(1) FIELD OF THE INVENTION
The present invention relates to a driving method and a driving device for selectively driving some of a plurality of light shutter elements together constituting a light shutter array, each of the elements being formed of such material as PLZT having an electro-optical effect.
(2) DESCRIPTION OF THE PRIOR ART
A known light shutter array of the above-noted type using such material as PLZT having an electro-optical effect, comprises a polarizer and an analyzer which are respectively constituted by a polarizing plate and are disposed at a light incident side and at a light emitting side with polarizing angles thereof crossing at a right angle to each other. For a light transmission therethrough, a driving voltage is applied to electrodes disposed at opposed sides of the shutter array, such that the light incident to the shutter through the polarizer undergoes 90 degrees polarization in the shutter so as to be transmitted through the analyzer.
Further, in order to increase a transmission light intensity difference between the ON state and OFF state of the shutter, a half-wavelength voltage for polarizing the light by 90 degrees is employed as the shutter driving voltage applied between the electrodes in accordance with the 90 degree polarizing angle difference between the polarizer and the analyzer.
Then, the inventors of the present invention thought of using such a light shutter array including a plurality of serially aligned light shutter elements in an electrophotographic printer and have conducted intensive research on this application.
In using such a light shutter array in an electrophotographic printer, as shown in FIG. 1, a polarizer 101 and an analyzer 102 are disposed respectively at the light-incident side and at the light-emitting side across the light shutter array 10. A light from a light source lamp 103 and its reflecting mirror 104 is guided through the polarizer 101 to the shutter array 10, in which some light shutter elements are driven depending on the image information so as to permit selective transmission of this light through the analyzer 102. This light through the analyzer 102 is converged by a converging rod lens array 105 to be irradiated onto a periphery of a photosensitive member 107 uniformly charged by a corona charger 106, whereby an electrostatic latent image is formed on the periphery. Then, this latent image is developed through a developing device 108 into a toner image, which image is transferred through a transfer device 109 onto a copy sheet while removing excess toner remaining on the peripheral surface of the photosensitive member 107 by an eraser 110 and a cleaner 111.
In the above-described construction of the light shutter array employed in the electrophotographic printer, the respective shutter elements need to be selectively driven by pulses in order to form the electrostatic latent image on the photosensitive member being rotated at a predetermined peripheral speed.
Thus, the inventors conducted research on ambient temperature variations in a transmission light intensity Ip under the ON state of the shutter and a leakage light intensity Id under the OFF state of the same. As shown in FIG. 2, the research revealed that the transmission light intensity Ip is hardly affected by any ambient temperature variations whereas the leakage light intensity Id increases with a decrease in the temperature and further that this leakage light intensity Id under the OFF state of the shutter significantly increases as shown in FIG. 3 if the light shutter is driven at a temperature lower than the room temperature.
Moreover, the research revealed, as indicated by a broken line in FIG. 4, that the leakage light intensity also increases due to generation of polarization in the respective shutter elements if the shutter array is continuously driven by constantly applying a unidirectional pulsate electric field thereto even if the ambient temperature is maintained constant.