Method and apparatus for forming a toner image in electrophotographic

Abstract

An electrophotographic printing apparatus employs a laminated, photosensitive medium including transparent supporting and conducting layers and a photosensitive layer defining the top surface of the medium, on which a toner image is formed. A first developer, maintained at a voltage of the same polarity as the toner particles, forms a uniform layer of charged toner particles on the surface of the photoconductive layer, the charged particles adhering to the surface. Simultaneously or subsequently, an optical beam is projected onto the photoconductive layer in accordance with the image to be printed, rendering the selectively exposed portions sufficiently conductive to permit charges of opposite polarity to be injected into the photoconductive layer from the conducting layer; the photoconductive layer establishes trapping potential levels therein, permitting the charges to traverse the layer to positions adjacent the top surface thereof. Upon extinguishing the optical beam, the photoconductive layer reverts to a nonconductive state, fixing the trapped charges in position as a laten electrostatic image. The latent electrostatic image establishes a field, or Coulomb force, which closely adheres the oppositely charged toner particles to the surface. In a second developing step, a developer, to which is applied a voltage of opposite polarity relative to that of the toner particles, then removes the toner particles from the nonexposed portions of the surface; the toner particles on the exposed portions of the surface are more firmly adhered thereto by virtue of the latent electrostatic image and thus a substantial portion thereof remain, providing a high contrast toner particle image for subsequent transfer to a suitable recording medium.