<p>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 has a waveguide portion. The apparatus further includes a light source of image-bearing light and two diffraction elements. Each diffraction element is configured for propagation of image-bearing light of the light source through the optical element. Each diffraction element is arranged to release the image bearing light from the optical element. The apparatus further includes a switching element for switching the image-bearing light repeatedly between the first diffraction element and the second diffraction element to provide a first visible image or a second visible image to the respective eye of a user.</p>
