A display device arranged to form a first picture that is 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, wherein the first spatially-modulated light corresponds to the first picture, wherein the picture generating unit comprises a controller arranged to control the first spatially-modulated light, a waveguide pupil expander comprising: an input port arranged to receive the first spatially-modulated light from the picture generating unit; a viewing window within which the first picture is viewable; and a pair of parallel reflective surfaces arranged to guide the first spatially-modulated light from the input port to the viewing window by a series of internal reflections therefrom, wherein a first reflective surface of the pair of parallel reflective surfaces is partially transmissive to light of the first colour, wherein the transmissivity of the first reflective surface is non-achromatic and the reflectivity / transmissivity of the first reflective surface changes with distance along the waveguide, and a viewer-tracking system arranged to determine a viewing position within the viewing window, wherein the first picture is formed by holographic reconstruction of a first hologram of the first picture; wherein the controller is arranged to change or recalculate the first hologram in response to a change in the viewing position determined by the viewer-tracking system; and wherein the first hologram is changed or recalculated to adjust a brightness of the first picture. a picture generating unit arranged to output first spatially-modulated light of a first colour, wherein the first spatially-modulated light corresponds to the first picture, wherein the picture generating unit comprises a controller arranged to control the first spatially-modulated light, a waveguide pupil expander comprising: an input port arranged to receive the first spatially-modulated light from the picture generating unit; a viewing window within which the first picture is viewable; and a pair of parallel reflective surfaces arranged to guide the first spatially-modulated light from the input port to the viewing window by a series of internal reflections therefrom, wherein a first reflective surface of the pair of parallel reflective surfaces is partially transmissive to light of the first colour, wherein the transmissivity of the first reflective surface is non-achromatic and the reflectivity / transmissivity of the first reflective surface changes with distance along the waveguide, and an input port arranged to receive the first spatially-modulated light from the picture generating unit; a viewing window within which the first picture is viewable; and a pair of parallel reflective surfaces arranged to guide the first spatially-modulated light from the input port to the viewing window by a series of internal reflections therefrom, wherein a first reflective surface of the pair of parallel reflective surfaces is partially transmissive to light of the first colour, wherein the transmissivity of the first reflective surface is non-achromatic and the reflectivity / transmissivity of the first reflective surface changes with distance along the waveguide, and a viewer-tracking system arranged to determine a viewing position within the viewing window, wherein the first picture is formed by holographic reconstruction of a first hologram of the first picture; wherein the controller is arranged to change or recalculate the first hologram in response to a change in the viewing position determined by the viewer-tracking system; and wherein the first hologram is changed or recalculated to adjust a brightness of the first picture. wherein the first picture is formed by holographic reconstruction of a first hologram of the first picture; wherein the controller is arranged to change or recalculate the first hologram in response to a change in the viewing position determined by the viewer-tracking system; and wherein the first hologram is changed or recalculated to adjust a brightness of the first picture.