A display device for use in ambient illumination, the display device comprising: a spatial light modulator arranged to output light; an output polariser arranged on the output side of the spatial light modulator, the output polariser being a linear polariser having an electric vector transmission direction; and an output diffuser structure arranged on the output side of the output polariser, the output diffuser structure comprising first and second structured output layers (608, 610) arranged on the output side of the output polariser, the first structured output layer being on the output side of the second structured output layer and having an output surface on the output side, and the first and second structured output layers (608, 610) comprising first and second transparent materials (601, 603) that have an interface surface therebetween, at least one of the first and second transparent materials (601, 603) being a birefringent material having an optical axis that is aligned parallel or orthogonal to the electric vector transmission direction of the output polariser, wherein: the output surface of the first structured output layer has a first surface relief profile; the interface surface has a second surface relief profile; the first surface relief profile and the second surface relief profile have the same, aligned shapes but with a relative scaling in amplitude along an axis normal to the plane of the output polariser so that the amplitude of the first surface relief profile is less than the amplitude of the second surface relief profile; for light output from the output polariser, the refractive index of the first transparent material is greater than the refractive index of the second transparent material, said relative scaling and said refractive indices of the first and second transparent materials (601, 603) are selected so that the output diffuser structure introduces no net angular deflection of light rays passed by the output polariser along an axis along a normal to the plane of the output polariser. a spatial light modulator arranged to output light; an output polariser arranged on the output side of the spatial light modulator, the output polariser being a linear polariser having an electric vector transmission direction; and an output diffuser structure arranged on the output side of the output polariser, the output diffuser structure comprising first and second structured output layers (608, 610) arranged on the output side of the output polariser, the first structured output layer being on the output side of the second structured output layer and having an output surface on the output side, and the first and second structured output layers (608, 610) comprising first and second transparent materials (601, 603) that have an interface surface therebetween, at least one of the first and second transparent materials (601, 603) being a birefringent material having an optical axis that is aligned parallel or orthogonal to the electric vector transmission direction of the output polariser, wherein: the output surface of the first structured output layer has a first surface relief profile; the interface surface has a second surface relief profile; the first surface relief profile and the second surface relief profile have the same, aligned shapes but with a relative scaling in amplitude along an axis normal to the plane of the output polariser so that the amplitude of the first surface relief profile is less than the amplitude of the second surface relief profile; for light output from the output polariser, the refractive index of the first transparent material is greater than the refractive index of the second transparent material, said relative scaling and said refractive indices of the first and second transparent materials (601, 603) are selected so that the output diffuser structure introduces no net angular deflection of light rays passed by the output polariser along an axis along a normal to the plane of the output polariser. the output surface of the first structured output layer has a first surface relief profile; the interface surface has a second surface relief profile; the first surface relief profile and the second surface relief profile have the same, aligned shapes but with a relative scaling in amplitude along an axis normal to the plane of the output polariser so that the amplitude of the first surface relief profile is less than the amplitude of the second surface relief profile; for light output from the output polariser, the refractive index of the first transparent material is greater than the refractive index of the second transparent material, said relative scaling and said refractive indices of the first and second transparent materials (601, 603) are selected so that the output diffuser structure introduces no net angular deflection of light rays passed by the output polariser along an axis along a normal to the plane of the output polariser.