Abstract: Disclosed is a reflecting wavelength plate that deflects and reflects a light path and adds a phase difference with respect to plural incident light having different wavelengths. The reflecting wavelength plate includes a substrate; a reflecting film laminated on the substrate; and a sub-wavelength concavo-convex structure that is laminated on the reflecting film and has a pitch less than or equal to the shortest wavelength of the plural incident light. The filling factor and the groove depth of the sub-wavelength concavo-convex structure are determined so as to add the phase difference obtained by (k?)/8, where k is an integer, to the plural incident light having the different wavelengths.

Abstract: An image forming method and an apparatus therefor applicable to a color copier. A signal value representative of a reflection and appearing when the output of a photosensor is minimum in response to a change in the amount of toner deposition on a photoconductive element is detected. This maintains the density of a predetermined reference pattern used to sense an image density constant despite the change in, for example, the reflection characteristic of the photoconductive element. A bias for development is so changed as to maintain the output of the photosensor constant, so that the effective bias remains constant relative to the charge potential on the photoconductive element.