Optical combiner with uniform illuminance throughput

A device, comprising:

• a display configured to generate a first image light;
• and
• a waveguide optically coupled with the display and comprising:
• an in-coupling element configured to couple the first image light into the waveguide as a second image light that propagates inside the waveguide via total internal reflection;
• and
• an out-coupling element configured with a predetermined diffraction efficiency gradient along a pupil-expansion direction of the device,
• wherein as the second image light propagates inside the waveguide via total internal reflection, the out-coupling element is configured to diffract, in the predetermined diffraction efficiency gradient, the second image light incident onto a plurality of portions of the out-coupling element with the same incidence angle out of the waveguide as a plurality of parallel third image lights.