Abstract
In military aviation helmet-mounted displays (HMDs) or head-up displays, light from the ambient scene is added to the symbology to create a complex mixture of colors, textures, and luminances. In the case of color mixing, the color of the transparent symbology symbols shifts toward the colors of the ambient background that the symbology overlays. The magnitude of the shift depends on the contrast of the symbology against the background. Against a darkened background, there is negligible shifting of symbology color. However, during daylight conditions, symbology colors shift toward the background hue. Using CIELAB distances between symbology colors as a measure of color discrimination, confusion contrast thresholds are calculated for each of seven symbology colors mixed with fourteen different background colors over a wide range of luminance contrasts. Confusion contrast thresholds are calculated for color normal and color vision deficient (CVD) observers. For CVD observers, colors are filtered using the RGB coefficients developed by Machado. Using the color discrimination data presented here as well as previous assessments of HMD luminance requirements based on observer ratings of the quality of symbology, luminance guidelines for see-through displays are presented, which correct for a calculation error made previously.
Highlights
Helmet-mounted displays (HMDs) provide pilots the ability to maintain a “head-up, eyes-out” awareness of the ambient scene during most flight maneuvers.[1,2] HMDs may provide symbology, other kinds of navigational aids such as synthetic imagery, and/or sensor imagery during day and night operations
HMD imagery must be of sufficient luminance that when combined with the background, it can be seen and interpreted with certain assurance that relevant visual information is communicated without visual ambiguity
Seven symbology colors were mixed with fourteen different background colors over the entire contrast range
Summary
Helmet-mounted displays (HMDs) provide pilots the ability to maintain a “head-up, eyes-out” awareness of the ambient scene during most flight maneuvers.[1,2] HMDs may provide symbology, other kinds of navigational aids such as synthetic imagery, and/or sensor imagery during day and night operations. For see-through displayed imagery to be legible and intelligible, it must be of sufficient contrast to stand out from the background, that is, to appear as an overlay upon the ambient scene. As color coding is introduced into HMDs, it is important to define the daylight requirements that will yield distinguishable symbology colors against any ambient scene. Given the additive nature of HMDs and head-up displays, two important questions regarding color discrimination arise. Under what combination of contrast and/or background conditions results in a symbology color no longer resembling the hue of the displayed color (e.g., what conditions result in a red symbol no longer appearing as reddish in color). The confusion contrast thresholds were used to further address daylight luminance guidelines for seethrough display systems. In the process of development of these guidelines, an error was discovered in Harding et al.[1] and Harding and Rash[2] previous calculations, and that error has been rectified here
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