1. Field of the Invention
This invention relates to a method of optimizing the electrostatic latent image formation process so as to stabilize an electrostatic latent image formed on a photosensitive medium by electrophotography, and more particularly to a method of quickly stabilizing the formed electrostatic latent image by quickly optimizing the latent image formation process including two kinds of electrostatically charging steps.
2. Description of the Prior Art
Various types of electrophotography have heretofore been proposed whereby an electrostatic latent image is formed on one of various types of photosensitive medium and transferred to a transfer medium after development or developed after transfer so that the formed image may be utilized, and several of these types including the Carlson process disclosed in U.S. Pat. No. 2,297,691 issued to C. F. Carlson have been put into practice. The image formed by electrophotography is readily affected by environmental conditions and stabilization of the electrostatic latent image formed thereby is very important in practice. First, the main factors contributing to the characteristic of the image formed by the common type of electrophotography include the characteristic of the photosensitive medium used, the characteristic of the charging means for sensitizing the photosensitive medium, the characteristic of the light source for exposure and its quantity of light, the developing characteristic, the image transfer characteristic, the characteristic of the transfer medium, the cleaning characteristic of residual developer, etc.
These characteristics are variable under the influence of temperature, humidity, contamination by dust, aging, etc., and this has intricately affected and varied the characteristic of the formed image.
For the stabilization of such image variation, a method of individually stabilizing each of the above-mentioned characteristic has heretofore been adopted and improvements of the respective characteristics have been advanced. However, ensuring the image in which different characteristics affect one another to be always stabilized is difficult to achieve by stabilizing the foregoing characteristics alone.
A method of stabilizing such an electrophotographically formed image is disclosed, for example, in U.S. Pat. No. 2,956,487, wherein in accordance with the so-called Carlson process, charge and image light are applied to xerography photosensitive medium to form an electrostatic latent image and when such image is developed and transferred, the quantity of light of the original image to which the photosensitive medium is to be exposed, the potential of the electrostatic latent image so formed or the density of the image after developed is detected so that the result of the detection is fed back to the charging and exposure means, etc. of the described process, thereby stabilizing the formed image. The factors making the electrostatic latent image unstable include variations in charging voltages, adherence of foreign materials to the charging electrode, aging of the charging electrode by its oxidation or the like, variations in characteristic of the corna discharge and in quantity of light of the image caused by temperature and humidity, fatigue of the photosensitive medium, variations in temperature and humidity characteristics of the photosensitive medium, etc. If these factors are within a predetermined range, it will be possible to stabilize the electrostatic latent image by measuring the potentials on the exposed and unexposed regions of the electrostatic latent image and varying the charging voltages and the quantity of exposure by the use of a feedback system.
The aforementioned U.S. patent controls the potential of the electrostatic latent image formed in accordance with the Carlson process, but once deterioration occurs to the photosensitive medium, the residual potential thereon is increased to vary the potential on the exposed region, thus making it difficult to stabilize the latent image.
Also, U.S. Pat. No. 3,586,908 discloses a system wherein the difference between the detected potential and the reference potential is applied to an integrator to control the output voltage in order to control the charging. Any of these methods is to control the Carlson process wherein latent image is formed by only one kind of charging step.
In contrast with such a latent image formation process which requires only one kind of charging step, the type of process which involves two or more kinds of charging steps for the electrostatic latent image formation is disclosed, for example, in U.S. Pat. Nos. 3,666,363 and 3,734,609, wherein the charging means for carrying out the respective charging steps are controlled to thereby enable the potentials on the exposed and the unexposed regions of the formed electrostatic latent image to be varied, and this type of process has thus been found to be suitable for the realization of stabilized image formation. Nevertheless, it has also been found that even if a feedback is applied from the heretofore contrived means for measuring the potential of the latent image to each charging means, a long time is required for the potential of the latent image to be converged and stabilized to a reference value. This is because, with only one of the charging means being set, the electrostatic latent image can not be stabilized and moreover, variation in one of the charging means affects the charging effected by the other charging means.