The HUD, or Head-up part of Kaiser's Vertical/Head-up Display, is an avionics device that collimates and projects symbology onto the real world at infinity. It enables a pilot to look through the aircraft windscreen while viewing command and status information without requiring visual accommodation changes. This experiment was undertaken to determine what symbol brightness is required to use the Head-up Display under high background brightness conditions. The anticipated worst situation (other than looking directly into the sun) consists of flight over sunlit clouds or snow, in which case there could be continuing background brightnesses on the order of 10,000 foot lamberts against which the display must be seen. Results indicate that pilots will want display contrasts of at least 20 to 35 per cent, i.e., perhaps 1800 to 3500 ft. L. display brightness reflecting from the HUD combining glass, assuming 90 per cent transmission by windscreen and combining glass and an external background luminance of 10,000 ft. L. The minimum brightness contrast for a barely visible, near-threshold display is on the order of 10 per cent, or 900 to 1,000 ft. L. reflected from the combiner. This will provide an extremely dim display, but one that most pilots can be expected to see more than 90 per cent of the time. (Author).