A pupil tracking system including a light source for illuminating a viewer's eye, an aperture located to pass light from the light source after retro-reflection from the viewer's pupil, the aperture sized and shaped to pass the light from the light source and block at least a portion of other light arriving at the aperture, and a light sensor for tracking a location of a spot produced by the light from the light source which passed the aperture. Related apparatus and methods are also described.