GB997047A - Improvements in or relating to devices for measuring the intensity of illumination - Google Patents

Abstract

997, 047. Light intensity measurement. MULLARD Ltd. Jan. 10, 1964 [April 26, 1963], No. 16531/63. Heading G1A. [Also in Division G2] A light intensity measuring device which may be used as a photographic exposure meter comprises a circuit giving an indication only when the input exceeds a certain value. A photo-sensitive device is connected to the circuit input and adjustable means provided to vary the level of the input signal it applies thereto. The Provisional Figure shows a cadmium sulphide photo-resistor 1 connected in series with a resistor 2. The junction between them is coupled through an inductor 5 to the base electrode of a PNP transistor 6 whose collector electrode circuit includes a transformer 9 magnetically coupled with inductor 4. The secondary of the transformer feeds a neon indicator 11 although this may be replaced by a relay or meter. When the potential on the transistor base electrode is below a predetermined value the circuit oscillates causing the neon to glow. The circuit may be adjusted by varying the resistor 2 or adding further resistors in series or parallel with photo-resistor 1. It is set up to oscillate for all light intensities above a relatively low value and light intensities above such a value are measured by inserting an optical wedge or opaque shutter to reduce the light falling on the cell until the neon just ceases to glow. The position of the optical attenuator then indicates light intensity. In a modification, instead of using an optical attenuator, resistor 2 is made variable and adjusted from a predetermined value to neon extinction point to determine light intensity. A lens may be placed in front of the photo-sensitive element and the circuit can be adapted for photo-voltaic devices. The optical attenator or variable resistor control may be coupled to adjust the aperture and/or exposure controls of a camera.