Ion modulator device and method of using in positive and negative modes

Claim 1

A process for producing from a graphic original copies displaying pagewise homogeneity and essentially free of background comprising the steps of: 1. applying an electrostatic charge to a photoconductive surface of an ion modulator while applying a large bias potential of the same polarity as that of the charge to said ion modulator, 2. projecting upon said ion modulator a pattern of light and shadow corresponding to said graphic original, 3. projecting ions upon a conductive surface of said ion modulator opposite from said photoconductive surface while applying a bias potential less than said large bias potential to said ion modulator, whereby ions are transmitted through said modulator in a pattern corresponding to said original, 4. creating a latent electrostatic image corresponding to said pattern upon a dielectric surface in the path of said transmitted ions, and 5. developing said latent electrostatic image, said ion modulator being a member selected from the group consisting of (a) a conductive screen having a photoconductor coated on at least one side thereof, an electrically insulating layer coated on the other side thereof and a thin metal film applied to said electrically insulating layer (b) a conductive screen overcoated with a photoconductive layer, said photoconductive layer being coated on one side thereof with a thin metal film (c) a conductive screen having an electrically insulating layer partially coated thereon, a thin metal film coated only on said electrically insulating layer and a photoconductive layer coated only on said metal film and (d) a conductive screen having a photoconductive layer partially coated thereon, a thin metal film coated only on said photoconductive layer and a photoconductive layer coated only on said metal film, said bias potentials being applied between said film and said screen during steps of applying an electrostatic charge and projecting ions and being defined by the relationships |V.sub.L - V.sub.B |≧ V.sub.block | with V B > V L > 0 and V block < 0 and |V.sub.D - V.sub.B |≧ V.sub.block | with V D < V B < 0, and V block < 0 when said ions are of negative polarity and by the relationships V.sub.L - V.sub.B ≧ V.sub.block > 0 with V B < V L < 0 and V.sub.D - V.sub.B ≧ V.sub.block > 0 with V D > V B > 0 where V L is the modulator potential in the light areas, V B is the bias potential, V block is the blocking potential and V D is the modulator potential in the dark areas when said ions are of positive polarity.