An optical waveguide system for a head up display, the system comprising first and second optical waveguides (301, 300) mounted such that the inner and outer faces of each of the first and second waveguides are all parallel; the first optical waveguide comprising a first diffraction grating and the second optical waveguide comprising a second diffraction grating the combination of the first and second grating providing pupil expansion in two dimensions, wherein the first and second gratings are arranged to be at right angles to each other such that pupil replication occurs in two substantially orthogonal directions and the first and second diffraction gratings at least partially overlap; an input region of the first waveguide arranged such that light can be projected into the first waveguide within an input angle range to strike the first diffraction grating without passing through the second diffraction grating, wherein the first diffraction grating diffracts portions of light trapped in the first waveguide by total internal reflection to an angle at which the diffracted light is not trapped by total internal reflection and at which the diffracted light propagates out of the first waveguide to strike the second diffraction grating and interact with the second diffraction grating, an output region of the first or second waveguide arranged such that light trapped in the first or second waveguide by total internal reflection and which strikes the first or second grating is diffracted into the input angle range to exit the optical waveguide assembly to an exit pupil without striking the other of the first or second grating; and wherein the pitch of the first grating remains the same as it extends. first and second optical waveguides (301, 300) mounted such that the inner and outer faces of each of the first and second waveguides are all parallel; the first optical waveguide comprising a first diffraction grating and the second optical waveguide comprising a second diffraction grating the combination of the first and second grating providing pupil expansion in two dimensions, wherein the first and second gratings are arranged to be at right angles to each other such that pupil replication occurs in two substantially orthogonal directions and the first and second diffraction gratings at least partially overlap; an input region of the first waveguide arranged such that light can be projected into the first waveguide within an input angle range to strike the first diffraction grating without passing through the second diffraction grating, wherein the first diffraction grating diffracts portions of light trapped in the first waveguide by total internal reflection to an angle at which the diffracted light is not trapped by total internal reflection and at which the diffracted light propagates out of the first waveguide to strike the second diffraction grating and interact with the second diffraction grating, an output region of the first or second waveguide arranged such that light trapped in the first or second waveguide by total internal reflection and which strikes the first or second grating is diffracted into the input angle range to exit the optical waveguide assembly to an exit pupil without striking the other of the first or second grating; and an input region of the first waveguide arranged such that light can be projected into the first waveguide within an input angle range to strike the first diffraction grating without passing through the second diffraction grating, wherein the first diffraction grating diffracts portions of light trapped in the first waveguide by total internal reflection to an angle at which the diffracted light is not trapped by total internal reflection and at which the diffracted light propagates out of the first waveguide to strike the second diffraction grating and interact with the second diffraction grating, an output region of the first or second waveguide arranged such that light trapped in the first or second waveguide by total internal reflection and which strikes the first or second grating is diffracted into the input angle range to exit the optical waveguide assembly to an exit pupil without striking the other of the first or second grating; and wherein the pitch of the first grating remains the same as it extends.