Patent GB-820361-A (GB)
Title: Improvements relating to multi-colour printing processes
Inventor: CROSFIELD JOHN FOTHERGILL
IPC: B41F33/00
Filed: 1955-03-18
Abstract: 820,361. Photo-electric control of colourprinting processes. CROSFIELD Ltd., J. F. March 16, 1956 [March 18, 1955], No. 7995/55. Class 40 (3). [Also in Groups XXXV and XL (c)] In an apparatus for indicating or controlling the quality of colour reproduction in printing in ink of one or more colours, a photo-electric scanning head including a photo-electric cell is provided for scanning one or more patches of the ink printed on to a surface to produce signals, related to the reflectance or spectral values of the patch or patches, which are applied to a control circuit connected to an indicator for indicating the colour quality, or to auxiliary equipment which in response to departures from a predetermined range of quality of colour reproduction restores a desired quality of colour reproduction to within said predetermined range in subsequent printing operations, and the photo-electric cell is additionally exposed to two different standardizing light intensities, at least one of which varies in accordance with changes in one or more factors which effect the signal or signals but are unrelated to the reflectance or spectral values of the patch or patches, the control circuit being responsive to variations in the difference between the signals representing the two standardising light intensities caused by changes in said unrelated factors to remove corresponding variations from the signals derived from the patch or patches. In Fig. 1 patches y, r, be, bk of the separate colours are printed on to a paper web 1. Light from a lamp 4 is reflected from the paper towards a photoelectric cell 5 enclosed in a rotary shutter 6 having a slot 7. The shutter 6 is driven in synchronism with the printing machine so that slot 7, for a period of each revolution of the shutter 6, admits to cell 5 light reflected first from the white paper and then from the colour patches in sequence. The resulting signal, at the output A of a cathode follower thermionic stage built into the scanning head, appears as shown at A in Fig. 2 in which vd represents the dark level and vw represents the white level. The signal at A is fed to an amplifier A1, which includes an AGC loop (see Group XL (c)) which feeds back to the input of the amplifier A1 a signal proportional to the difference between the voltage levels of the black and white signals, by which means the output of the amplifier A1 is rendered independent of changes due to variation in brightness of the lamp, ageing of the photo-cell, dust on the glass of the scanning head aperture and other factors. The output of the amplifier A1, at the point B is fed to pulse gates G1, G2, G3, G4 each of which is opened to one of the separate colour pulses which are re-combined at F to form the signal shown at F in Fig. 2. The signal at the point B is also applied through a rectifier 18 to a condenser k1 so that during exposure of the photo-cell 5 the point D is at a potential of very nearly vw as indicated at D in Fig. 2. Between the exposure the condenser k 1 is charged to the level vd minus vw by a waveform C, Fig. 2, applied over a rectifier 18a. The point D is connected to the ends of four potentiometers Py, Pr, Pbe, Pbk which at the opposite ends are connected to a point E at a stabilized voltage vw. The potentiometer sliders, which are ganged to the sliders of four other potentiometers Py<1>, Pr<1>, Pbe<1>, Pbk<1>, are connected to gates G1A, G2A, G3A, G4A which are opened in sequence. The outputs of the gates G1A, G2A, G3A, G4A are connected to the point G at which a signal appears as shown at G in Fig. 2. The signals at F and G are fed into a difference amplifier DA which has an output signal I consisting only of colour pulses, as shown at I in Fig. 2. The signal at I is amplified by a second amplifier A2 to which feed-back is applied from potentiometers Py<1>, Pr<1>, Pbe<1>, Pbkl which are adjustable so that the pulses appearing at its output K are of equal amplitude, as shown at K in Fig. 2. The output of the amplifier A2 is clamped to earth to produce the waveform shown at L in Fig. 2, this waveform being applied to the gates G1B, G2B, G3B, G4B which are opened in sequence to apply each individual pulse of the waveform L to a separate one of the D.C. voltage measuring circuit PM1, PM2, PM3, PM4. An electrometer pentode is provided in each of the circuits PM1, PM2, PM3, PM4, the output of which is applied, in opposition to a stabilized potential produced in a unit BV to a centre zero voltmeter, IY, IR, IBE, IBK. In use the potentiometers Py<1>, Pr<1>, Pbe<1>, Pbk<1> are adjusted so that, at the required colour quality, each of the volt meters reads zero. Any deflection above or below zero then gives a quantitative indication of the departure of the quality of colour print. The outputs of the instrument are also connected to a four-track record RR. The gating circuits are controlled from a cold cathode selector tube ST. A Schmitt sequence trigger TR1 produces square waves of width t1 minus t7 which are differentiated at 27 and clipped by a rectifier 28 to leave a sharp negative pulse at the time t2 which is applied to the first cathode of the tube ST. A Schmitt switching trigger TR2 is fed with positive pulses at times t3, t4, t5, t6 from points W1, W2, W3, W4, and the output pulses are differentiated and clipped to leave sharp negative pulses which are applied to the switching terminals of the tube. The cathodes of the tube are connected each to a Schmitt trigger TR3, TR4, TR5, TR6, the first cathode by the way of a delay circuit DC. The triggers TR3, TR4, TR5, TR6 each have an output U, Fig. 3, delivering positive square pulses to the gates G1, G2, G3, G4, G1A, G2A, G3A, G4A and an output V, Fig. 3, delivering negative square pulses to the gates G1B, G2B, G3B, G4B. In the case wherein colour printing has to be carried out on metal foil, cellophane or other uniformly coated material which will not reflect light to give a satisfactory white signal a further aperture 7a, Fig. 10, is provided in the rotary shutter 6 to allow light to pass directly from the lamp 4 to the photo-cell 5 just before the colour patch y passes the main spot of light 1a on the web 1. An arrangement for automatically controlling the quality of ink is shown in Fig. 12 in which the volt meter Iy is in series with a relay 85 having an arm 87 which makes contact with one or other of two contacts 88 when the quality of it deviates one way or the other from the standard quality by a predetermined amount. The arm 87 and contacts 85 are included in a circuit containing a periodically closing cycling switch 91 and a pair of relays 93, 94 one or other of which is operated when the arm touches one of the contacts 88. When energized the relays 93, 94 open valves 101, 102 respectively allowing ink of double strength from a tank 103, or inkreducing medium from a tank 105, to flow into the ink duct 104 of the printing press. The quality of colour printing in letter press printing may be automatically controlled by causing the deviation of voltage in the indicators IY, IR, IBE, IBK to vary the speed of the inking roller. In lithograph printing the density of ink printed may be varied by a change in either the water or the ink reaching the printed surface, or by changing the pressure of air blown over the printing surface. In a sheet fed printing press the scanning head SH, Fig. 13, is traversed across the sheet and back to scan the patches y of one colour. The x-plates 114, Fig. 14, of a cathode-ray tube CR are connected to the slider 115 of a potentiometer 116 driven in step with the scanning head SH and the tips of the pulses thereby obtained are applied to y-plates 113 of the cathode-ray tube CR to produce signals on the tube CR corresponding to the pulse tips. Permanent lines on the cathode-ray tube indicate permissible high and low values of the pulses. The y-plates of a cathode tube may be fed with the output of the amplifier A1 so that the cathode-ray tube displays a waveform as shown at B, Fig. 2. In some cases, in continuous web feed printing, wherein it is not permissible to print colour patches in sequence to form a track extending in the direction of travel of the web, for example in magazines in which there is no margin to be subsequently cut away, the colour patches are placed in a line running across the web. Several scanning spots produced from one lamp from a composite lens system are then arranged along a line inclined at an angle, for example 45 degrees, to the direction of feed. A second composite lens system reflects the light dispersed from the several scanning spots to separate slightly circumferentially spaced points around a rotary shutter having a corresponding number of exposure slits and the photo-cell yields an output similar to that shown at A in Fig. 2. In order to provide a measure of the spectral values as well as the densities of the colour patches, the light illuminating the web may be passed through one of a number of filters each corresponding to one of the coloured inks, the filters being brought into the operating position in sequence and in alternation with periods when no filters are used. When a filter is in an operative position the output is switched from the density measuring meter to a colour measuring meter.