GB1097735A - Improvements in or relating to printing - Google Patents

Abstract

1,097,735. Copying by scanning. D. J. KYTE. Sept. 16, 1965 [Sept. 23, 1964], No. 38781/64. Heading H4F. To produce a screened photographic reproduction of a monochrome or coloured original 4 an image of the face of a cathode-ray tube 11 is focused on to a sheet of moving unexposed film 13, the light spot on the face of the tube being deflected in a periodic series of patterns such that a regular array of dots are exposed on the film, the size and/or the brightness of the light spot being altered according to a photo-electric signal produced by the scanning of the original. The original 4 and unexposed film 13 are on rotating drums 1 and 2 driven by shaft 3, and the original is scanned by a light source 5 and scanning system 7, the resultant electrical signals from the photo-cells in the scanning system being fed to a computer 8 which performs the functions of contrast alteration, tonal correction, colour masking &c. The output of the computer represents a function of the optical density and is in the form of a varying D.C. signal, this signal being fed to suppressor circuit 28. During scanning drums 1 and 2 rotate in synchronism whilst the scanning system 7, 5 and exposing system 11, 12 preferably move relative to the drums in a direction parallel to the longitudinal axis. To provide equal spacing of the exposed dots round the circumference of the rotating drum the pattern generator is synchronized with the rotation of the drum by means of a transparent timing disc 14 attached to shaft 3. This disc 14, Fig. 2, not shown, has a number of alternately opaque and transparent slits of equal width equal to the number of dots which would correspond to a complete circumference of the drum. By means of a light source 15 this concentric timing track is focused on to an aperture plate 18, Fig. 3, not shown, and a photo-cell 19 upon which the emergent light falls produces a series of electrical pulses corresponding to the number of lines around the timing disc per revolution. These pulses are fed to a sawtooth generator 21 forming part of the circuits for generating the required pattern on the cathode-ray tube. Generator 21 generates a constant amplitude sawtooth waveform of a frequency determined by the pulses from photo-cell 19. A further generator 22 produces a constant amplitude waveform at higher frequency than that produced by generator 21 and this high frequency output is amplitude-modulated to a depth of preferably 50% by the low frequency sawtooth from generator 21 in modulator 25. This amplitude-modulated signal is fed to suppressor circuit 28 which allows only those parts of the input waveform on conductor 27 which are greater in amplitude than the amplitude of the incoming D.C. voltage from computer 8 to pass to the output conductors 29 and 30. Figs. 5, 6 and 7, not shown, illustrate the waveforms on conductor 29 for high, medium and low valves of the input D.C. voltage. The waveform on conductor 30 is similar to that on conductor 29 but of opposite phase, and the two conductors are connected to one pair of deflecting plates, or a magnetic deflecting coil, on the cathode-ray tube 11. The moving film 13 gives the time axis and thus successive lines of exposed images will produce a range of exposed dots of varying sizes according to the D.C. output signal from the computer 8, in the form of dot patterns, Figs. 9, 10 and 11, not shown. Such screened reproductions are of a form suitable for letterpress and lithographic printing since the dot size, but not the dot density, varies with the optical density of the original. In a second embodiment, Fig. 12, the pulses from the photo-cell 19 are fed to a sawtooth generator 36 which generates an asymmetrical sawtooth waveform. As before, the high frequency sawtooth generator 22 generates a symmetrical sawtooth waveform which is fed to the X deflector plates of the cathode-ray tube 11. The asymmetrical sawtooth output of generator 36 is fed to a summing circuit 37 where it is added to the varying positive D.C. voltage from computer 8 and the output of circuit 37 is fed to one of the Y deflector plates of tube 11. On the face of tube 11 is mounted a thin opaque mask having a triangular cut-out as shown at A in Fig. 13, the X direction of the tube being horizontal in Fig. 13. The high frequency sawtooth output of generator 22 causes the light spot to continuously trace a line in the X direction, and the mask is so disposed that in the absence of any Y deflection, or with some predetermined fixed voltage applied to the Y plates, the line of light occupies some such position as that of the line marked R-R in B, Fig. 13. Moreover, the image of this line formed on the film 13, Fig. 1, is parallel with the axis of the rotating drum. With the output of sawtooth generator 36 applied to the Y plates, the output of the computer being assumed zero, then the line of light will move periodically between such extremes as lines S-S, S<SP>1</SP>-S<SP>1</SP> at B in Fig. 13. The sawtooth waveform produced by generator 36 is asymmetrical to compensate for the movement of the film on the rotating drum in order that the line of light moves faster in the downward direction since the film is moving in the opposite direction. If the computer produces a D.C. output and this is added to the sawtooth waveform it will cause the mean value of Y deflection to shift towards the apex of the triangular mask such as T-T and a pattern of dots is produced on the film as before depending on the magnitude of the computer D.C. output, Figs. 9, 10 and 11, not shown. In a third embodiment, Fig. 15, not shown, suitable for producing screened reproductions for the making of printing plates for photogravure printing in which the successive lines of exposed dots are required to be interlaced, Fig. 16, not shown, the phase of the low frequency sawtooth generator 36, Fig. 12, is altered by 180 degrees for each complete rotation of the drum. In this embodiment the D.C. output from the computer is fed to the cathode of the cathode-ray tube via a suitable non-linear amplifier (46) and also to the summing circuit (37) via switch (45). With switch (45) open the dots remain constant in size, but their optical density varies according to a function of the optical density of the scanned original. With switch (45) closed, the size and optical density of the exposed dots varies according to the computer output. In this case it is arranged that the exposed dots do not overlap.