Increasing overlap in angular space between footprints 24 advantageously increases the likelihood of seeing a combination of light from the different SLMs 15 from an increased number of viewing angles. Typically, increasing overlap in angular space between footprints 24 additionally assists with reducing speckle. If the optical system (e.g. optical system 17) comprises an asymmetric aperture, light from one or more SLMs 15 will be cut disproportionately more than from one or more other SLMs 15.