An eye tracking system for detecting position and movements of a user's eyes in a head-mounted display (HMD). The eye tracking system includes at least one eye tracking camera, an illumination source that emits infrared light towards the user's eyes, and diffraction gratings located at the eyepieces. The diffraction gratings redirect or reflect at least a portion of infrared light reflected off the user's eyes, while allowing visible light to pass. The cameras capture images of the user's eyes from the infrared light that is redirected or reflected by the diffraction gratings.