In the art of xerography, a photosensitive member comprising a binder layer, such as zinc oxide particles dispersed in a film forming insulating resin, is uniformly electrostatically charged in the dark and then exposed to a pattern of activating radiation to form a latent electrostatic image on the surface of the binder layer. This latent image may then be developed by immersing the photosensitive member in a liquid developing solution which contains toner particles. The toner particles are attracted to and adhere to the areas containing the latent electrostatic image. After removal from the developer bath, the photosensitive member is dried and the image fused to form a permanent reproduction of the original radiation pattern or image.
It is well known in the art of electrophotography that it is difficult to adjust the properties of a photosensitive layer to render it suitable for continuous tone reproduction where the photosensitive layer is imaged in the conventional sequence of charging, exposure, and development with toner. In the art of photography, there are available grades designated No. 1 to No. 5 of printing paper of silver salt photography having the characteristic curve for soft to hard tones. In electrophotography, however, the conventional photoconductor member is characterized by a straight line in that the characteristic curve is shorter and the incline of the curve usually larger, which means harder. This characteristic curve is prepared by plotting the logarithm of the exposure strength at the abscissa and relative residual potential or developer concentration at the ordinate.
Where a photosensitive layer contains photoconductive particles dispersed in an insulating resin binder, it is known that the incline of the characteristic curve cannot be largely adjusted, even by changing the ratio of the binder and photoconductive member, or by other process variations in the preparation of the binder layer.
In order to avoid the above problems, the art has adapted various process techniques. One of these comprises a photosensitive layer which is prepared by kneading photosensitive zinc oxide sensitized with pigment absorption, and unsensitized zinc oxide with a binder. This process is more fully described in Japanese Patent Publication No. 11710/1966. U.S. Pat. No. 3,003,870 teaches the use of two kinds of zinc oxide having different sensitivities which are included in a continuous layer. British Pat. No. 967,690 teaches the use of several distinct binder layers sensitized by pigments having different sensitivities.
It can be seen from the above prior art, that complicated manufacturing processes, requiring much labor and resulting in lower efficiency in production, are required in order to obtain photoconductors exhibiting a soft characteristic suitable for continuous tone reproduction.
Conventional electrophotography is usually used for obtaining a positive image from a positive original. It is well known, however, that positive members usually have rather hard tone. Accordingly, by the use of conventional techniques described above, it is difficult to prepare photoreceptors exhibiting tone soft enough to meet requirements for a continuous tone reproduction.