The present invention relates to a copier capable of specifying a minimum necessary area of a photoconductive area to be erased in relation to a paper size, document size, trimming size and magnification, so that latent images within the specified area only may be erased to produce a quality copy.
In a copier, it has been customary to reproduce a document by an electrophotographic process which includes steps of charging, exposing, erasing, developing and transferring. During the erasing step, eraser means of the copier performs front end erasure, side erasure and rear end erasure so as to remove needless ones of electrostatic latent images which are formed on a photoconductive element. A typical implementation for erasure control known in the art is optically erasing needless latent images on a photoconductive element based on the size of papers which is sensed by a paper size sensor, which is associated with a paper cassette. Another known implementation relies on a sensor sensitive to the size of a document.
The problem with any of such prior art erasure control schemes is as follows. Assuming that the erasure control is performed on, for example, a paper size basis, where the document size is smaller than the paper size, stains in those portions of a document feed belt, document pressing plate and others which are located outside of the document area are formed as images and transferred onto papers to degrade the reproduction. Meanwhile, where the document size is smaller than the paper size, even needless regions of the photoconductive element which will not be transferred to papers are developed by toner, aggravating toner consumption as well as the loads on a cleaning unit.