Holographic Combiner Design To Obtain Uniform Symbol Brightness At Head-Up Display Video Camera

A typical head-up display (HUD) system incorporates a video camera for recording the HUD symbology and the scene outside the cockpit. When using a HUD video camera (HVC) with a zero-power holographic combiner, the brightness of the HUD symbology seen by the camera changes significantly as a function of vertical field angle because the holographic combiner's reflectance characteristics are angularly sensitive and optimized for the pilot's eye position. A holographic combiner design is presented that overcomes this problem while simultaneously maintaining high reflectance of the phosphor's light to the pilot and high visual transmittance. The combiner contains an additional holographic layer tuned to the blue emission of the P53 phosphor as viewed from the HVC, taking advantage of the HVC's high sensitivity in the blue. The reflectance of the additional hologram is tapered to achieve minimum brightness variation at the HVC. The response of the additional hologram as viewed by the pilot shifts towards the ultra-violet and is thus nearly invisible. Theoretical and measured performance of the combiner are presented.