Device and method for producing a number of copies by electrophotographic techniques

摘要

A device for electrophotographically copying wherein for writing a charge pattern into a suitable photo-conductive layer with differing electron mobility and hole mobility a relatively long wave length of light is used which has a penetration depth comparable with the thickness of the layer and which has a quantum energy which corresponds to the energy of deep traps for the less mobile kind of charge carriers. Due to the long life of the charge carriers in the deep traps, the newly applied charges of opposite polarity remain on the exposed locations for a relatively long time since the charge has been caught in the traps whereas the unexposed locations are repeatedly charged again to maintain an undisturbed potential such that the charge pattern is retained for a longer time even with continued exterior charging. So as to erase the charge pattern a correspondingly light beam of shorter wave length is utilized which has a penetration depth that is small relative to the thickness of layer so as to cause the deep traps to be quickly removed through recombination of charges.