An enlarger head for multigrade paper is described which comprises a light mixing system for mixing light from two sources.The system comprises disposed along a longitudinal axis a solid light-mixing member of an optically clear material which has as a light-input face a generally planar sulcated face having grooves therein of equilateral triangular cross-section and a substantially smooth light-output face, said two light sources being diametrically opposed relative to the longitudinal axis and each being adapted to direct light onto said light-mixing member from an angle substantially 60.degree. relative to said longitudinal axis, the general plane of said sulcated face of the light-mixing member being disposed normal to the said longitudinal axis and the grooves therein having smooth polished surfaces and extending normal to a plane containing both light sources and the longitudinal axis, light from the two sources passing through the light-mixing member to emerge therefrom substantially along the longitudinal axis.