Illumination system for copying apparatus

An illumination system for a photographic copying apparatus for mixing light from two sources (1a, 1b), which illumination system comprises disposed along a longitudinal optical axis (5) a light-mixing mirror (16) which has as the light-mixing face (16a) a generally sulcated face having grooves (17) therein of triangular cross-section and raised portions of triangular cross-section separating the grooves such that alternate facets (17a, 17b) are parallel, said two light sources (1a, 1b) being diametrically opposed relative to the longitudinal axis (5) and located below the sulcated face (16a) and each being adapted to direct light onto said light-mixing mirror (1b) from an angle (.beta.) relative to said longitudinal axis (5) which is twice the complement of the angle (.alpha.) made by one of the sets of said parallel facets (17a, 17b) relative to the longitudinal axis (5), the general plane of the sulcated face (16a) of the light-mixing mirror (16) being disposed normal to the said longitudinal axis and the grooves (17) therein having smooth polished reflecting surfaces (17a, 17 b) and extending normal to a plane containing both light sources and the longitudinal axis (5), light from the two light sources striking the light-mixing mirror (16) and being reflected therefrom substantially along the longitudinal axis.