GB529907A - Improvements in photometric apparatus for timing photographic exposures - Google Patents

Abstract

529,907. Photo-electric photographic exposure timing devices. BING, J. M. May 26, 1939, No. 15756. Convention date, March 6. [Class 40 (iii)] [Also in Group XX] An automatic exposure control system for photographic apparatus comprises a photoelectric cell 12 subjected to the scene brightness and connected to a bridge circuit 23, 24, 25 supplied with audible frequency current and provided with an audible indicator of balance 29 as described in Specification 506,219, a bridge adjusting member 30 the adjustment of which is a measure of scene brightness, and a resistance 31 adjustable by the same member 30 to determine the exposure time automatically. As shown, the control system is applied to an enlarger 10. In operation, switches 44, 43 are first closed to light the lamp in the enlarger and to energize the bridge circuit from a transformer 27. When the bridge has been balanced by the adjustment of member 30, the switches 44, 43 are opened, a piece of enlarging paper is placed on the easel 11, and then a switch knob 39 is pressed down and is held down by a catch 41. The lamp circuit is at once closed by contacts 37, and at the same time the circuit of resistance 31 is closed by contacts 34. After a given time, a thermal device 32, 33 deflects the catch 41 and thereby releases the switch knob 39 which is raised by a spring 45 and opens the contacts 37, 34, completing the exposure. The Specification as open to inspection under Sect. 91 describes also a reflex camera containing adjustable bridge resistances associated with its objective ; a sound generator or buzzer, a small battery for operating the buzzer, a telephone receiver, and further bridge resistance in the bottom of the casing ; and a photo-electric cell on the ground glass covered by the collapsable hood. This subject-matter does not appear in the Specification as accepted.