Patent GB-886903-A (GB)
Title: Photographic printing apparatus
IPC: G03B27/80
Filed: 1958-01-31
Abstract: 886,903. Photographic printing apparatus. AGFA A.G. Jan. 31, 1958 [Feb. 7, 1957], No. 3332/58. Classes 98(1) and 98(2). A photographic printing apparatus including a photo-electric exposure regulator having a control means requiring adjustment in accordance with characteristics of individual transparencies to be printed, is provided with a device available for operation in connection with replacement of a transparency for printing, the said device and control means being so coupled that on operation of the device the control means is returned to a zero position. The device may be either a switch contact for a transparency inspection lamp or a member which must be operated to enable a transparency to be replaced. As shown. in Fig. 1 a printing frame 1 placed on the base board of an enlarger carries a hinged cover 21 and houses the photo-electric cell of an exposure control system activated by light passing through the focusing screen 2 of the frame. A knob 4 alters a resistor in the exposure control circuit to take account of printing paper sensitivity while a knob 5 is adjustable against spring action in opposite directions from a central zero position to increase or decrease the photo-electrically set exposure time to allow for negatives of abnormal density distributions, this knob being retained in the manually selected position by a spring-urged pawl engaging a toothed disc on the shaft of the knob. The parts are in the positions occupied at the end of an exposure. Further prints from the same negative are produced merely by inserting a fresh sheet of photographic paper in the printing frame 1 and closing the cover 21 by means of the arm 25 which is latched by a detent 33 on the pivoted lever 32, switch 43 thereby being operated to cause the photo-electrically controlled mechanism to control the exposure, at the end of which a solenoid 35 is momentarily energized to cause a pin 41 (Fig. 3 not shown) to trip the latch lever 32 and so cause the frame cover .21 to spring open. When a fresh negative has been inserted in the projection position the inspection light is switched on by closing switch 44 so that a dropping resistor is placed in series with the projection printing lamp 45 and solenoid 50 is energized operating transfer contacts 50a, 50b. A solenoid 14 is thereby momentarily energized tripping the pawl for the disc on the shaft of the knob 5 which returns to its zero position. The knob 5 is then re-set if necessary to an appropriate position, whereupon the switch 44 is opened again, releasing solenoid 50 so that solenoid 35 is energized to allow the cover 21 to re-open. Prints are then made from the new negative, switches 48, 55, 59 controlling printing light, discharging of condenser 54 in series with solenoid 35, and dark room lamp 58 being controlled from the exposure control circuit. In the modification of Fig. 5 a contact printing box includes a cover 64 which houses the photo-cell of the exposure regulating device and the negative is held on a transparent plate 62 by a member 65 movable through a lever system by a footoperated lever 73. If on changing the negative the setting of the knob 5 is not to be altered the lever 73 is moved only so that a notch 78 in it is engaged by a spring 79 but if re-setting is required lever 73 is moved so that arm 77 of a bell-crank lever connected to it depresses contact 76 to energize the solenoid 14. A normally-closed manual switch may be placed in series with the contact 76 so that the return device for the knob 5 may cut-out. The printing paper is held over the negative by the pivoted member 75. Instead of the use of the inspection light switch 44 or the negative holding member 65, the return of the knob 5 may be effected by a part of the negative support moved on insertion of the negative, and the knob 5 and its return device as well as parts of the exposure control mechanism may be housed separately from the printing apparatus.