The copying processes of the charge transfer type where in the latent electrostatic image formed on the photoconductive layer is transferred to the second receptor by the electrical discharge across the air gap can be devided into the sequential and the simultaneous charge transfer processes. The important parameters for these processes are the capacitances of the photoconductive and dielectric layers and are expressed in the term of the air-equivalent dielectric thickness for the analysis. The effects and the contributions of these parameters are discussed to obtain the optimum copying conditions for these processes mainly from the practical point of view. The desired characteristics of photoconduclor and toning are described based on the results of the analysis of the characteristics of the latent image formation and transfer with referring to the practical technological subjects for these processes. The sensitometry of the total image reproduction characteristics is, as a summary, described with the aid of the practical electro-graphic (EG) office copier.

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Various electrometers for measuring surface charges on insulating materials have known. In the study of electrophotographic films, however, the instrument must be capable of measuring accurately the rapid decay of surface potential during exposure to light.
From the point of view, electrometers utilizing electrostatic induction are of great value.
Some sort of vacuum tubes or MOS FET having extremely high input impedance are available for this purpose.
In this experiment an operational amplfier named μPC 152 is used for the electrometer input stage.
It has a nominal input impedance fo 10¹³ (ohm) and shows a time constant of about 500 (sec) when connected with the senser electrode system. The surface potential of an electrophotographic film placed opposite to the electrometer probe is indicated on an X-Y recorder connected to μA 741, which amplifies the output of μPC 152. Caliblations is given using a standard electrode in the place of the test sample. Furthermore, the decay rate of surface potential during light exposure is recorded simultaneously with the ligth decay by differentiating the output of μA 741 electrically.
The mechanisms of photoconduction are discussed about test sample on the basis of effective photocurrent which is propotional to the surface voltage decay rate. The error due to finite input time constant and imperfect differentiation are calculated theoretically, showing that under the present measuring conditions their effects are negligible.

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By using transient photoconductivity techniques the transport properties of charge carriers have been studied in a photoreceptior consisting of a multilayer structure (SeTe/Se). The transient current is relatively constant at the initial stage, subsequently starts to increase at a critical time and decreases after arrival at a peak value. Analysis of the current has been made, assuming that there is no potential barrier for carrier injection at the interface between the layers and that both layers are subjected to a uniform electric field devided by their capacities. The analysis explains experimental results to a considerable extent; the transient current essentially depends on drift mobility and fall time of the tail current for each layer alone. A multi layer photoreceptor, also, may serve for measuring drift mobilities by using transient photoconductivity techniques.

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