The present invention relates to an electrostatographic apparatus such as an electrostatic copying machine.
In a dry process electrostatic copying machine a light image of an original document is focussed on an electrostatically charged photoconductive member to produce an electrostatic image through localized photoconduction. A dry toner substance is then applied to develop the electrostatic image and produce a toner image which is transferred and fixed to a copy sheet to provide a permanent reproduction of the original document.
To prevent white background areas of the original document from appearing gray in the copy, a bias voltage is applied to a developing unit which typically comprises a magnetic brush. The bias voltage is slightly lower than the potential of the background areas of the electrostatic image on the drum. In addition, to produce copies of proper density and contrast the light image intensity must be optimally controlled. This has been accomplished in the prior art by means of visual evaluation of the copies and manual control of the bias voltage and light image intensity. The latter may be controlled by adjustment of the intensity of illumination of the document or the width of an exposure aperture.
However, the problem is compounded by the fact that different original documents have different values of density and contrast. Proper setting of the bias voltage and light image intensity after visual evaluation of the document can only be accomplished by a highly skilled operator. An unskilled operator must make several trial copies and progressively adjust the settings until a satisfactory copy is obtained. Such a process is wasteful and time consuming.
To overcome this problem and provide automatic control of bias voltage and light image intensity, several attempts have been made heretofore. One prior art expedient is to increase the scanning stroke of the optical system so that the optical density of part of the document is sensed prior to imaging. The bias voltage and light image intensity are then set based on the sensed density and maintained constant during imaging. This system is disadvantageous in that the increased scanning stroke substantially increases the length of time required for the copying operation.
Another prior art proposal is to sense the optical density of a leading edge portion of the document during a first portion of the imaging operation and then set the light image intensity and bias voltage for the remainder of the imaging operation. Whereas this system does not increase the time required for copying, it is disadvantageous in that the density and constrast of the copy change abruptly at the point where the light image intensity and bias voltage were set.