1. Field of the Invention
This invention relates to an electrographic apparatus comprising a photosensitive screen composed of an insulating layer, two electrically conductive layers coated on opposite sides of the insulating layer and a photosensitive layer coated on one of the electrically conductive layers, a corona discharge device arranged at one side of the photosensitive screen and emitting a flow of corona ions, and a dielectric coated record sheet arranged at the other side of the photosensitive screen, whereby an electrostatic latent image produced on the photosensitive layer of the photosensitive screen causes the flow of corona ions directed from the corona discharge device through the photosensitive screen toward the dielectric coated record sheet to modulate so as to produce, on the dielectric coated record sheet, an electrostatic charge image corresponding to the electrostatic latent image on the photosensitive layer of the photosensitive screen.
2. Description of the Prior Art
Various kinds of electrographic apparatus have been proposed which can modulate a flow of corona ions by an electrostatic latent image produced on a photosensitive screen so as to produce on a dielectric coated record sheet an electrostatic charge image.
One of these prior art apparatus makes use of a photosensitive screen of four layer construction composed of an insulating layer, two electrically conductive layers coated on opposite sides of the insulating layer and a photosensitive layer coated on one of the electrically conductive layers. Such apparatus can perform a first step of uniformly charging the photosensitive layer, a second step of illuminating the photosensitive screen with a light image and producing thereon an electrostatic latent image, and a third step of applying a high voltage between a field electrode arranged at the rear of a dielectric coated record sheet which is opposed to and spaced apart from the photosensitive layer of the photosensitive screen on the one hand and the photosensitive layer of the photosensitive screen on the other hand and directing a flow of corona ions from a corona discharge device arranged at the side of the electrically conductive layer of the photosensitive screen through the photosensitive screen toward the dielectric coated record sheet while applying a bias voltage between the two electrically conductive layers and producing on the dielectric coated record sheet an electrostatic charge image corresponding to the electrostatic latent image produced on the photosensitive layer of the photosensitive screen.
This apparatus has the advantage that the picture image can be controlled by adjusting the bias voltage applied between the two electrically conductive layers in dependence with the number of copies set beforehand and that a plurality of copies can be reproduced by repeating the third step after the electrostatic latent image has been produced on the photosensitive screen.
In practice, however, a concentration of the picture image becomes changed in dependence with the number of copies set beforehand owing to a dark decay characteristic of the photosensitive layer and to an undesirous detour of the flow of corona ions emitted from the corona discharge device. As a result, copies each having a picture image having a good quality are limited in number and it is impossible to obtain a large number of copies each of good quality.
In order to obviate such drawback, another prior art electrographic apparatus has been proposed. In this apparatus, in the above mentioned third step, the bias voltage applied between the electrically conductive layers of the photosensitive screen can be changed in response to the number of copies set beforehand and hence can compensate for the decay of the electrostatic latent image on the photosensitive layer. In such electrographic apparatus constructed as above described, in the case of obtaining a plurality of copies from the electrostatic latent image produced on the photosensitive screen by repeating the third step, if the concentration of the first picture image becomes incorrect, the concentration of subsequent picture images becomes also incorrect.