An optical waveguide in which, in operation, image-bearing light propagates therein by total internal reflection, the image-bearing light having a polarisation angle relative to a plane of the optical waveguide, the waveguide comprising at least first and second diffraction regions (14, 16), the first diffraction region being arranged to diffract image-bearing light propagating along the waveguide so as to expand it in a first dimension and to turn the image-bearing light towards the second diffraction region, the second diffraction region being arranged to diffract the image-bearing light so as to expand it in a second dimension and to release it from the waveguide as a visible image, wherein the first diffraction region is formed by a first diffraction grating embedded within the waveguide, wherein the first diffraction grating is configured to, at a first interaction with the image-bearing light impinging from below or above the first grating: diffract a first portion of the image-bearing light such that a polarisation angle of the first portion is rotated relative to the plane of the optical waveguide to direct the first portion towards the second diffraction region; characterized in that the first diffraction grating is configured to: diffract a second portion of the image-bearing light above, respectively below the first grating such that a polarisation angle of the second portion is rotated relative to the plane of the optical waveguide to propagate the second portion along the first diffraction grating, wherein the first diffraction grating is further configured to, at a second interaction with the second portion of the image-bearing light: diffract a third portion of the image-bearing light such that a polarisation angle of the third portion is rotated relative to the plane of the optical waveguide to direct the third portion towards the second diffraction region; and diffract a fourth portion of the image-bearing light below, respectively above the first grating such that a polarisation angle of the fourth portion is rotated relative to the plane of the optical waveguide to propagate the fourth portion along the first diffraction grating, the first diffraction grating having a periodic physical grating structure arranged to present a substantially similar grating profile to the image-bearing light when incident upon the grating from a given angle above or below the grating such that any polarisation changes incurred by the image-bearing light through an interaction with the first diffraction grating when incident thereon from above are substantially equal in magnitude but of opposite sense to any polarisation changes incurred through an interaction with the grating when incident thereon from below, and such that the variation in polarisation of the image-bearing light following interaction with the first diffraction region is thereby reduced and the image-bearing light is thereby diffracted by the second diffraction region with substantially uniform efficiency, increasing the uniformity of intensity of the visible image released from the waveguide. diffract a third portion of the image-bearing light such that a polarisation angle of the third portion is rotated relative to the plane of the optical waveguide to direct the third portion towards the second diffraction region; and diffract a fourth portion of the image-bearing light below, respectively above the first grating such that a polarisation angle of the fourth portion is rotated relative to the plane of the optical waveguide to propagate the fourth portion along the first diffraction grating, the first diffraction grating having a periodic physical grating structure arranged to present a substantially similar grating profile to the image-bearing light when incident upon the grating from a given angle above or below the grating such that any polarisation changes incurred by the image-bearing light through an interaction with the first diffraction grating when incident thereon from above are substantially equal in magnitude but of opposite sense to any polarisation changes incurred through an interaction with the grating when incident thereon from below, and such that the variation in polarisation of the image-bearing light following interaction with the first diffraction region is thereby reduced and the image-bearing light is thereby diffracted by the second diffraction region with substantially uniform efficiency, increasing the uniformity of intensity of the visible image released from the waveguide.