Patent GB-1159633-A (GB)
Title: Improvements relating to Image Reproduction Systems
IPC: H04N7/00
Filed: 1968-02-02
Abstract: 1,159,633. Television. INTERNATIONAL POLAROID CORP. 2 Feb., 1968 [8 Feb., 1967; 22 Jan., 1968], No. 5474/68. Heading H4F. The invention relates to a method of reproducing an image of a subject utilizing the fact that visual perceptions are not determined principally by the absolute brightnesses of different areas of a field of view, but rather by the interrelationship of all the ratios between. brightness levels on opposite sides of significant visual boundaries or edges and, on the basis of all these edge ratios, assigns to the various areas of the subject a rank order on a lightness scale which does not in general correspond to the rank order of the various areas in terms of brightness. It has been observed that the visual complex ignores gradual brightness gradients across a surface and only responds to the aggregate edge or boundary information, i.e. any bounded region of an image tends to appear uniform in lightness (constant luminous reflectance) regardless of the luminous energy variations within that region. In a television system (see later) utilizing this principle the resulting image does not necessarily contain all the brightness gradation of the original subject but is a representation of the original subject in terms of comparative lightness. In such a system the bandwidth requirements are reduced. In a first embodiment, Figs. 1 and 2, an image orthicon 14 is scanned, under the control of sweep circuits 18, preferably in a zigzag fashion from left to right then from right to left and similarly from top to bottom and from bottom to top, or spirally from the centre outwardly and then from the outside inwardly, so that the scanning beam which has a constant velocity never leaves the image area. Circuits 18 are controlled by sync. circuits 22 which may also combine synchronizing impulses with the output video signal from the orthicon. The video signal which is a continuous record of the brightness of the subject is amplified, 24, and fed to a logarithmic amplifier 26, the output of which is represented by curve A, Fig. 2, the nearly vertical portions of which corresponding to sudden transitions in brightness levels produced when the scanning beam crosses a significant boundary line in the original subject. This output is differentiated, 28, curve B, and passed through a bottom-clipper circuit 30 to produce curve C (edge ratio signal), the amplitude of each pulse therein representing an edge ratio between the brightness of an incremental area on one side of a visual boundary line and the brightness of another closely spaced area immediately on the other side of the boundary line, i.e. the spaced positive and negative pulses do not represent the absolute brightnesses of the different image areas, but instead convey information concerning the relative lightnesses of image components on opposite sides of significant boundary lines. The received (demodulated) edge ratio signal C from receiver circuitry 38 is integrated, 40, to produce a signal D, Fig. 2, defined by a series of switched amplitude signals which represent on a logarithmic scale a sequential multiplication of edge ratios to be translated into intensities in the image to be reproduced (the integrator adds the logarithmic edge ratio signals), the gradual brightness variations or gradients between visual edges in the original subject being ignored. The output signal from integrator 40 is passed through antilog amplifier 42 and via video amplifier 44 to kinescope 46. In a second embodiment, Fig. 4 (not shown), the signal which characterizes edge ratios and which by the sequention multiplication of edge ratios is used to reconstruct the lightnesses of different image areas is derived by feeding the output signals from the logarithmic amplifier over branching circuit lines to a delay line (52) and to an inverter (54), the output signals from which being summed algebraically and passed into the bottom clipper circuit. The delay line and inverter may both be in one side of the branched circuit. Because of the logarithmic character of the signals, the algebraic sum of the delayed and undelayed signals represents a true ratio of the brightnesses between the two closely spaced points. The visually significant boundary lines are characterized by a significant change in brightness between two spaced apart points and the system may be made more sensitive by adjusting the amplitude above which the bottom clipper operates or by replacing the bottom clipper by a high band-pass filter between the logarithmic amplifier and the branched circuit which discriminates against low-frequency signals representing gradual brightness changes. In a third embodiment, Fig. 5, an optical imaging and scanning arrangement 60 is used including scanning mirrors 64 and 65 and two photosensors 66 and 67, spaced apart a distance dx, which generate signals having logarithmic characteristics. One of the detector's outputs is inverted, 72, and then the outputs are summed and fed to the bottom clipper circuit 30. To reproduce coloured images, Fig. 6 (not shown), three image orthicons each associated with a colour filter (red, green or blue, e.g. 20, Fig. 1) may be used, the red, green and blue video signals being fed via edge ratio circuits and sequential multiplication circuits to the three guns in a shadow-mask tube. With this arrangement the colour quality of the image produced is almost independent of the colour balance of the illumination of the original scene.