An apparatus including a first substrate including a first incoupling diffractive optical element configured to couple light into the first substrate, and a first outcoupling diffractive optical element configured to output, from the first substrate, light that has been coupled into the first substrate; and a second substrate including a second incoupling diffractive optical element configured to couple light into the second substrate, and a second outcoupling diffractive optical element configured to output, from the second substrate, light that has been coupled into the second substrate; wherein the first and second incoupling diffractive optical elements are substantially inverse of each other and the first and second outcoupling diffractive optical elements are substantially inverse of each other.