An optical structure for use in an augmented reality or virtual reality display, comprising: a waveguide; an input diffractive optical structure configured to receive light from a projector and couple the received light into the waveguide; an output diffractive optical structure configured to receive light from the input diffractive optical element in an input direction, wherein the output diffractive optical structure comprises at least a first diffractive optical element and a second diffractive optical element with different respective diffraction efficiencies, wherein the first diffractive optical element has a relatively high diffraction efficiency and the second diffractive optical element has a relatively low diffraction efficiency and the first and second diffractive optical elements are overlaid on one another in or on the waveguide, wherein the first diffractive optical element is configured to receive light along the input direction and couple it towards the second diffractive optical element which can then act as an output diffractive optical element, providing outcoupled orders towards a viewer, and wherein the second diffractive optical element is configured to receive light along the input direction and couple it towards the first diffractive optical element which can then act as an output diffractive optical element, providing outcoupled orders towards a viewer; wherein the output diffractive optical structure comprises a first portion and a second portion, wherein in the first portion the first diffractive optical element is configured to couple light from the input direction towards the second portion and wherein the second diffractive optical element is configured to couple light from the input direction away from the second portion, and in the second portion the first diffractive optical element is configured to couple light from the input direction towards the first portion and wherein the second diffractive optical element is configured to couple light from the input direction away from the first portion, wherein the first diffractive optical element in the first portion has a grating vector which is substantially parallel to the grating vector of the second diffractive optical element in the second portion. a waveguide; an input diffractive optical structure configured to receive light from a projector and couple the received light into the waveguide; an output diffractive optical structure configured to receive light from the input diffractive optical element in an input direction, wherein the output diffractive optical structure comprises at least a first diffractive optical element and a second diffractive optical element with different respective diffraction efficiencies, wherein the first diffractive optical element has a relatively high diffraction efficiency and the second diffractive optical element has a relatively low diffraction efficiency and the first and second diffractive optical elements are overlaid on one another in or on the waveguide, wherein the first diffractive optical element is configured to receive light along the input direction and couple it towards the second diffractive optical element which can then act as an output diffractive optical element, providing outcoupled orders towards a viewer, and wherein the second diffractive optical element is configured to receive light along the input direction and couple it towards the first diffractive optical element which can then act as an output diffractive optical element, providing outcoupled orders towards a viewer;