Patent GB-891978-A (GB)
Title: Photoelectric unsharp masking apparatus
IPC: H04N1/58
Filed: 1960-09-22
Abstract: 891,978.. Copying by scanning. FAIRCHILD CAMERA & INSTRUMENT CORPORATION. Sept. 22, 1960 [Oct. 9, 1959], No. 32612/60. Class 40(3). The invention relates to "unsharp" masking apparatus for use in a photo-electric scanning image-reproducing system and has application to both monochrome and colour reproduction. In accordance with the invention, signals representative of the luminance value of light beams derived through "sharp" and "unsharp" apertures are derived photoelectrically and combined differentially to produce a control signal which is applied to modify the reproduction so as to sharpen the boundaries of a reproduced image, the modification being effected by applying the signal as gain-control feedback to the photo-electric devices which are of the photo-multiplier type. Fig. 1 shows a colour reproducing system in which the image from a scanner 10 is formed by means of a half silvered mirror 15 on a mask 13 containing a "sharp" aperture and a mask 14 containing an "unsharp" aperture. The light beam passing the "sharp" aperture is split into blue, green and red component beam by means of dichoric or like reflectors 17 and 18, the component beams being detected by photo-cells 20B, 20G, 20R having associated filters 19B, 19G, 19R. The photo-electric cell signals pass through amplifiers 23B, 23G, 23R to imagereproducing devices 25B, 25G, 25R. The light beam passing the "unsharp" aperture is detected by a photo-electric cell 21, a filter 16 having a spectral response corresponding to the eye being included in the light path so that photo-electric cell 21 generates a luminance type of signal. An adder circuit 26 combines the blue, green and red signals from amplifiers 23 in the proportions of 6%, 73% and 21% to produce a "sharp" luminance signal. The "unsharp" and "sharp" luminance signals are then combined differentially in subtractor circuit 28. A gamma control circuit 30 produces a signal corresponding to the instantaneous maximum value of the red, green and blue component colour signals. The output signal from subtractor circuit 28 is combined with the output signal from gamma control circuit 30 in adder circuit 32. The final signal is then added in series with the high voltage supply from circuit 22 for the photo-multiplier dynodes of photo-electric cells 20B, 20G, 20R, 21. The effect of adding the control signal in series with the supply from circuit 22 is to modify the gain of the photo-electric cells. The effect of the invention is to increase the luminance gradient of a reproduced image at a boundary between areas of substantially different luminance so that the reproducing image appears to have sharpened boundaries. A detailed circuit for carrying out the invention is illustrated in Fig. 2 (not shown). Gamma control circuit functions to compress the range of amplitude variation of the colour separation signals to a range appropriate to the particular image reproducing apparatus. Where the invention is applied to a monochrome image-reproducing system, only one "sharp" signal channel is provided. Adder circuit 26 is omitted and the gamma control circuit simplified to respond to the signal from the single monochrome channel. Specification 692,349, U.S.A. Specification 2,455,849 and German Specification 1,039,842 are referred to.