GB1352412A - Electrographic apparatus incorporating automatic contrast control - Google Patents

Abstract

1352412 Automatic current control - electrophotographic processes EASTMAN KODAK CO 26 April 1971 [24 April 1970] 11361/71 Heading G3R [Also in Division G2] In an electro-graphic apparatus for document copying and having automatic contrast control, a uniform negative charge controlled by a grid 5 and exposure time is set up on a photo-conductive grid 6 (Fig. 1) by means of a corona source 4. The grid 6 is then exposed to a radiation pattern derived from the subject matter (e.g. a film transparency) to be reproduced, whereby the negative charge in the grid areas corresponding to bright regions of the pattern, is decreased, and subsequently the charge pattern on the grid is used to modulate ion current flow between the corona source 4 and a direct current source 3 (e.g. including a constant current diode, a vacuum photo-diode or a transistor, whereby the current may be maintained constant) through a receiving copy sheet 2. Contrast control is achieved in that when multiple copies are being made, any charge decay in grid 6 will result in a larger ion current flow to receiver sheet 2 which in turn will cause an increased volt drop across the current source 3. Thus grid 6 will go more negative and reduce the ion current flow passing to the sheet 2. Thereby the current is maintained substantially constant. In a further embodiment Fig. 3, the voltage drop across current source 3 is detected by an amplifier 9 and a network 10 converts the sensed voltage to a control current which is integrated with respect to time by a circuit 11. The output of the integrator is indicative of the charge produced on receiver sheet 2 since charging began. A comparator 12 compares this output with a desired contrast signal set up on potentiometer 13. When the total charge received by the sheet 2 reaches the desired value a relay coil 14 is de-energized and a switch 15 opens, the switch having been held closed to pass ion current through the sheet between the current source 3, conductive base-plate 1 and corona source 4. Thereby automatic contrast control is achieved when producing multiple copies, by change in the length of periods during which successive sheets receive ion current.