Patent GB-619512-A (GB)
Title: Improvements relating to automatic exposure control for photographic printing
IPC: G03B27/80
Filed: 1946-03-21
Abstract: 619,512. Automatic control systems. KODAK, Ltd. March 21, 1946, No. 8806. Convention date, March 21, 1945. [Class 38 (iv)] [Also in Group XL (b)] In apparatus for continuous projection printing of photographic negatives, e.g. for airgraph letters, a " sample " of the light proceeding from the negative is directed on to a photo-electric cell controlling a reversible motor through thermionic valve circuits, this motor in turn maintaining the intensity of illumination of the printing paper substantially constant by variation of the size of the aperture through which the light from the negative passes or of the position of an optical wedge interrupting this light. Referring to Fig. 1, the negative passes from the supply spool 18 to the take-up spool 22 passing over an exposure gate through which light from the exposure head 12 passes. As shown in Fig. 7, the exposure gate 26 is U-shaped, the limbs of the " U " facing in the direction from which the negative proceeds. The printing paper P passes in the reverse direction being held tightly to the periphery of the drum 30 by rollers 28, 32, 33 and 31. Printing takes place immediately below the exposure slit 34. Mounted to one end of the exposure slit 34 are a pair of mirrors 47 upon which the light passing through the part of the negative immediately above the arms of the U-shaped exposure gate falls, this part of the negative not yet having arrived at the printing position which is the long bottom of the U. The light rays affecting the photo-cell pass through the edge only of the strip negative so that the exposure intensity is controlled in accordance with background density of the film. It is also stated that if some of these rays pass through the written part of the negative clearer printing of the written part is attained. This light is reflected to a photo-cell 45 forming part of a thermionic circuit and should its intensity be outside certain predetermined limits a servo-motor M runs to close or open an adjustable diaphragm mounted with the projection lens 17 by rotating through step-down gearing a pin 51 fixed to the adjusting ring of the diaphragm. Alternatively the motor may rotate an optical wedge until a darker or lighter part of the wedge is interposed in the printing beam. This operation is so timed that the light is adjusted by the time the trial " sample " part of the negative arrives in the exposure gate proper. Circuit details. Fig. 9. The photo-electric cell 45 forms one arm of a bridge circuit of which the other arms are one or more of the tapped resistances R1, R2, R3, R4, which may be used to adjust the light intensity limits for four different grades of paper and resistances R14 and R15 which form part of a potentiometer supplied from a full wave rectifier through smoothing circuits and incorporating voltage stabilizers V5 and V6. Variation of the cell resistance due to variation in the intensity of the light falling in the printing paper, increases the potential of one of the control grids of a double triode amplifier V1 and reduces the potential of the other. Due to anode loads R7 and R8, the anode potentials alter in opposite sense to these of the grids and this variation is passed direct to the grids of a second double triode amplifier V2. The voltage across the anode loads R9 and R10 of this valve are then applied to the grids of a pair of thyratrons V3 and V4 and should the change of resistance of the photo cell have been sufficient one of the thyratrons will fire causing current to flow through one of the fields of a split field motor M to alter the intensity of illumination of the cell and rebalance the bridge. A resistance R13 connected across the motor fields prevents hunting and condensers C1 and C2 are provided to introduce a delay in the system in the event of very dark margin lines passing through the exposure gate. In a modified circuit only one stage of amplification is used and the potentiometer supplying the thyratron bias of voltage is not, as in Fig. 9, in series with those supplying the bias of the amplifying stages. Independent adjustment of these biasses and alteration of the range of the circuit are therefore possible.