Patent GB-1199850-A (GB)
Title: Screening Unit for Half-Tone Color Reproduction
IPC: G03G15/01
Filed: 1968-12-04
Abstract: 1,199,850. Using half-tone screens. RCA CORPORATION. Dec. 4, 1968 [Dec. 27, 1967], No.57586/68. Headings G2H and G2M. A half-tone screen for use in colour reproduction is mounted on a rotary member, and supported at a predetermined distance from the photo-sensitive surface on which the colour image is produced. Fig. 1 shows the arrangement applied to electrophotographic copying apparatus, light from an enlarger head 16 being projected through one of a number of colour filters in a wheel 19 on to the photo-conductive surface 18 of a recording strip 14. At the exposure station 100, the light passes through a half-tone screen unit 10, shown in more detail in Fig. 5. The screen 90 itself comprises a meshwork of lines at right angles to each other, ruled on glass, and mounted on the underside of a tubular member 24. This is itself rotatably mounted in a central opening 26 of a carriage 22 which can run on wheels along a base platen 12, the screen being maintained at a fixed distance from the strip 14. In producing a colour image, a plurality of half-tone images of different colours are superimposed, one after the other, and in order to avoid the formation of moire patterns the half-tone screen is rotated through a predetermined angle after each exposure. This is effected automatically by providing a ring of ratchet teeth 66 round the top of rotary member 24. A spring- loaded pawl 64 on the end of a bar 54 secured on carriage 22 engages against a fixed bracket 74 when the carriage reaches the left hand end of its traverse, and so causes member 24 (and hence the screen) to index round one position. The screening unit can be used in co-operation with a processing head 20, in which a number of processing fluids can be applied sequentially through a number of separate tubes 116-120, controlled by separate solenoid valves. The excess fluids are removed by vacuum means through a hose 136. The head also contains a corona discharge wire (140) Fig. 4 (not shown), for applying an electrostatic charge to the photoconductive surface 18. The head 20 can also, like the carriage 22, be reciprocated along the base 12, so that, in one direction of movement, an electrostatic latent image is toned with an appropriate toner, and in the opposite direction of movement the toned image is rinsed and the surface re-charged for the formation of a subsequent image. The head 20 carries two electromagnets 174, 176, which co-operate with brackets 82, 84 of magnetic material attached to carriage 22, so that head and carriage can be coupled together. In operation, the two units are moved together to the left, during which time the surface 18 is charged. The two units are now moved to the extreme right, so that unit 10 is pulled into the exposure position 100, and an exposure made. On moving to the left again, the latent image is toned by the processing head, while the pawl 64 is actuated to rotate the screen. On moving the two units to the right once more, the image on surface 18 is rinsed, and the surface re-charged for a further exposure. By de-energizing the magnets 174, 176, an exposure can be made without using the half-tone screen if desired.