For symbology to be viewed in a see-through helmet-mounted display (HMD) or head-up display (HUD), the luminance of the symbology must be sufficient to distinguish it from the see-through background. This is true whether or not symbology is displayed on a monochromatic or full-color display. When the contrast of the transparent symbology is sufficiently high, the symbology appears as an overlay on the ambient scene. In order for an HMD or HUD to be usable in an operational environment, the luminance requirements must take into consideration the type of displayed imagery (e.g., symbology, situational maps, target sights), the tasks (e.g., targeting, navigation, obstacle avoidance), the operational setting (e.g., day/night, terrain features), additional hardware (e.g., visors, windscreens, laser protection), and other considerations. For a color HMD, symbology color overlaid on an ambient scene should consider luminance and color contrast, which requires information about the spectral content of the landscape or ambient scene. However, optical designers must consider all terrain features when designing a see-through optic as the military can be deployed to any geographical location around the globe; this fact may simplify the development of luminance requirements for see-through, full-color HMDs. The reason that this will simplify the development of a contrast requirement is that luminance contrast, and not color contrast, is the determining factor.