Patent GB-856423-A (GB)
Title: Improvements in or relating to photographic colour reproduction apparatus
Inventor: KILMINSTER ARTHUR ROY
IPC: G03B27/73
Filed: 1957-09-24
Abstract: 856,423. Copying by scanning; photo-electric printing control. HUNTER-PENROSE Ltd. Sept. 17, 1958 [Sept. 24, 1957], No. 30013/57. Class 40(3). [Also in Group XL(c)] Photographic colour reproduction apparatus comprises a scanning cathode-ray tube 3, Fig. 1, operable to direct a spot of substantially white light on to a coloured original 1, time-base circuits 4 connected to deflector means for the cathode-ray tube and operable to form a scanning raster on the face thereof, and control means connected to the time-base circuits and arranged to be activated by the primary colour components of light transmitted from the original to control modulation of the speed of scan to control the amount of light transmitted from the original. The control means comprise photo-electric cells 8, 9 and 10 associated respectively with blue, green and red filters 11, 12 and 13, and an electronic control circuit 14 which responds to the signals from the cells and in accordance with both the colour and tone of the original produces a control signal for application to timebase circuits 4 to effect a desired correction of the reproduction. The control circuit (see below) comprises a first combination of a cathode-ray tube and photo-electric cell in conjunction with a variable density screen to introduce colour correction and either a second such combination or an electronic circuit to introduce tone correction. The time base circuits (see below) are controlled by varying the scanning oscillator discharge rates. A camera 2 is exposed directly to the light transmitted from the original, reference being made to the production of coloured prints and colour separation negatives for use in the production of the yellow, magenta, cyan and black plates for a four-colour printing process. The apparatus shown in Fig. 1 is intended for use with a transparency which is not light scattering. This permits the light path 19 to the photo-electric cells to be diverged from that, 18, to the camera. The cell signals have a slight time lead over the camera exposure which compensates for delay in the control circuit. A modified apparatus is described, Fig. 13 (not shown), for use with an original which tends to be light scattering, the scanning light from the cathode-ray tube being imaged on to a diffusing screen placed in contact with the original, and light for the photo-electric cells 8, 9 and 10 being extracted from the light path to the camera by a semi-silvered mirror. Reference is made (without giving details) to the use of the invention with an opaque original. The control signal for the time-bases may be applied to a monitoring cathode-ray tube, not shown, so that the amount of correction can be observed by an operator. The time-base circuits comprise conventional blocking oscillators and associated amplifiers for producing line and frame direction scans. In one circuit, Fig. 12 (not shown), the control signal is applied to vary the potential on the discharge resistor of both oscillators. In another circuit, Fig. 16 (not shown, see Group XL(c)) only the line scan oscillator is controlled and the frame scan oscillator is controlled in steps from the line scan oscillator. The control signal is either applied to the discharge resistor or is employed to modulate the intensity of a light source which is exposed to a photo-electric cell connected in the oscillator discharge path. Electronic colour and tone control circuit. In a first embodiment, Fig. 14, the blue, green and red component signals from photo-electric cells 8, 9 and 10 are amplified in stages 26, 27 and 28 and produce outputs through cathode-followers 29, 30 and 31 on lines 32, 33 and 34 which are connected to logarithm deriving circuits comprised of resistor-rectifier combinations 104-105, 110-111 and 114-115 and associated amplifiers 109, 112 and 116. The red component signal is then applied in common to deflector plates 52 and 54 of cathode-ray tube 36 whilst the blue and green component signals are applied respectively to plates 51 and 53. The spot on the tube face assumes a position, Fig. 6 (not shown), determined by the colour of the original at each elemental area as it is scanned, the position being independent of the brightness of the light from the scanning cathode-ray tube as a result of the differential connections to plates 51-54 and the use of logarithm deriving circuits. The spot is exposed to a photo-electric cell 56 via a screen 55, the distribution of optical density of which (see Fig. 7, not shown), varies in accordance with the colour correction to be produced and the type of printing process used. A separate screen is utilized for production of each negative in a four-colour process. The signal from photo-electric cell 56 is applied via push-pull amplifier 119 to plates 71, 72 of a second cathode-ray tube 66. A portion of the signal is also applied via potentiometer 124 as negative feedback to the cathode of cathoderay tube 36. Plates 73 and 74 of cathode-ray tube 66 are supplied from a push-pull amplifier 125 and, dependent upon the colour negative being produced, receive either the "blue", "green" or "red" signal from line 32, 33 or 34, or in the case of the black printer, a signal formed by combining the three signals in a network 37. The signals are selected by switch 35 and applied to the input of amplifier 125 via a logarithm deriving circuit formed of resistor 129 and rectifier 130. The spot on the face of cathode-ray tube 66 assumes a position,'Fig. 9 (not shown), which varies in one co-ordinate with the tone of the original for the colour selected and in the other co-ordinate dependent upon the amount of colour correction to be applied. The spot is exposed to a photo-electric cell 70 via a screen 69 having a density distribution arranged in accordance with the correction to be applied, Figs. 10 and 11 (not shown). A different screen is used for the production of each negative. The signal from photo-electric cell 70 comprises the control signal for the time-base circuit and is applied to produce an output on lead 120 via amplifier 134. An output from the amplifier is also applied via potentiometer 131 as negative feedback to the cathode of cathode-ray tube 66. A meter 43 is employed in conjunction with screen potentiometers 100, 101 and 102 for amplifiers 26-28 to balance the apparatus under "no-signal" conditions. A second embodiment, Figs. 2 and 5 (not shown), is generally similar but dispenses with the logarithm deriving circuits. A third embodiment, Fig. 15 (not shown), dispenses with the second cathode-ray tube 66 and photo-electric cell 70 and employs a mixer circuit for combining the output of photo-electric cell 56 with a combined signal derived from network 37. A fourth embodiment, Fig. 17 (not shown), also dispenses with the second cathode-ray tube and photo-electric cell and derives the tone correction by applying the combined signal from network 37 to modulate the anode potential of the first photoelectric cell 56. In this embodiment, the control signal for the time-base circuit is derived directly from photo-electric tube 56.