Patent GB-1287626-A (GB)
Title: Engraving apparatus for intaglio printing plates
IPC: B41C1/05
Filed: 1969-08-27
Abstract: 1287626 Copying by scanning QUANTONIX CORP 27 Aug 1969 [28 Aug 1968] 42761/69 Heading H4F [Also in Division H1] The spactial density of equal sized depressions or cells produced in an intaglio printing plate at 31, e.g. by a laser 79 or electron beam, corresponds to the optical density of an original image at 30 which is being copied. In the preferred embodiment, an electro-optic deflector, of BaTiO 3  for example, causes a photo-electric sensor 49 to perform a superimposed scan over an interval of, say, three resolvable cells on either side of a main helical scan produced by rotation of the copy cylinder 30 and translation of the sensor 49. The output of the sensor is fed to a signal processor 32, shown in detail in Fig. 5 (not shown), including a density computer where it is sampled six times per deflection i.e. once per cell, and the samples digitized according to their magnitude, e.g. from 0-36, when 37 tone values are resolved. The density computer may be manually adjusted to take paper and ink parameters into account. The digitized samples are then fed to a resolution converter, Fig. 6, where they are continuously added in a circuit 61, 65, 62, until the sum exceeds a threshold, whereupon a circuit 63, 64 delivers a logical "1" output signal and a circuit 66, 67, 68 subtracts the maximum possible digital signal (36) from the total at 61. In the present case the threshold decision device 63 preferably operates at digital 18, but any arbitrary level may be used. A succession of logical "0" and "1" signals is thus produced at the output of the resolution converter where they are converted to parallel form and stored on 12 tracks of a 14-track tape recorder 51. An encoder 41 produces a signal for every 15 microns of travel of the surface of cylinder 30, which is fed via a clock pulse generator to a timing generator, whose output is used to control the revolution converter and which is also recorded, together with a once per revolution sync. pulse from the encoder 41, on the other two tracks of the recorder 51. In the engraving half of the system, the timing generator signals are used to control a deflector 76, e.g. similar to 47 if a laser is used, and the sync. pulses are compared at 83 with the output of a shaft encoder 81 to control the speed of a tape reader 72. The logic signals are re-converted to serial form, and the "1" pulses control the firing of a laser 79, the laser pulse intersities being controlled by a system 91, 92. It is stated that the process may also be used for producing flat plates. In a modification, the tone level of a whole resolution area of the original is sensed, i.e. 36 possible cells in this instance, and the resulting signal used to control the frequency of operation of the laser. The laser 79 may be controlled by Q-switching, Fig. 8 (not shown), or be mode locked and employ a gate c.s. a Kerr cell, which opens for each "1" pulse, Fig. 9 (not shown). Alternatively, Fig. 10 (not shown), a combination of Q switching and mode locking may be used. A variable loss modulator, e.g. a Kerr cell, KDP, or lithium niobate, in the optical resonator receivers mode locking signals from an oscillator via a gate under the control of the engraving logic signals, the resonator being switched to a high Q and mode locked state when a "1"  signal's received. Since the absorption of laser pulses by the metal printing plate's greatly increased when the second harmonic of the laser light's used, if a Nd : YAG laser's used for instance, a second harmonic generator of barium sodium niobate is disposed between the laser 79 and cylinder 31. Alternatively, Fig. 11 (not shown), the second harmonic generator may be placed inside a mode locked laser and use made of the fact that refractive indices must be watched for good conversion. The output reflector is made reflective at the fundamental wavelength and substantially transparent to the second harmonic, so that a logical "1" causes the generator to be tuned and gives a large output pulse. In a further alternative, a quarter wave rotator and Nicol prism may be set up inside a mode locked laser to normally transmit the light between 100% reflective mirrors. A logical "1" at the rotator would cause the plane of polarization to be rotated by 90 degrees and the light to be deflected out of the cavity by the prism. Reference has been directed by the Comptroller to Specification 1,035,342.