In the electro photographic technique, a method is known in the past, in which a photoconductive layer is vacuum-deposited on an electrode layer, and the exposure is performed after the photoconductive layer is totally charged, electrostatic latent image is optically formed on the photoconductive layer by leaking electric charge on the exposed portion, the toner having electric charge of opposite polarity is attached on the residual electric charge, and the image is electrostatically transferred on paper or the like. This method is primarily used for duplicating purpose, while, according to this method, electrostatic charge retaining period on photoconductive layer is shortened, and toner development is performed immediately after electrostatic latent image is formed. Thus, this cannot be used for photographing purpose because of low sensitivity.
In the television imaging technique, line sequential scanning is required to take up and record electric image signals obtained through image pickup tube. Line sequential scanning is performed by electron beam in the image pickup tube and by magnetic head in video recording. Because resolution depends upon the number of scanning lines, the resolution is extremely decreased compared with planar analog recording such as conventional photography.
Further, the television imaging system using solid image sensor developed in recent years is also essentially the same as far as resolution is concerned. The problems with these techniques are: The higher quality and resolution the image recording has, the more complicated the process is, and the simpler the process is, the more it lacks the memory function or image quality is basically decreased.
Also, there is an electronic imaging technique. According to this method, electrode is vacuum-deposited on a photoconductive layer, and the entire surface of photoconductive layer is electrically charged by corona charging in dark place. Then, it is exposed to strong light to turn the exposed portion of the photoconductive layer to electrically conductive. By leaking electric charge on such portion, electrostatic latent image is optically formed on the surface of the photoconductive layer. The toner having electric charge of opposite polarity (or electric charge of the same polarity) is attached to the residual electrostatic charge, and the image is electrostatically transferred on paper or the like. This is mostly used for duplicating purpose, whereas it cannot be used for photographing because of low sensitivity. Because electrostatic charge retaining time on photoconductive layer as recording medium is short, toner development is usually performed immediately after electrostatic latent image is formed.
Further, a method is known, by which thermoplastic substance layer having selenium particle layer is furnished on a transparent electrode. After total surface corona charging and image exposure, the information is reproduced as visible information through heat development. The accumulated electric charge information is of permanent nature, while corona charging is required for information recording, and the information is reproduced by visualization. (U.S. Pat. Nos. 3,520,681, 4,101,321, and 4,496,642)
In the electrostatic information recording medium having electrode layer and electric charge retaining layer, electrostatic information can be recorded in the form of information electric charge on the electric charge retaining layer through exposure under voltage application, and the accumulated information can be reproduced by reading, amplifying and outputting the surface potential on the surface of electric charge retaining layer. The object of the present invention is to improve electrostatic information retaining property on electrostatic information retaining medium and also to offer an electrostatic information recording medium having excellent electrostatic information (electric charge) retaining property and an electrostatic information recording and reproducing method.