GB1207830A - Automatic xerographic development control - Google Patents

Abstract

1,207,830. Developing electrostatic images. RANK XEROX Ltd. Dec. 29, 1967, [Jan. 3, 1967], No. 59080/67. Heading G2H. The potential applied to a development electrode 24 is controlled by a first circuit which is activated solely during the exposure of a charged xerographic plate to produce a first potential which is a direct function of exposure time and intensity and approximates the drop in plate voltage in fully exposed areas and a second circuit which subtracts the first potential from the initial potential and applies the resulting potential between the plate and electrode to eliminate the field therebetween. In one embodiment, xerographic plate 14 is exposed to a document, photograph or other subject 19 illuminated by lamps 20. A photo-sensitive device 30 is exposed to a sample of background material 32 having the same optical density as the background areas of the subject. The device 30 is connected to an amplifier 34 connected through a resistor 36 to a capacitor. Capacitor 38 is charged during exposure at a rate proportional to the intensity of illumination as detected by the device 30. The development electrode 24 is swung into position in front of the plate 12 and a switch 28 is moved to right to connect power supply 40 to the capacitor 38. The supply 40 had a voltage and polarity equal to the voltage of the initally charged plate, such that the potential of the development electrode is equal to the potential of supply 40 minus the potential across capacitor 38 and so approximates the potential in image areas appearing as highlights. No electro-statically attractable particles are deposited in these areas, since there is no electric field between the plate and electrode. In a modification, the exposed plate is removed and placed in a support (66) Fig. 3 (not shown) beneath a developer hopper (68) controlled by a solenoid valve (70). In this case an amplifier (60) replaces the power supply 40, Fig. 1. In another embodiment capacitor (38) Fig. (not shown) is connected to a variable resistance circuit (78) connected to the electrode (24). In one form resistance of circuit the output of a divided circuit (90) Fig. 7 (not shown) is fed to an amplifier (92) which generates an output signal fed to the base of transistor (80) connected to the capacitor.