Patent GB-986531-A (GB)
Title: Facsimile method and apparatus
IPC: H04N1/40
Filed: 1962-05-15
Abstract: 986,531. Copying by scanning. TIME Inc. May 15, 1962 [May 16, 1961], No. 18666/62. Heading H4F. A transparency or other original picture 204, Fig. 2, carried by a drum 203 is illuminated by a source 200 and the transmitted light directed in part (90%) to a main scanning aperture 213 and by reflection from a partially silvered mirror 211 in part (10%) to an axuliary scanning aperture 215, the arrangement being such that the auxiliary aperture views an area 205, Fig. 4, which is concentric with the spot 214 scanned by the main aperture and is at least twenty times larger in diameter. The light passing the main scanning aperture enters a photoelectric colour analyzer and results in red, green and blue component colour signals, whilst the light passing aperture 215 passes to a further photo-electric cell and results in a signal representative of the average tone density over the area seen by the aperture. In accordance with the invention, at least one of the component colour signals is modified as a function of the signal from this further photoelectric cell so as to increase the local contrast in reproductions of the transparency made with the aid of the signals. The invention is described as applied to the four-colour facsimile reproduction system disclosed in Specification 827,865, the increase in contrast being effected by a circuit arrangement as shown in Fig. 3 in which the signal derived from the further photo-electric cell 225 is combined with the linear black signal on lead 71 (i.e. the signal corresponding to the greatest intensity of the component colour signals) before it is applied over lead 31 to the undercolour removal modulators in the respective component colour signal channels. The signal combination is effected by applying the signals to the opposite ends B and C of a potentiometer network comprising a linear resistor 235 and a non-linear thyrite resistor 236, the lead 31 being connected to the intermediate point 0. By appropriate choice of the spectral characteristics of the light source and photo-electric cell 225, or by the inclusion of suitable filters in the light path, the signal from the photo-electric cell is arranged to be ortholuminous (i.e. corresponding to visual response). The photoelectric cell signal is passed through an exponential compressor 228 whose characteristic is preferably matched to the characteristics of the compressors in the colour channels. The Specification discusses with references to Figs. 5-7 (not shown) the manner in which the invention increases the local contrast. In fourcolour reproduction it is necessary that the black printer production should be related to the amount of under-colour removal. With the present invention since the under-colour removal signal is subject to variation, it is necessary to control the black printer signal correspondingly. To this end, the signal at point 0 is applied through a cathode-follower 271 to the grid of a pentode 277 through which the linear black signal on lead 71 is applied to the black printer correction circuit. A circuit is described with reference to Fig. 8 (not shown) for preventing a reduction in the densities of the reproduced colour components in coloured areas lacking tone variations such that the main and auxiliary scanners produce identical signals. The circuit is connected in the colour channels before the final amplifiers and is controlled by an ortholuminance signal, produced by combining the original blue, green and red component signals in the proportions 5%, 75% and 25% respectively, and functions to reduce a colour signal (equivalent to increasing the final colour density) when the signal exceeds the ortholuminous signal. The invention may be applied to a threecolour system in which a red filter 220, Fig. 2, is inserted in the optical path from aperture 215 and the resulting photo-electric cell signal is employed to control red and green undercolour removal modulators. No control is applied to the blue modulator. The circuit shown in Fig. 3 is employed by operating switch 240 so that lead 31 is connected directly to point B. The system is based on the four colour system described in Specification 827,865, and the Specification discusses certain other operational adjustments it is necessary to make to permit three-colour operation. Specifications 765,443, 765,444, 794,201, 986,532, 986,533 and 986,534 also are referred to.