GB472147A - Instrument for determining exposures for photographic purposes - Google Patents

Abstract

472,147. Photo-electric photometers. TRUECOLOUR FILM, Ltd., DOLGORUKI, E. SANDERS-, and WHALEN, F. C. Jan. 26, 1937, No. 2257. [Class 40 (iii)] [Also in Group XX] A photo-electric photometer comprises a photo-electric cell, means for estimating the intensity of the light falling thereon, an adjustable focusing lens, and a mask in the focal plane of the lens embodying a small aperture positioned in front of the cell whereby the optical field of the photometer is restricted to a narrow pencil to permit a reading to be obtained of the intensity of light reflected from a standard surface located at or adjacent the scene to be photographed. As shown, the apparatus comprises a lens 2, at the focal plane of which is a mask with a square aperture, in a mounting 3 slidable in a tube 7. A tube 8, connected to the mounting 3 by pins 9, is slidable on the tube 7, and has a rack 10 engaged by a pinion on the spindle 12 provided with an operating knob 13. The metal case 16 encloses a photo-electric cell P and an electrometer triode T. A mirror 18 carrying a ruled square 20, may be moved by a rod 28 to throw an image on to the screen 19 or to inoperative position. The circuit is shown in Fig. 4, P being the photo-electric cell, T the electrometer triode and V a triode valve. The anode of T is connected through the battery B to the control grid of V, which is also connected through a resistance r to the cathode. In the anode circuit of V is a relay R<1>, the movable contact a of which is connected to a battery B<1> and the fixed contact b of which is connected to the cathode of P and the control grid of T. When a negative charge is applied to P through the contacts a, b, this charge, which is also applied to the control grid of T, stops the anode current in T, and accordingly the grid of V is substantially at cathode potential, and the anode current holds relay R<1> open. As the charge leaks away through P, an increasing negative potential is applied to the grid of V and the anode current decreases to a point at which relay R<1> closes to apply a fresh negative charge to P. A second relay R2, across which is connected a rectifier W to minimize sparking, operates a counter. The contact b of the relay R<1> is supported by a quartz tube. A carrying case for the instrument may be provided with a universal mounting and also with a rotary carrier for colour filters.