<title>Luminance contrast and color recognition in helmet-mounted displays</title>

Abstract

Although luminance contrast requirements for legibility of text are fairly well established, luminance contrast requirements for helmet-mounted display (HMD) color recognition are not. This is a significant issue for color HMDs, such as Helmet-Mounted Sight Plus (HMS+), because the symbol colors will sum with the outside scene, causing them to shift from their intended CIE chromaticity coordinates. Obviously, if the HMD luminance contrast is `sufficient', the colors will remain recognizable--but how much is sufficient? We generated representative outside scenes on a graphics system with superimposed Visually- Coupled Acquisition and Targeting System (VCATS) symbology, using the color code that can be generated by HMS+. For simplicity we used only the target designator box (TD box) from VCATS, and varied the luminance contrast in incremental steps from low to high until the observer could identify the color of the symbology. Further, to test luminance contrast color requirements against legibility requirements, observers then attempted to correctly identify the number at the top of the TD box (target degrees before break lock). If unsuccessful, the luminance contrast was raised until the observer could correctly identify the number. We discuss results in terms of luminance contrast requirements for color recognition as well as legibility.