GB898678A - Photographic printing apparatus with device for the automatic regulation of the time of exposure - Google Patents

Abstract

898,678. Photographic printing apparatus. AGFA A.G. Oct. 1, 1958 [Oct. 23, 1957], No. 31357/58. Class 40(3). In a photographic printing apparatus in which the time of exposure is automatically regulated by a photo-electric arrangement, a further light sensitive circuit adjusts the intensity of the printing light to a predetermined value. With switch bar 25 raised, light from lamp 1, supplied via resistor 9, is transmitted via negative and adjustable diaphragm 6 to printing easel 5. The output voltage of a photo-cell 14 which is energized by light reflected from the easel, is applied to a circuit 13 in which it controls the gain of a valve to which a small A.C. input is applied. The output of the valve is compared in a thyratron control circuit with the output of a similar valve, whose gain is controlled in an opposite sense and to which a similar input is applied. The circuit is initially set up so that each valve supplies the same output when the intensity of the printing light has a predetermined value. The control circuit drives a motor 20 controlling the adjustable diaphragm which is adjusted until both valves provide the same amplitude of output signal. With switch bar 25 depressed, printing paper is inserted and a switch 32 operated. The full mains supply is connected to the printing lamp and the photo-cell is connected in an exposure time control circuit 12 of known form which terminates the exposure when the integrated value of the light measured by the cell reaches a predetermined value. Specification 873,826 is referred to.