Patent GB-879875-A (GB)
Title: Improvements in or relating to methods of and apparatus for colour-corrected projection for photographic colour printing
IPC: G03B27/73
Filed: 1958-02-24
Abstract: 879,875. Photo-electric control of photographic colour printing. GANSHIRT, L. Feb. 24, 1958 [Feb. 25, 1957], No. 5855/58. Class 40(3) [Also in Group XX] In the production of a photographic print from a coloured transparency, correction filters are added to the transparency so as firstly to equalize the densities at the three component colours, and secondly to introduce corrections in accordance with the printing material. The operation is carried out automatically under the control of a photo-electric cell which measures light passing through the transparency in succession at each of the three component colours and controls the adjustment of complementary colour filter wedges until equal light intensities are achieved. Thereafter the wedges are further adjusted to introduce density values preset in accordance with the character of the printing material. At each step the wedges are reset to bring the least dense to zero so as to eliminate any neutral correction component. At the completion of the correction, the photo-electric cell is removed from the light path through the transparency and turned to face the print material where it serves by reflected light to control the exposure time in a conventional manner. General. The principle of the invention is illustrated in Fig. 2 where light from a source 1 projects through a transparency 34 to print on material P. To carry out the correction a pushbutton is operated whereupon a photo-electric cell 35 is positioned in the light path in front of the projection lens 53 and simultaneously a filter drum 2 is set into rotation to cause a blue (3), a green (4), a red (5) and finally a clear (6) filter section to be inserted in succession in the light path. As each of filters 3, 4 and 5 enters the light path a contact carried by the drum completes a circuit and causes the appropriate one of three drums 15, 16 and 17 with a complementary colour filter wedge also to be moved into the light path and set in rotation. The output from photo-electric cell is connected to a galvanometer 36 and when a predetermined density is reached the needle 37 obscures the light path through an aperture 40 from a lamp 38 to a photo-electric cell 39. The resulting cell signal stops the rotation of the filter wedge and causes it to be tilted out of the light path in the rotational position to which it is adjusted. In this way each of filter wedges 15, 16 and 17 is separately adjusted and when the clear section 6 of drum 2 arrives they are all moved back into the light path in their adjusted positions and are available to correct the three colour components simultaneously. Firstly, however, they are rotated together in the direction of decreasing wedge density until the one of least density reaches zero density, so removing any neutral correction component, and they are then rotated into the opposite direction until they engage stops preset in accordance with desired corrections for the print material. Thereafter they are again rotated in the direction of decreasing density to remove any neutral component. The filters are now correctly set and available for printing. At this stage photo-electric cell 35 is moved automatically to a position out of the light path through the projection lens where it views the print and controls the exposure through a capacitor timer 54. Construction. Figs. 1 and 2. Filter drums 2, 15, 16, 17 are mounted concentrically for rotation about source 1, drums 15, 16, 17 also being mounted for tilting so that they may be moved individually into an out of the light path. Sections 4, 5 on drum 2 are adjusted for equal density with section 2 by the addition of neutral filters 7, and 8. Each of Sections 3, 4 and 5 is associated with an adjustable neutral density wedge 9, 10, 11 which may be set manually to modify the colour correction which is introduced in accordance with the nature of the subject. These wedges rotate with drum 2. Each of drums 15, 16 and 17 is associated with a stationary colour wedge 20, 25, 30 whose density varies in the opposite direction to the drum and which serves to compensate for unequal colour distribution across the light beam. Two of each group of filters 15, 16, 17 and 20, 25, 30 include neutral sections to equalize densities. The filter wedges are formed of uniform density filter foil cut in the shape of a comb with triangular teeth, Fig. 5 (not shown). Stops 45, 46 and 47 for introducing the corrections for the character of printing material are set by knobs 42, 43 and 44. The value of the density to which the apparatus operates may be adjusted by moving the position of aperture 40 in the galvanometer of knob 41.