Patent GB-1206424-A (GB)
Title: Photographic colour printer
Inventor: NEALE DENIS MANKTELOW
IPC: G03B27/73
Filed: 1967-07-20
Abstract: 1,206,424. Photographic colour printing. ILFORD Ltd. July 12, 1968 [July 20, 1967], No.33482/67. Heading G2A. A method of printing positives from multi-colour negatives comprises continuing printing for each colour component of printing light, red, green and blue, until a predetermined integral of light against time for each colour has been administered to the print material as measured by an exposure control means, the integral being constant for negatives representing subjects integrating to grey, but being reduced for at least two colours in response to means for detecting an abnormally low ratio of integrated transmittance to the negative of blue light relative to light of a longer wavelength. Photocells 8, 9, 10 Fig. 1 sensitive respectively to red, green and blue light are connected to a photoelectric integrator arranged to operate a shutter to terminate exposure of print material 7 to the corresponding colour component of printing light, additional photo-cells 11, 12 being responsive respectively to blue light and a light of longer wavelength, e.g. yellow. A double optical wedge 13 is movable by a motor 19 into the path of printing light and out of the path of reflected light incident on photo-cell 11, or vice versa. Photo-cells 11, 12 are connected in to form two arms of a bridge circuit, Fig.2, (not shown), two resistors (17), (18) one being adjustable, forming the other arms. The junction of the photo-cells is connected to one input of a differential amplifier (16), the other input being connected to the junction between the resistors. The output of the ampliflier is connected to the motor 19. An alternative embodiment of the invention is shown in Fig.4. Photo-multiplier tubes 24, 25, 26 sensitive respectively to blue, green and red light provide currents proportional to the respective light intensities to charge capacitors 36, 37, 38 until valves 39, 40, 41 conduct whereupon the exposure of the print material to the corresponding colour is terminated by a shutter. Upon commencement of an exposure cycle switches 33, 34, 35, 44, 45, 46 open. The relative intensities of the blue, green and red printing light are arranged so that equal exposure times of these components produce satisfactory prints of negatives representing subjects integrating to grey. The retardation of the rise in voltage of capacitors 37, 38 due to i 30  remains at a constant value when a negative being printed represents a subject integrating to grey or yellow. However, when the negative represents subjects integrating to blue, i 30  will be small and will result in the green and red printing exposures terminating at abnormally small exposure integrals, and the blue exposure proceeding to the normal value and thus giving a print that will appear too yellow. This fault is eliminated by not permitting exposure to blue light once both green and red exposures have terminated, e.g. by mechanical or electrical inter-connection of the exposure controlling shutters. In a third embodiment, Fig.5 (not shown) a printer of known type has filter elements (52), yellow, (53), very pale magenta, and (54), blue, are mounted side by side upon a wire which is movable by a motor to selectively position a part of the filter system in the printing light beam. A variable alternator (62) is coupled to the wire and positioned in front of a photo-cell (61) receiving scattered light from the negative (58) to control the exposure time, the variable attenuator being so arranged that upon displacement of the filters to adjust the printing light colour the attenuator moves in front of the photo-cell and allows a greater amount of light to strike the photo-cell when a negative representing a blue subject is being printed thus reducing the exposure time compared with a negative representing a subject integrating to grey or yellow.