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 (700) on the light receiving surface (125), wherein the image comprises a primary image region (410) comprising the information for a user and a secondary image region (720) different from the primary image region. The detector is arranged to detect the optical power of the secondary image region of the image. Some embodiments relate to a system controlling the image brightness or colour balance of the holographic projection.