GB902115A - Improvements relating to light measuring apparatus - Google Patents

Abstract

902,115. Measuring intensity of light. OPTISCHE INDUSTRIE DE OUDE DELFT N.V. April 21, 1960 [April 22, 1959], No. 14048/60. Class 40 (3). The sensitivity of an apparatus for measuring light intensity is controlled by means including a reference light source which radiates on a photo-cell. The cell output under the reference radiation is compared with a reference signal, the difference being used to control the sensitivity. The sensitivity is adjusted only during standardization intervals, which are short compared with the measuring intervals, and is held constant during the measuring intervals. The adjusting means may be, for example, a variable stop or variable density diaphragm interposed in the light beam, or a variation in the cell voltage. When a photo-multiplier tube is used, the voltages on the dynodes is used to control the sensitivity. In the embodiment illustrated ganged switches 3, 13 are closed during the standardization interval and a reference light source 2 illuminates a photomultiplier 1. The photo-multiplier produces a voltage across resistor 15 and is adjusted until this voltage slightly exceeds that of a reference source 16. If, for example, the voltage across resistor 15 is too low, the cathode follower 11 becomes more conductive and the potential on the grid of valve 9 increases. Thus, a higher voltage is produced between the photomultiplier cathode and earth, increasing the current in resistor 7-the dynode supply source- and increasing the multiplication factor of the tube 1 until the voltage across resistor 15 reaches equilibrium. During standardization capacitor 10 charges, and the charge remains substantially constant during the measuring interval, when switches 3, 13, which may be thyratrons, are open. The circuit formed of elements 8, 9, 10 now acts as a stabilized H.T. supply. A meter 17 indicates the light flux falling on the photo-multiplier during the measuring interval. The meter may be replaced by an integrating circuit for timing a photographic exposure, e.g. in X-ray photofluorography.