Rotating diffuser for improved image quality

A head-up display comprising:

• a spatial light modulator arranged to display a hologram of a first image;
• a light source arranged to illuminate the hologram displayed on the spatial light modulator to form spatially modulated light encoded with the hologram;
• a scattering diffuser disposed at an image reconstruction plane of the hologram such that an image reconstruction of the first image is formed on the scattering diffuser, the scattering diffuser arranged to diffusely scatter each pixel of the image reconstruction, wherein different pixels of the image reconstruction are scattered differently by the scattering diffuser to create a plurality of different local scattering profiles, each local scattering profile characterised by a shape and orientation, and wherein the plurality of different local scattering profiles combine to create a global scattering profile forming an output of the scattering diffuser;
• a rotation apparatus arranged to rotate the scattering diffuser at a rotation speed such that a signal-to-noise ratio and/or speckle contrast of the image reconstruction of the first image is reduced owing to the image reconstruction forming on different parts of the scattering diffuser within the integration time of the human eye, each different part of the scattering diffuser forming a different speckle pattern across the image reconstruction, wherein the scattering diffuser and rotation are coordinated such that the shape and orientation of the respective local scattering profile associated with each pixel of the image reconstruction is constant with rotation of the scattering diffuser;
• and
• a waveguide pupil expander having an input port coupled to the output of the scattering diffuser, wherein a shape and orientation of the input port of the waveguide pupil expander matches the shape and orientation of the global scattering profile.