US Patent 10474229 Folded viewing optics with high eye tracking contrast ratio

An apparatus includes a display configured to emit display light, an optical system configured to provide the display light to an eye of a user and an eye tracking system. The optical system includes a plurality of optical surfaces. The optical system is disposed between an eye tracking light detector and the eye of the user such that a portion of the eye tracking light that is reflected from the eye of the user and is transmitted through the optical system and also reflects from an optical surface of the optical system to generate one or more parasitic reflections of the eye tracking light. At least one of the plurality of optical surfaces is configured to reduce an intensity of the one or more parasitic reflections as measured on a surface of the eye tracking detector.