There is provided a holographic projection system comprising a light receiving surface (125), a light source (110), a spatial light modulator (140) and a detector. The light source is arranged to output light. The spatial light modulator is arranged to receive the light from the light source and output spatially modulated light in accordance with a computer generated hologram addressed to the spatial light modulator to form an image on the light receiving surface, wherein the image (700) comprises a primary image region (710) comprising the information for a user and a secondary image region (720). The detector is arranged to detect the optical power of light travelling to or from the secondary image region of the image, wherein an active light detecting area of the detector and the secondary image region have a corresponding shape. In embodiments, the system allows optical alignment of the holographic reconstruction.