For an image formation apparatus having supplemental light sources in addition to an optical system for providing an image forming light beam reflected from an original to be copied, a control unit controls exposure by these supplemental light sources according to the photosensitivity characteristics of microcapsules of different kinds with which a light-receiving sheet is coated and which respond differently to individual beams from the supplemental light sources. The control unit may also be designed to take into consideration the effects of ambient humidity and the user's choice for particular coloring.