Apparatus for forming a positive electrostatic image

Claim 2

Apparatus for forming an electrostatic image that corresponds to an optical image comprising a composite screen defining a plurality of spaced apart apertures and having an outer conductive layer forming the obverse face of said screen, an insulative layer superposed on said outer conductive layer, an inner conductive layer superposed on said insulative layer, and a photoconductive layer superposed on said inner conductive layer and forming the reverse face of said screen, means connected between said outer conductive layer and said inner conductive layer for establishing a voltage gradient across said insulative layer, said voltage gradient forming a first electrostatic field that extends into said apertures to form blocking fields within said apertures polarized in a first direction, means for establishing a charge on said reverse face that forms in cooperation with said inner and outer conductive layers a second field in said apertures that has a polarity and magnitude sufficient to counteract said first field when said photoconductive layer is in a dark condition, and means for impressing the optical image on said reverse face so that relatively light regions of said image cause corresponding regions of said photoconductive layer to become conductive and discharge corresponding portions of said second field.

Claim 3

Apparatus for forming a positive image of an object on a surface comprising a plurality of particles, means for forming an electric field between said particles and a particle receiving surface for accelerating the particles on a path toward said surface, a particle pervious screen disposed in said path in substantial parallelism with said surface, said screen having a reverse face facing said surface, an obverse face opposite said reverse face, and a plurality of apertures extending therethrough between said obverse face and said reverse face, said apertures being of sufficient size to admit said particles therethrough, said screen including an electrically conductive outer layer on said obverse face, an electrically insulative layer underlying said outer conductive layer, aN electrically conductive inner layer underlying said insulative layer and a photoconductive layer underlying said inner conductive layer and defining said reverse face, means for establishing a first electric field between said inner and outer conductive layer so polarized as to repel particles approaching the apertures from the direction of said obverse face, means for establishing a second electric field between said inner conductive layer and said photoconductive layer that has a polarity and magnitude sufficient to counteract said first field, and means for projecting an image of the object on said reverse face so that portions of said photoconductive layer that are illuminated will establish a conductive path to discharge said second field in the illuminated portions, whereby particles are repelled by said first field at apertures in such illuminated portions of the screen.

Claim 4

Apparatus for forming a positive image of an object on a surface comprising a plurality of particles, means for forming an electric field between said particles and a particle receiving surface for accelerating the particles on a path toward said surface, a particle pervious screen disposed in said path in substantial parallelism with said surface, said screen having a reverse face facing said surface, an obverse face opposite said reverse face, and a plurality of apertures extending therethrough between said obverse face and said reverse face, said apertures being of sufficient size to admit said particles therethrough, said screen including an electrically conductive outer layer on said obverse face, an electrically insulative layer underlying said outer conductive layer, aN electrically conductive inner layer underlying said insulative layer and a photoconductive layer underlying said inner conductive layer and defining said reverse face, means for establishing a first electric field between said inner and outer conductive layer so polarized as to repel particles approaching the apertures from the direction of said obverse face, means for establishing a second electric field between said inner conductive layer and said photoconductive layer that has a polarity and magnitude sufficient to counteract said first field, and means for projecting an image of the object on said reverse face so that portions of said photoconductive layer that are illuminated will establish a conductive path to discharge said second field in the illuminated portions, whereby particles are repelled by said first field at apertures in such illuminated portions of the screen.