Patent GB-660099-A (GB)
Title: Improvements in or relating to a method and apparatus for making photographic colour prints
IPC: G03B27/73
Filed: 1947-05-09
Abstract: 660,099. Photo-electric exposure control. KODAK, Ltd. May 9, 1947 [May 10, 1946; June 20, 1946], Nos. 12637/47 and 12638/47. Class 40 (iii). [Also in Group XX] A method of making three-colour prints on light-sensitive multi-colour photographic material from multi-colour negative or positive transparencies whose overall colour integrates to a tinted grey comprises the step of adjusting the colour of the printing light for each print so that when all of the printing light transmitted by the whole of the negative is integrated and employed to expose such material, the exposed material after processing is grey. As shown, a projection printer having a white light source 10 regulable by a variable resistor 17 is provided with a carrier 11 for a colour negative 14, a three-colour filter disc 15, projection lens 12, shutter 18, a photo-emissive cell 16, and means for holding a sheet 13 of light-sensitive multi-colour photographic material in the image plane of the lens 12. The light from the negative passing through one of the outer set of red, green and blue filters R1, Gl, B1 is measured by the photo-cell, and the resistor 17 is adjusted until the cell current reaches a standard value. The shutter is then opened for a standard time to print on to the sheet 13 with light which passes through the corresponding member of the red, green and blue filters R, G, B, corresponding members of the two sets of filters being so related that the product of the relative sensitivity of the cell to each wavelength transmitted by the cell filter and of the transmission of the cell filter at each wavelength is equal to the product of the sensitivity of the printing paper at each same wavelength and of the transmission of the filter through which the print is made. The filter disc 15 may be replaced by a disc in which the related filters are placed side by side in pairs R, R1; G, G1; B,B1, circumferentially of the disc, the cell 16 may be replaced by red, green, and blue-filter covered pairs of barrier layer cells arranged so that members of each pair are disposed at diametrically opposite points around the objective pupil, and the light intensity adjusted by any well known means, e.g. diaphragm, grey wedge, crossed polarizers. Fig. 8 shows one embodiment of an automatically operating printer. On closing the switch S2 the motor MS rotates the cam-shaft 43 which, by means of cams 80, 61, 42, 84, closes respectively hold-on contacts 81 for the motor MS; that contact of the double-throw switch 60 in the circuit through the coil CCW; switch SR connecting barrier layer cell CR covered with a filter related to the printing filter R as the filter R1 above, to a galvanometer 65; and switch 83 completing the circuit to the lamp 10 through the resistor 17 which is normally set so that the light emitted by the lamp is insufficient to print even the least dense negative. The closure of the circuit through the winding CCW of the shaded-pole motor MP which rotates the shaft of the variable resistor 17 causes the motor to rotate it counter-clockwise and so reduce the resistance in the lamp circuit until the current through the cell CR causes closing of the switch 64, which in turn causes opening of a switch 59 to stop the motor. The cam 76 then closes contacts 77 to energize the solenoid 53 which opens the pivoted blade shutter 50 so that the red component image of the negative 14 is projected by the lens 12 on to the printing paper (not shown). At the end of this exposure, cam 78 closes switch 79 momentarily, so energizing solenoid 29 to free the disc 1511 of the latch 25 to allow the motor MD to drive through onerevolution clutch 28 the disc clockwise to bring the next filter sector G into the printing position. The cycle is completed by the next hump on cam 76 opening contacts 77 so that the shutter 52 closes again, and the follower of the cam 61 dropping into a hollow on the cam opening the circuit of the motor winding CCW and closing that of the winding CW so that the motor MP returns the movable arm of the variable resistor 17 to its normal high resistance setting. The continued rotation of the shaft 43 repeats the sequence just described to print the green and blue images, cam 42 closing switches SG, SB to bring the cells CG, CB into operation, and the filter disc 1511 being rotated to bring the filters B, R into the printing beam at the end of each cycle, and finally cams 80, 84 break the circuits to the cam-shaft motor MS and the printing lamp 10 to return the apparatus to rest. In practice the galvanometer-controlled switch 64 typically comprises a light-beam reflected from the mirror of the galvanometer 65 which is deflected across a slit covering a photo-emissive cell and the cell current generated is employed to energize a relay which opens the switch 59. In another embodiment the filter disc 1511 is replaced by a disc 15, as shown in Fig. 2, and only a single photo-electric cell is used, the filter R1, G1, or B1 in front of which is changed simultaneously with the printing filter R, G, or B, and the cam 42 and switches SR, SG, SB are dispensed with. Instead of the galvanometer control of the motor for the printing lamp variable resistor described above, the photo-cell current is amplified by a multi-stage D.C. amplifier the output of which energizes the relay 63 opening the switch 59. In a further embodiment, Fig. 6 (not shown), the printing filter and shutter are incorporated in a single disc, opaque sectors separating the colour filters, and this disc is rotated continuously for one revolution by a motor through reduction gear and a one-revolution clutch, e.g. that described in Specification 405,033, the adjustment of the lamp intensity being carried out while one of the shutter sectors is in the printing beam. In this construction, cams 76, 78 controlling the shutter and intermittent rotation of the printing filter disc are dispensed with, the filter disc being latched by a solenoid in series with the motor when the single notch in the disc has returned to its original position after a complete revolution, while the cam 60 is replaced by a single recess cam closing a switch for a complete revolution. This allows the shaded-pole motor MP to be replaced by a reversible two-phase motor, one winding of which is fed with current in quadrature to the supply voltage while the other is fed with current either in phase or in antiphase with the supply from the secondary of a transformer, opposite halves of whose primary are alternately fed, through a double throw switch, operated by a mains fed vibrator, with the difference in potential between the cell output and that of a comparison source. The initial setting of the comparison source, which determines the maximum intensity of each of the coloured printing beams, is made in accordance with the appearance of trial prints of either a neutral grey or a perfect colour negative. If desired, the simultaneous integration of the three colours and the printing of each colour either successively or simultaneously may be effected by the use of a separate light source and photo-cell for each colour. Instead of employing a constant printing exposure time with a variable printing light intensity, a variable length of exposure at a constant intensity may be used though such an arrangement 'is not suitable for automatic operation.