An optical structure for use in an augmented reality display, the optical structure comprising a first waveguide and a second waveguide, wherein the first waveguide comprises: a first input diffractive optical element configured to receive light from a projector and diffract the received light so that diffracted wavelengths of the light in first and second primary colours are coupled into the first waveguide, to be totally internally reflected within the first waveguide, and so that diffracted wavelengths of the light in second and third primary colours are coupled out of the first waveguide in a direction towards the second waveguide; and a first output diffractive optical element configured to receive and diffract totally internally reflected light within the first waveguide in order to couple the totally internally reflected light out of the first waveguide towards a viewer; wherein the second waveguide comprises: a second input diffractive optical element configured to receive diffracted light that is coupled out of the first waveguide by the first input diffractive optical element, and to diffract the received light so that diffracted wavelengths of the light in the second and third primary colours are coupled into the second waveguide, to be totally internally reflected within the second waveguide; and a second output diffractive optical element configured to receive and diffract totally internally reflected light within the second waveguide in order to couple the totally internally reflected light out of the second waveguide towards the viewer; wherein the diffractive optical elements have a period which is the separation of diffractive optical features, and wherein the inverse of the period of the second output diffractive optical element is equal to the inverse of the period of the first input diffractive optical element plus the inverse of the period of the second input diffractive optical element. a first input diffractive optical element configured to receive light from a projector and diffract the received light so that diffracted wavelengths of the light in first and second primary colours are coupled into the first waveguide, to be totally internally reflected within the first waveguide, and so that diffracted wavelengths of the light in second and third primary colours are coupled out of the first waveguide in a direction towards the second waveguide; and a first output diffractive optical element configured to receive and diffract totally internally reflected light within the first waveguide in order to couple the totally internally reflected light out of the first waveguide towards a viewer; wherein the second waveguide comprises: a second input diffractive optical element configured to receive diffracted light that is coupled out of the first waveguide by the first input diffractive optical element, and to diffract the received light so that diffracted wavelengths of the light in the second and third primary colours are coupled into the second waveguide, to be totally internally reflected within the second waveguide; and a second output diffractive optical element configured to receive and diffract totally internally reflected light within the second waveguide in order to couple the totally internally reflected light out of the second waveguide towards the viewer; wherein the diffractive optical elements have a period which is the separation of diffractive optical features, and wherein the inverse of the period of the second output diffractive optical element is equal to the inverse of the period of the first input diffractive optical element plus the inverse of the period of the second input diffractive optical element. a second input diffractive optical element configured to receive diffracted light that is coupled out of the first waveguide by the first input diffractive optical element, and to diffract the received light so that diffracted wavelengths of the light in the second and third primary colours are coupled into the second waveguide, to be totally internally reflected within the second waveguide; and a second output diffractive optical element configured to receive and diffract totally internally reflected light within the second waveguide in order to couple the totally internally reflected light out of the second waveguide towards the viewer; wherein the diffractive optical elements have a period which is the separation of diffractive optical features, and wherein the inverse of the period of the second output diffractive optical element is equal to the inverse of the period of the first input diffractive optical element plus the inverse of the period of the second input diffractive optical element.