An apparatus, comprising: an optical engine configured to receive an image from an image source and generate an input optical beam therefrom; a substrate of optical material, said substrate comprising a first part and a second part; two means for diffraction (14a, 14b) configured to receive the input optical beam, wherein the first part comprises one of the two means for diffraction and the second part comprises another of the two means for diffraction; and two further means for diffraction (16a, 16b), wherein the first part comprises one of the two further means for diffraction and the second part comprises another of said two further means for diffraction; wherein the two means for diffraction (14a, 14b) are configured to in-couple, via diffraction, at least part of the input optical beam into the substrate to provide two diffracted optical beams substantially equally divided between the first part and the second part, respectively; wherein the two further means for diffraction (16a, 16b) are configured to out-couple, via diffraction, at least part of the diffracted optical beams in each of the first part and the second part (12a, 12b) out of the substrate to provide substantially identical two output optical beams with an expanded exit pupil in one or two dimensions; characterized in that: the substrate of optical material is a split substrate of optical material wherein the first part and the second part are physically separated and configured to rotate relative to each other around a line which separates the first and the second parts in a predetermined angle range (α), and wherein said two means for diffraction (14a, 14b) are substantially next to each other and adjacent to said line. an optical engine configured to receive an image from an image source and generate an input optical beam therefrom; a substrate of optical material, said substrate comprising a first part and a second part; two means for diffraction (14a, 14b) configured to receive the input optical beam, wherein the first part comprises one of the two means for diffraction and the second part comprises another of the two means for diffraction; and two further means for diffraction (16a, 16b), wherein the first part comprises one of the two further means for diffraction and the second part comprises another of said two further means for diffraction; wherein the two means for diffraction (14a, 14b) are configured to in-couple, via diffraction, at least part of the input optical beam into the substrate to provide two diffracted optical beams substantially equally divided between the first part and the second part, respectively; wherein the two further means for diffraction (16a, 16b) are configured to out-couple, via diffraction, at least part of the diffracted optical beams in each of the first part and the second part (12a, 12b) out of the substrate to provide substantially identical two output optical beams with an expanded exit pupil in one or two dimensions; the substrate of optical material is a split substrate of optical material wherein the first part and the second part are physically separated and configured to rotate relative to each other around a line which separates the first and the second parts in a predetermined angle range (α), and wherein said two means for diffraction (14a, 14b) are substantially next to each other and adjacent to said line.