A head-mounted or helmet-mounted display apparatus is provided. The apparatus includes an optical element which in use is disposed in front of the eyes of a user. The optical element comprises a waveguide portion. The apparatus further includes a first light source and a second light source of image-bearing light. The apparatus further includes first diffraction element for propagation of image-bearing light of the first light source through the optical element. The apparatus further includes a second diffraction element for propagation of image-bearing light of the second light source through the optical element. The first light source and the second light source is disposed such that relative angular movement of the first light source and the second light source about an axis of symmetry of the optical element adjusts the inter-pupillary spacing of a first visible image and a second visible image presented to a respective eye of a user.