Patent GB-643330-A (GB)
Title: Improvements in or relating to the half-tone reproduction of documents or the like
IPC: H04N1/405
Filed: 1947-05-20
Abstract: 643,330. Copying telegraphy ; modulating tight. ETABLISSEMENTS E. BELIN. May 20, 1947, Nos. 13509 and 13510. Convention dates, May 8, 1946 and April 24, 1947. [Class 40 (iii)] In a system for reproducing a document, means are provided for converting each shade of the original into a current whose magnitude is a function of the shade and then converting this modulated current into a plurality of intermittent luminous signals which are constant in intensity but variable both in duration and spacing. In a system designed to produce half-tone plates for printing on fabrics the multi-shade original is scanned photo-electrically in a known manner and the modulated picture currents are applied to the deflecting system of a mirror oscillograph. A light source A, Fig. 4, of constant intensity is focused on to an arcuate slot F, the image of which is reflected from the oscillograph mirror M through a mask G and rotating obturating disc D on to a photo-electric cell C. The amplifier output from this cell is applied to an oscillograph which reflects light through a slotted mask on to a photo-sensitive recording surface B in known manner. The light issuing from the obturating disc may instead impinge directly on a photo-sensitive receiving surface, or the output from the photo-electric cell may control an engraving tool. The mask G has a triangular aperture H, Fig. 3, and the height at which the arcuate image S impinges on this aperture and therefore the length of image exposed is dependent on the amplitude of the picture signal, i.e. is a function of the shade being scanned. The obturating disc D comprises a number of slots J, Fig. 2, and other apertures K so arranged that the number of apertures passing the image S is proportional to the distance of the image from the centre O of the disc. Thus a black signal deflects the image S to the base of the triangular aperture H producing a wide beam which is repeated twelve times per revolution of the disc D, an almost white signal deflecting the image S to the top of the aperture H and producing a narrow beam which is repeated nineteen times. In a second embodiment the obturating disc D<SP>1</SP>, Fig. 7, comprises four rings of slots J<SP>1</SP>, comprising 14, 18, 24 and 28 slots respectively. The mask G is formed with an additional rectangular aperture I for each possible transition from one ring to the next and a second, rectangular, image S<SP>1</SP> is reflected by the mirror M on to the mask G. When the image S is moving downwards during a transition to a darker shade and begins to overlap two rings of slots J<SP>1</SP>, the auxiliary image S<SP>1</SP> begins to impinge on an auxiliary slot I to energize an auxiliary photo-electric cell and feed back a current which causes the mirror M to increase the downward movement of the main image S. This feed-back arrangement ensures a rapid transition of the image S from one ring of slots J<SP>1</SP> to the next, until with the auxiliary slot I completely illuminated a state of equilibrium is reached since any further downward movement tends to decrease the illumination of the auxiliary slot and make the images rise again. If, now a lighter tint is scanned, the images S, 8<SP>1</SP> tend to rise, the auxiliary slot I receives less illumination and the resultant feed-back current assists the rapid rise of the image S to the upper ring of slots J<SP>1</SP>. If, however a darker tint is scanned, the image S tends to move downward and this tendency is retarded by the resulting feed-back current due to the decrease in light passing through the auxiliary slot I. The only ill-effect of this retardation is to reproduce a rather lighter shade than desired until the image S<SP>1</SP> has left the auxiliary slot 1. Since, with a flat-tint original, any variations in the positioning of the main image S within the limits of a particular slot J<SP>1</SP> are due to unwanted irregularities in the tint being scanned a stepped form of triangular aperture H may be employed to reproduce such an original, Fig. 9. This irregular movement of the image S will then give rise to no change in the width of the light beam uncovered by the mask G. The recording cylinder B may be mounted on the same shaft as that carrying the original document.