A head-mounted display device comprising: a light source (410; 530; 630) adapted to generate modulated light; a rendering engine configured to generate a hologram representative of a three-dimensional object, wherein the hologram includes depth information such that the three-dimensional object is rendered in use with a focus that is determined by the depth information and wherein the rendering engine is configured to generate the hologram in the form of a phase map; a spatial light modulator (430; 520; 650) that is caused by the phase map to modulate in use light from the light source (410; 530; 630) as indicated by the hologram by modulating the phase of the light to generate a three-dimensional view of the three dimensional object, wherein the phase map comprises a two-dimensional array of phase shifts, wherein each entry within the array is associated with a pixel within the spatial light modulator (430; 520; 650); a switchable transmission hologram (440; 620) comprising multiple stacked switchable gratings, wherein each of the stacked switchable gratings is associated with: a particular emission pathway (600, 610) of the modulated image light which is received from the spatial light modulator (430; 520; 650), and one or more resulting exit pupil locations (710a-710d, 720a-720d) on a viewing surface (420b; 700; 810); an eye tracker configured to track in use the gaze direction of a user's eye and to map in use the gaze direction to a viewing location on the viewing surface (420b; 700; 810); and a processor configured to activate in use a particular switchable grating within the switchable transmission hologram (440; 620) that is associated with a target exit pupil location (710a-710d, 720a-720d) that aligns with the viewing location and matches the gaze direction: wherein each exit pupil location shows a correct image that is specific to the exit pupil location and depicts a virtual, three-dimensional object with a correct depth of field. a light source (410; 530; 630) adapted to generate modulated light; a rendering engine configured to generate a hologram representative of a three-dimensional object, wherein the hologram includes depth information such that the three-dimensional object is rendered in use with a focus that is determined by the depth information and wherein the rendering engine is configured to generate the hologram in the form of a phase map; a spatial light modulator (430; 520; 650) that is caused by the phase map to modulate in use light from the light source (410; 530; 630) as indicated by the hologram by modulating the phase of the light to generate a three-dimensional view of the three dimensional object, wherein the phase map comprises a two-dimensional array of phase shifts, wherein each entry within the array is associated with a pixel within the spatial light modulator (430; 520; 650); a switchable transmission hologram (440; 620) comprising multiple stacked switchable gratings, wherein each of the stacked switchable gratings is associated with: a particular emission pathway (600, 610) of the modulated image light which is received from the spatial light modulator (430; 520; 650), and one or more resulting exit pupil locations (710a-710d, 720a-720d) on a viewing surface (420b; 700; 810); an eye tracker configured to track in use the gaze direction of a user's eye and to map in use the gaze direction to a viewing location on the viewing surface (420b; 700; 810); and a processor configured to activate in use a particular switchable grating within the switchable transmission hologram (440; 620) that is associated with a target exit pupil location (710a-710d, 720a-720d) that aligns with the viewing location and matches the gaze direction: wherein each exit pupil location shows a correct image that is specific to the exit pupil location and depicts a virtual, three-dimensional object with a correct depth of field. a particular emission pathway (600, 610) of the modulated image light which is received from the spatial light modulator (430; 520; 650), and one or more resulting exit pupil locations (710a-710d, 720a-720d) on a viewing surface (420b; 700; 810); an eye tracker configured to track in use the gaze direction of a user's eye and to map in use the gaze direction to a viewing location on the viewing surface (420b; 700; 810); and a processor configured to activate in use a particular switchable grating within the switchable transmission hologram (440; 620) that is associated with a target exit pupil location (710a-710d, 720a-720d) that aligns with the viewing location and matches the gaze direction: wherein each exit pupil location shows a correct image that is specific to the exit pupil location and depicts a virtual, three-dimensional object with a correct depth of field.