Patent GB-1029161-A (GB)
Title: Photographic colour printing apparatus
IPC: G03B27/73
Filed: 1964-01-17
Abstract: 1,029,161. Photographic colour printing control. EASTMAN KODAK CO. Jan 17, 1964 [Jan 18, 1963], No. 2119/64. Heading G2A. [Also in Division G1] The light illuminating the transparency to be copied is regarded as being divisible into three spectral regions, and two separately movable rows of filters are located between the transparency and the light source, movement of one row modifying the ratio of the light intensities from the first and second spectral regions, and movement of the other row modifying the corresponding ratio from the first and third regions; an electro-optical device provides a signal having two components proportional respectively to the difference between the intensities of light from the first and second regions transmitted by the transparency and to the corresponding difference between the light intensities from the first and third regions, the said signals controlling the two rows of filters respectively, to move them across the light beam until each signal reaches a predetermined value. The right-hand side of Fig. 1 shows the optical arrangement. Light from a source 15 passes first through a heat-absorbing filter 17, then in succession through two rows 21, 22 of filters, and then into a light-scrambling device such as a mirror box 23. An exit aperture 38 at the other end of the box allows the light to pass through the negative or transparency 13 which is to be printed, then through an objective lens 25,to be reflected by a mirror 29 on to a sheet of light-sensitive material 11. A shutter 27 operates to make the exposure. The filter rows 21,22 each have the tripartite arrangement shown in Fig. 2. Filter 21 consists of a segment 32 passing cyan and absorbing or reflecting red, and a segment 34 passing yellow and absorbing or reflecting blue. Between them is a corrector segment 33 which transmits a higher percentage of all colours than either of the other two segments, but the ratio of its transmittance of blue and green is the same as the corresponding ratio for cyan filter 32, and the ratio of its transmittance of green and red is the same as the corresponding ratio for yellow filter 34. As filter row 21 moves so that the opening 39 of box 23 is blocked either by the whole of cyan filter 32 or partly by filter 32 and partly by filter 33,the ratio of blue and green light entering the box will not alter, but only the transmittance of red light, according to how much of the opening is covered by the cyan filter. Similarly, the ratio of the transmittance of green and red light by the corrector segment 33 is the same as for the yellow filter 34, so that when opening 39 is blocked by either the whole of the yellow filter or partly by the yellow filter and partly by filter 33, only the transmittance of blue light is affected by the amount of opening 39 covered by the yellow filter. Filter row 22 has a similar arrangement to row 21, but consists of a magenta filter 35 and a cyan filter 37, separated by a corrector filter segment 36. The arrangement and manner of operation of filter row 22 is similar to that of row 21, so that the amount of red light only which enters box 23 is controlled by the position of cyan filter 37 across entrance 39, and the amount of green light only is controlled by the position of magenta filter 35 across the entrance. Thus movement of filter row 21 controls the proportion of red to blue light passing without affecting the proportion of green, while movement of filter row 22 controls the proportion of red to green light passing without affecting the proportion of blue. The ratios of red to green and red to blue may therefore be adjusted simultaneously by adjustment of both filter rows. The central corrector filters of each row may in some cases be clear glass, or omitted altogether. A light probe 41 is directed at a particular spot on the transparency, and focuses the light on to a photomultiplier cell 43, the light being modulated by passage through a spinning filter disc 45 having a 180 degree sector 48 which passes only red light, and two 90 degree sectors 49, 50 passing respectively only green and blue light. The output of photocell 43 has two A.C. components 90 degrees out of phase with each other, one proportional to the intensity difference between red and blue light and the other to the intensity difference between red and green light. Part of this output is fed through an amplifier 57 to the control windings of two two-phase servo-motors 63,65, powered from the same supply which drives the filter disc 45. The phase of the signal of motor 63 is altered by 90 degrees by means of a capacitor 67 and potentiometer 69. Thus the two motors react to the respective A.C. components which are 90 degrees out of phase with each other. The motors are mechanically coupled to the filter rows 21,22 respectively, and can move either of these by a rack and pinion arrangement. The desired colour balance is monitored by biasing the output signals of photocell 43 from manually operated units 79, 80, and thereafter the servomotors maintain the desired colour balance by applying automatic correcting adjustments to the positions of the two filter rows. After the proper colour balance has been achieved, and the servomotors come to rest, a slow-acting relay S operates, and switches the D.C. component of the output of cell 43 through a contact 89 to an exposure timer 93 which operates the shutter 27 to make the exposure.