The invention relates to a method for maintaining a constant potential ratio in the exposure of electrostatically charged light-sensitive layers on carriers, on which an electrostatically latent image of an original is formed during the exposure.
Long-term uniform quality of printing plates, on the charged light-sensitive layers of which electrostatically latent images are obtained by exposure and are developed with a toner, requires that the mutual potential ratios of charge, residual charge, and counter-voltage are precisely maintained. In order to achieve this, the electro-photographic layer must be produced within narrow tolerances and an exposure device, for example a camera, must be adapted to working conditions which may differ from one printing plate to another. The exposure control of conventional cameras consists in general of a timer which switches off the shutter and the light source after a preselected time. An improved control is possible with so-called light dosimeters which measure the light arriving in the zone of the original and correct the exposure time accordingly. In this way, different lamp outputs, caused by light source agin, by fluctuations in the supply voltage, or by dirt on the reflectors, are partially compensated for. The setting required for a desired residual voltage of the exposed printing plate can only be found with the aid of trial plates which are exposed for different periods, so that the exposure step is terminated at different residual voltages of these trial plates. That trial plate which allows satisfactory printing also gives the requisite setting for the desirable residual voltage at the end of exposure. The setting thus obtained cannot be retained, however, since the residual voltage resulting at an unchanged setting varies with the charging and the sensitivity of the light-sensitive layer of the printing plate. These parameters depend on manufacturing tolerances, type of plate, atmospheric humidity, temperature, pre-exposure, dirt on the corona, and the like.
German Offenlegungsschrift No. 3,049,339 has disclosed an electrostatic recording device with a measuring device in the form of an electrometer for measuring the surface potential of a light-sensitive layer. The surface potential of a latent image on the light-sensitive layer is measured, the surface potential being registered as an a.c. voltage signal, and various conditions for producing the image, such as, for example, the charging voltage and the developing bias, are controlled from the measured surface potential. For this purpose, a first control device contains a stored program for a sequence control of the device generating the latent image, and a second control contains a stored program for controlling the conditions for the formation of the latent image by means of the device generating the latent image or for controlling the conditions for development by the developing device by means of output signals from the surface potential-measuring device.
German Patent No. 2,857,218 has disclosed a method for keeping the optimum conditions constant in electrophotographic duplicating, wherein, for the formation of an electrostatic latent image on a light-sensitive carrier, the carrier is charged electrostatically and exposed and, next to the electrostatic image of the original on the carrier, an electrostatic latent reference image is formed, the potential of which is measured and an adjustable parameter for the duplication is adjusted in accordance with the measured potential reference image. The reference image is generated by forming light and dark areas on the light-sensitive carrier and accordingly has areas of low potential and high potential. The potentials of both areas are measured and, in the event of a deviation of the potentials of the reference image from given set values, the adjustable parameter, allocated to the particular area, is changed until the potentials of the electrostatic latent reference image have been brought to the given set values. If the exposure of the light-sensitive carrier is insufficient (in which case the potential is in general too high), signals are generated for automatically correcting the voltage of the illumination device, the width of the slit opening, or the like, or to provide a corresponding correction figure for the potential. If the charge of the light-sensitive carrier is insufficient, that is to say the potential is too low altogether, signals are provided for automatically increasing the voltage of the charging device or for carrying out a corresponding correction of the potential. If both the exposure and the charge are not at desired levels, signals are provided for correcting both parameters in such a way that, after the passage of a few copies, the given set values can be reached. This means that follow-up control of the voltage of the charging device and/or an increase in the exposure intensity takes place, the starting point for these corrections being the measurement of the surface potential of an electrostatic latent reference image on the light-sensitive carrier.
European Patent Application No. 0,098,509 describes a method for controlling the electrostatic charging of a photoconductor surface by means of a corona-charging device. In this case, the charged photoconductor surface is partially discharged by exposure, and signals are measured which allow a comparison of the exposed and unexposed areas of the photoconductor surface. The charging device is controlled according to these comparison signals. The comparison signals are also used for regulating the light intensity of an exposure lamp and the duration of exposure. For this purpose, when the measured signal agrees with a stored set value of the unexposed area of the photoconductor, the corona-charging voltage is regulated to a value corresponding to the matched levels of measured signal and set value. The measured signal belonging to this corona-charging voltage in the exposed area of the photoconductor regulates the exposure lamp in agreement with stored data of the lamp characteristics, which data take into account the aging of the lamp, non-linear influences due to voltage fluctuations, a shift in the color temperature of the exposure lamp relative to the photoconductor sensitivity, and the like, in order to charge the exposed area sections of the photoconductor to the desired surface voltage. In this method, the corona-charging voltage, the exposure intensity and the duration of exposure are regulated in such a way that, at the start of toner application to the photoconductor, a given voltage exists in the exposed areas. A continuous measurement of the decreasing photoconductor voltage and of the exposure process in the exposed area sections does not take place.
U.S. Pat. No. 3,438,705 describes an exposure and developing device, in which the background density of a copy is automatically controlled by means of a photosensitive device which scans the material to be copied. The potential obtained on scanning the background is applied during the exposure to the development plate, in order to prevent overcharging of the plate. In this device, the exposure which the plate to be developed receives in a background area is measured. From this measured exposure value and the initial potential of the plate to be developed, a new potential is obtained which is equal to the potential resulting from the exposure of the plate, and this potential is applied to the development electrode and to the plate during the development. Control of the duration of exposure, based on the voltage contrast between the exposed and unexposed areas of the plate to be developed is not envisaged. Rather, the potential of the latent image on the printing plate is registered and, by means of the signal corresponding to this potential, the conditions which are necessary for generation of the image are controlled. Thus, there is follow-up control after the measurement until the desired set values have been reached.
The known methods and devices share the common feature that the predetermined duration of exposure does not adequately take into account any possible changes in the light-sensitive layers from one carrier to another and there is no control executed via the residual charge of the exposed layer of the carrier or of the exposed plate.