An apparatus, comprising: a substrate of optical material having a first surface and a second surface, said substrate comprising a first area and a second area substantially adjacent to each other along a line; two means for diffraction (14a, 14b), disposed on the first or the second surface, configured to receive respectively two sequentially switched input optical beams (32-1b, 32-2b) comprising an optical image of a display such that one of the two sequentially switched input optical beams is received by only one of the two means for diffraction and another of the two sequentially switched input optical beams is received by only another of the two means for diffraction, wherein one of the two means for diffraction is disposed on the first area and another of the two means for diffraction is disposed on the second area, respectively, wherein said two means for diffraction (14a, 14b) are substantially next to each other and adjacent to said line; two further means for diffraction (16a, 16b) disposed on the first or the second surface, wherein one of the two further means for diffraction is disposed on the first area and another of said two further means for diffraction is disposed on the second area, respectively; and means for optical delivery (20,22, 24, 26, 28) configured to sequentially switch said two input optical beams (32-1b, 32-2b) comprising the optical image of the display between said two means for diffraction (14a, 14b); wherein the apparatus is configured such that: at least part of each of the two sequentially switched input optical beams (32-1b, 32-2b) at any time is diffracted only in one of the two means for diffraction (14a, 14b) to provide a diffracted optical beam in the same area with said one of the two means for diffraction substantially within the first and second surfaces, and at least part of the diffracted optical beam in the first or the second area (12a, 12b) is further coupled out of the substrate by diffraction in one of said two further means for diffraction (16a, 16b) for providing two sequentially switched and substantially identical output optical beams (32-1d, 32-2d), each comprising the optical image of the display with an expanded exit pupil in one or two dimensions. a substrate of optical material having a first surface and a second surface, said substrate comprising a first area and a second area substantially adjacent to each other along a line; two means for diffraction (14a, 14b), disposed on the first or the second surface, configured to receive respectively two sequentially switched input optical beams (32-1b, 32-2b) comprising an optical image of a display such that one of the two sequentially switched input optical beams is received by only one of the two means for diffraction and another of the two sequentially switched input optical beams is received by only another of the two means for diffraction, wherein one of the two means for diffraction is disposed on the first area and another of the two means for diffraction is disposed on the second area, respectively, wherein said two means for diffraction (14a, 14b) are substantially next to each other and adjacent to said line; two further means for diffraction (16a, 16b) disposed on the first or the second surface, wherein one of the two further means for diffraction is disposed on the first area and another of said two further means for diffraction is disposed on the second area, respectively; and means for optical delivery (20,22, 24, 26, 28) configured to sequentially switch said two input optical beams (32-1b, 32-2b) comprising the optical image of the display between said two means for diffraction (14a, 14b); wherein the apparatus is configured such that: at least part of each of the two sequentially switched input optical beams (32-1b, 32-2b) at any time is diffracted only in one of the two means for diffraction (14a, 14b) to provide a diffracted optical beam in the same area with said one of the two means for diffraction substantially within the first and second surfaces, and at least part of the diffracted optical beam in the first or the second area (12a, 12b) is further coupled out of the substrate by diffraction in one of said two further means for diffraction (16a, 16b) for providing two sequentially switched and substantially identical output optical beams (32-1d, 32-2d), each comprising the optical image of the display with an expanded exit pupil in one or two dimensions. at least part of each of the two sequentially switched input optical beams (32-1b, 32-2b) at any time is diffracted only in one of the two means for diffraction (14a, 14b) to provide a diffracted optical beam in the same area with said one of the two means for diffraction substantially within the first and second surfaces, and at least part of the diffracted optical beam in the first or the second area (12a, 12b) is further coupled out of the substrate by diffraction in one of said two further means for diffraction (16a, 16b) for providing two sequentially switched and substantially identical output optical beams (32-1d, 32-2d), each comprising the optical image of the display with an expanded exit pupil in one or two dimensions.