Patent GB-1206681-A (GB)
Title: Electronic reproduction of continuous image by half tone dots
IPC: H04N1/405
Filed: 1968-03-06
Abstract: 1,206,681. Copying by scanning. PRINTING DEVELOPMENTS Inc. 6 March, 1968 [22 March, 1967], No. 10781/68. Heading H4F. In a system for reproducing an image as a half-tone image formed by dots varying in size in accordance with the tone density of the image on a recording medium, means are provided for varying from nominal standard positions the locations of the centres of area of the dots in response to predetermined changes in condition associated with the scanning of the original image. By this means the ragged edge which occurs in a boundary between black and white is smoothed out to sharpen the reproduced image. General description. A motor 41, Fig. 2 drives an opaque drum 45, carrying a sheet of photosensitive material 61, and a transparent hollow drum 46 carrying a transparency 60 and a grating type pattern 50 of black bars alternating with white bars. A light projector 54, 55 inside the drum 46 co-operates with a scanner 56 containing a similar grating type mask to provide an output in the form of a triangular wave 190, Fig. 9, which is supplied to a half-tone dot generator 90. A light projector 70 co-operates with a scanner 62 which scans the transparency 60 and the film 61 is scanned by an image recording means 100, the devices 62, 100 being mounted on a slidable carriage 64 which between each scan is stepped leftward by the width of one scan track by a carriage drive 66. The spacing of the bars in strip 50 is the same as the width of each scan track whereby the raster pattern is graduated into square dot zones. The scanner 62 derives an area signal via a phototransistor which represents the integral of the tonal detail within the illuminated area which is supplied via lead 74 to the dot generator 90, and through a beam splitter co-operating with a pair of phototransistors, Fig. 6 (not shown), derives left and right half-image signals on leads 81, 81<SP>1</SP>, which represent the tonal detail within the portions left and right respectively of the centre line of the scan track of the illuminated area. Within the unit 90 the respective half-image signals are modified by the area signal to boost local contrast, combined with the half-image signal differentiated, inverted and differentiated again to accentuate the density edges in known manner, and combined with the cyclical signal on lead 57 to form left and right deflecting signals on leads 91, 91<SP>1</SP>, to control the deflection in opposite directions of a pair of ribbons and thus expose a corresponding amount of a dot forming an aperture in the light valve 100, Figs. 10 to 12 (not shown), of the recording means, to alter the spot size accordingly. These dots i.e. with uniform tones, normally are positioned to be symmetrically arranged about the centre-line of the scan track but under certain conditions, Figs. 18 to 22 (not shown), a tone density edge in the transparency 60 will cause the centre of area of the spot to be deflected to the right or left of the centre line so as to give a smoother appearance to the tone density edge on the film 61. These conditions are detected by a comparison of the left and right half-image signals and the appropriate off-centre potentials derived in the dot generator 90, Fig. 8 (not shown), and in these cases the ribbons are deflected by different amounts in the same direction. Fig. 9 shows the combination of the sawtooth signal 190 from scanner 56 with a half-image signal under various conditions. Assuming that the left halfimage signal is at white level 191 then a deflecting signal is provided only for the short interval when the level of signal 190 exceeds that of signal 191. If the half-image signal is at gray level 192 the output will be a triangular wave equivalent to the wave 190. With the half-image signal at black level 193 the output would be signal 190 superimposed on level 193 but a limiter in the circuit clips of the top of this waveform. The system may be designed to produce images differing in size from the orginals and the necessary modifications are discussed. Other scanning techniques such as a cathode-ray tube may be used.