Patent GB-967690-A (GB)
Title: Materials for electro-photography
IPC: G03G5/09
Filed: 1962-06-07
Abstract: 967, 690. Electrophotographic processes. EASTMAN KODAK CO. June 7, 1962 [June 7, 1961 No. 22177/62. Heading G2H. [Also in Division H1] An electrophotographic material comprises a support bearing at least two superposed continuous layers containing photoconductive zinc oxide particles dispersed in an insulating binder, the speed of the layer nearest the support have a speed in a given spectral region greater than that of the uppermost layer, any layers between these two having a speed in the given region less than that of the layers nearest the support and having a speed in the given region or in a different spectral region greater than that of the uppermost layer, and in which each layer transmits indication to the layer or layers lying between it and the support to which those layers are sensitive. Such materials can produce an exposure scale greater than that of single layer zinc oxide-based materials. In the examples dye sensitization is used to obtain changes in the degree of sensitivity or spectral region of greatest sensitivity of the layers but other methods are discussed in the general description. Two main types of material are considered. In one the layers are sensitized in different spectral regions. For example, a two layer material may have the layer nearest the support sensitized to green light while the top layer is unsensitized. Both layers will be sensitive to ultra-violet light because of the inherent sensitivity of zinc oxide in this region. Fig. 4 shows the relationship between exposure and developed density for such a material which has undergone sequential exposure to first green light and then to ultra-violet. A simultaneous exposure made with both green and ultra-violet light can be made to produce a smooth curve of extended exposure scale, as shown in Fig. 5, which is double that of a single layer zinc oxide-based material. A similar material may be made with three layers, that nearest the support being red sensitive, the next green sensitive, and the top layer being unsensitized. These independent exposures with red, green, and ultra-violet light will produce a tripled exposure scale which, if the relative layer thicknesses and sensitivities are chosen correctly will give a smoother exposure curve longer than but generally like that of Fig. 5 when simultaneous exposure is made with selected amounts of light in all three regions. In the second type of material all the layers are sensitive to ultra-violet light and are additionally sensitized in a particular spectral region with, of course, the layer nearest the support being the most heavily sensitized and the top layer being the least sensitized. Fig. 9 shows the family of exposure characteristics obtained with a three layer material in which all the layers are sensitized in the green region of the spectrum and in which the relative layer thicknesses and sensitivities are chosen to give smooth curves. Curve 41 shows the exposure characteristic obtained by using ultra-violet light alone (this is of course of the same exposure scale as a single layer material) while curve 44 shows the extended exposure scale obtained by using green light alone. Curves 42 and 43 show that intermediate exposure scales may be obtained by using green and ultra-violet light simultaneously, the exact exposure scale depending on the relative proportion of each used. With all the materials described simultaneous exposure to light in different regions may be carried out using separate tungsten sources each with its associated filter or a single source may be used in conjunction with a special filter. The development process is mentioned briefly and in two of the examples use is made of magnetic brush development respectively with a pigmented resinous powder and with sulphur powder containing a black pigment such as carbon black. Specification 877,432, U.S.A. Specifications 2,493,748, 2,727,807 and 2,727,808 are referred to.