Abstract: An exposure detecting device for a copying machine having an optical system which produces an image of an original on a photosensitive drum using an image forming lens and at least one mirror. At least one mirror in the optical system is implemented with a multilayer film interference filter which transmits light rays in a spectral range in which said photosensitive body is not sensitive and reflects light rays in other ranges. A photosensor is arranged on the rear surface of the multilayer film interference filter. The output of the photosensor is applied to control the intensity of a light source used for forming the image of the original on the photosensitive drum.