There are two means of exposing electrophotographic plates. The first utilizes an inorganic photoconductor, typically selenium or cadmium sulfide, applied to the surface of a drum. A corona sensitizes the drum to light using an electrostatic charge. Light reflected by non-image areas of the original dissipates the charge from the non-image portions of the drum, the charge remaining in the image portions. A dry or liquid toner, oppositely charged, is attracted to the charged (image) portions of the drum and adheres to it, transferring then to the paper (or other substrate) passing beneath the drum. Plates produced in this fashion require special treatment, and are used typically for only very short runs or in reprography.
A second variety of electrophotographic plate utilizes the same principle outlined above, except that the photoconductor (organic in this case) is applied directly to the surface of the desired substrate rather than a drum. The toner is either fused directly to the surface of the photoconductor, or is transferred to another substrate. Plates produced in this fashion for high-speed laser imaging have a base of anodized aluminum, and the photoconductive coating must be removed from non-image areas prior to printing. Non-printing areas need to be sensitized using gum arabic and fountain etch to impart the desired level of water-receptivity. This chemical treatment, however, tends to damage halftone dots, making these plates not ideally suited for fine screen work. (See Plate: Offset Lithography.)