Luminance contrast is a ratio of two luminances that measures how well something stands out against its background. Specifically, the lower a color's contrast, the more vulnerable it is to fading under g. Luminance converts a measure of light energy into a number that relates to the light's brightness in much the same way that the decibel converts a measure of sound energy into a number that relates to the sound's loudness. In both cases, human sensitivity to different wavelengths is used to yield a number that better describes the human perception. The luminance contrasts that can be produced on a cockpit's display depend on the colors in use.
Color displays produce color by mixing red, green, and blue light.
Green usually has the highest peak luminance and blue has the lowest. So, using a dark gray background, for example, green symbols can be presented at higher contrast than blue symbols, making the green symbols less likely to fade and the blue symbols more likely. Two patches of color having similar luminances, seen side by side, tend to blend together due to their low contrast, as in yellow and green. In other cases, low contrast symbols tend to blend into their background, as with cyan contrasted against a white background. Understanding these effects and the role of display limitations will be important to designers of aircraft cockpit symbology, who use standardized computer symbols and situational indicators.