GB822444A - Improvements relating to printing processes - Google Patents

Abstract

822,444. Photo-electrically indicating the quality of colour printing. CROSFIELD Ltd., J. F. Aug. 30, 1957 [Sept. 5, 1956], No. 27209/56. Class 40 (3). Apparatus for indicating changes in the quality of colour reproduction in printing with ink of one or more colours comprises a photoelectric scanning head, translating means for traversing the scanning head across a moving printed surface transversely to the direction of movement of the surface whereby the scanning head is enabled to scan one or more of a number of monitoring patches of said ink spaced at selected points across the width of the surface, indicating means responsive to the output of the scanning head and gating means to ensure that the indicating means does not receive signals corresponding to the printed surface between the monitoring patches. Across a colour printed sheet 10, Fig. 1, which is about to pass round a rotating drum 12, are arranged a number of monitoring areas in the form of patches 16 of ink of the various colours used, applied thereto by the printing rollers. Each patch 16 lies at the foot of a longitudinal strip of the sheet, and its colour corresponds to a colour which is laid down in large quantity in that strip. In the example the patches 16<1> are of yellow ink, the other patches being of red, blue and black ink. A photo-electric scanning head 24, which is driven transversely by a rotating screw 26, carries a projection 48. Stops 44 in line with the patches are arranged along four vertically displaceable bars 46 disposed one behind another, one bar for each colour, the stop in alignment with any patch 16 being provided on the bar corresponding to the colour of the patch. When the projection 48 engages a stop 44 the corresponding bar 46 is lifted and thereby one of four microswitches 51, 52, 53, 54 each corresponding to one of the colours, is operated to energize a relay, Fig. 6 (not shown), which disconnects a " forward " magnetic clutch 32 which is located between the feed screw 26 and its driving motor 30 so that rotation of the feed screw 26 and consequently the transverse movement of the scanning head 24 are interrupted. The scanning head 24, Fig. 3, contains a light source 62, photo-electric cell 70 and a synchronously driven rotating shutter 68, which are arranged so that the photo-electric cell is illuminated only by light reflected from the patch 16 (and its immediate white surround) corresponding to that stop member 44 with which the projection 48 is engaged. Under control of a counter operated by the photo-cell output, the scanning head remains stationary whilst several printed sheets pass round the roller, the corresponding patch on each sheet causing a pulse in the photo-electric cell. The pulses after amplification, shown at A in Fig. 5, are fed to gating circuits which produce an output, shown at L, Fig. 5, which is proportional only to the colour patch content c of the pulse shown at A. The pulses shown at L, derived from corresponding patches on the several sheets which pass round the roller whilst the scanning head 24 is stationary, are added and an output proportional to their average value is fed to a recorder 112 which includes a galvanometer and record paper 116. Also fed to the recorder 112 in series with the averaged colour signal voltage is a bias voltage corresponding to the colour of the patches being scanned, the bias voltage being selected by the relay energized over the contacts of the corresponding one of the micro-switches 51, 52, 53, 54. This bias voltage is such as to move the galvanometer pointer 114 to one of four different datum readings corresponding to the four colours, and the resultant position of the pointer 114 is dependent upon the algebraic sum of the bias voltage and the average colour signal voltage. Under control of the counter, the passage of the required number of sheets operates a reset relay which causes a chopper bar 120 to depress the galvanometer pointer 114 on to an ink ribbon 118 to mark the recording paper 116. The reset relay also operates contacts by means of which transverse movement of the scanning head 24 is re-established and the various circuits cleared for the next cycle of operation which commences when the projection 48 engages the next stop 44. By this means the paper 116 receives markings indicating the variation in colour density across the sheet for each colour. A " reverse " magnetic clutch 34 is provided for returning the scanning head and a switch 40 is operated through stop members 37, 42 to change-over the clutches 32, 34 at the ends of travel of the scanning head. A pulley- and-cable drive may replace the screw-feed mechanism for traversing the scanning head. The amplifier for the photo-electric pulses includes an A.G.C. circuit which maintains its largest output signal at a constant value so that it compensates for variation in lamp brightness, photo-cell sensitivity, dust on the scanning head and other factors. In a modification, the scanning head may be given continuous travel at a speed such that several monitoring areas in a single longitudinal line pass the scanning area. The movement of the scanning head operates means to send the colour signals to the appropriate recording or indicating channels. Alternative indicating or recording systems which employ a cathode ray tube or a group of astatic meters are referred to. Specification 820,361 is referred to.