The invention relates to a method of exposing films and other light-sensitive materials in an image setter of the internal drum type. According to the invention, a light beam is conducted from a light source with the wavelength lambda 1 to a first exposure point on the light-sensitive material inside the image setter drum via an optical system comprising at least one input end and one output end as well as at least one rotating reflector arranged relatively to the optical output end, the invention being unique in that at least one additional light source generates an additional light beam with a wavelength lambda x which is different from lambda 1, said additional light beam with the wavelength lambda x being conducted via the input end of the optical system to the output end of the optical system, so that at least two light beams with different wavelenghts are conducted geometrically coincidently from the output end of the optical system to an additional exposure point on the light-sensitive material inside the image setter drum in dependence on lambda x or the mutual difference between the wavelengths DELTA lambda .