For the titled purpose from practical viewpoint, DQE was resolved into two factors, primary quantum efficiency F and noise figure p ; DQE= F/p ² where p represents the extent of noise increase within the system, and F is the rate of photon use. By plotting the maximum DQE values of various systems of a F -1/ p ² plane, characteristics of chemical and physical imaging systems has been distinctively depicted; while the former are associated with low values of F except AgX, most of the latter suffer from low values of 1/ p ² due to noises involved in the permanent image formation. Physical systems, which are based on reversible photo response, can further be classified by the permanence of the image recorded. Comparison of the vidicon and electrophotography disclosed an intrinsic conflict involved in the tone reproduction by the latter. This conflict arises from the combination of the linear photoconductive response with the image reconstruction by light absorption.