Waveguide pupil expansion

A method of controlling light propagation in a light engine, to form an image visible from a viewing window, wherein the light engine comprises a display device, a waveguide pupil expander and a viewing system comprising the viewing window, wherein the method comprises:

• displaying, on the display device, a hologram of the image;
• illuminating the display device to spatially modulate light in accordance with the hologram, wherein the hologram is configured to angularly distribute spatially modulated light of the image in accordance with position of image content, such that angular channels of the spatially modulated light correspond with respective continuous regions of the image;
• arranging the waveguide pupil expander to receive the spatially modulated light and to provide a respective plurality of different light propagation paths for the spatially modulated light from the display device to the viewing window;
• and
• controlling propagation of the plurality of different light propagation paths using a control device disposed between the waveguide pupil expander and the viewing window, wherein the control device comprises at least one aperture;
• wherein the step of controlling propagation of the plurality of different light propagation paths comprises configuring the control device so that a first viewing position within the viewing window receives a first channel of light spatially modulated by the hologram in accordance with a first region of the image and a second viewing position within the viewing window receives a second channel of light spatially modulated by the hologram in accordance with a second region of the image.