circuit 25, 26 to produce an anode current pulse of sufficient duration to operate relay OR and cause contact 24 to move over to 23 which is at a potential sufficient to hold the relay energized. The negative half of the next pulse is fed through rectifiers 21 to lower the potential of point 23 to cause the relay to relapse and contact 24 to return to 22 and so on. A further set of change-over contacts operated by the relay is connected to a telemeter which thus receives pulses with a mark to space ratio of unity at a rate dependent on illumination. The lower end of the meter scale may be expanded by wiring a limiting resistor in series with the cell 9. In a modification for measuring the integrated illumination the binary divider is dispensed with, the pulses being fed through rectifier 20 and the RC delay circuit to the control grid of the second half of valve 1 to operate a pulse counter in its cathode circuit. In another modification a fixed resistor is used as a grid leak but is returned to the tapping point on a potentiometer across the H.T. supply formed by a fixed resistor and a photo-cell. The fixed resistor may be replaced by another photo-cell in which case the differential illumination of the cells determines the pulse rate. In the above embodiments the thermionic valve may be replaced by a junction transistor appropriately connected. In a further embodiment utilizing a gas-filled cold cathode tube in a tuned anode Meissner oscillator the cell is used as a trigger leak. The circuits described may be used for assessing the demand for lighting recording the content of solids in flue gases, in radiation pyrometry, using infra red or visible light, examination of boiler feed water by reflected light, as a low level alarm for opaque liquids and finely divided solids and for remote observation of water level in a gauge glass. Where visible light is to be measured the photo-cell may be of the vacuum or gas-filled photo-emissive type and for infra-red radiation a lead sulphide or germanium photo-conductive cell may be used. Specifications 177,808, [Class 40 (v)], and 264,792, [Class 40 (i)], are referred to.