A display device arranged to form a first picture and second picture that are visible at all viewing positions within a viewing window, the display device comprising: a picture generating unit arranged to output first spatially-modulated light of a first colour and second spatially-modulated light of a second colour, wherein the first spatially-modulated light corresponds to the first picture and the second spatially-modulated light corresponds to the second picture, wherein the picture generating unit comprises a controller arranged to control the first spatially-modulated light and the second spatially-modulated light, a waveguide pupil expander comprising: an input port arranged to receive the first spatially-modulated light and the second spatially-modulated light from the picture generating unit; a viewing window within which the first picture and the second picture are viewable; and a pair of parallel reflective surfaces arranged to guide the first spatially-modulated light and the second spatially-modulated light from the input port to the viewing window by a series of internal reflections therefrom, wherein the reflectivity of a first reflective surface of the pair of parallel reflective surfaces is provided by a graded coating that is partially transmissive to light of the first colour and light of the second colour, wherein the transmissivity of the graded coating is non-achromatic, and a viewer-tracking system arranged to determine a viewing position within the viewing window, wherein the controller is arranged to change the brightness of the first and/or second spatially-modulated light based on the viewing position determined by the viewer-tracking system. a picture generating unit arranged to output first spatially-modulated light of a first colour and second spatially-modulated light of a second colour, wherein the first spatially-modulated light corresponds to the first picture and the second spatially-modulated light corresponds to the second picture, wherein the picture generating unit comprises a controller arranged to control the first spatially-modulated light and the second spatially-modulated light, a waveguide pupil expander comprising: an input port arranged to receive the first spatially-modulated light and the second spatially-modulated light from the picture generating unit; a viewing window within which the first picture and the second picture are viewable; and a pair of parallel reflective surfaces arranged to guide the first spatially-modulated light and the second spatially-modulated light from the input port to the viewing window by a series of internal reflections therefrom, wherein the reflectivity of a first reflective surface of the pair of parallel reflective surfaces is provided by a graded coating that is partially transmissive to light of the first colour and light of the second colour, wherein the transmissivity of the graded coating is non-achromatic, and an input port arranged to receive the first spatially-modulated light and the second spatially-modulated light from the picture generating unit; a viewing window within which the first picture and the second picture are viewable; and a pair of parallel reflective surfaces arranged to guide the first spatially-modulated light and the second spatially-modulated light from the input port to the viewing window by a series of internal reflections therefrom, wherein the reflectivity of a first reflective surface of the pair of parallel reflective surfaces is provided by a graded coating that is partially transmissive to light of the first colour and light of the second colour, wherein the transmissivity of the graded coating is non-achromatic, and a viewer-tracking system arranged to determine a viewing position within the viewing window,