Helmet-Mounted Displays: - USAARL - The - U.S. Army

Figure 5.2. A cyclical luminance pattern.
Optical Performance 107
As the ratio of L t / L b increases, the significance of subtracting the value of
1 becomes meaningless and Equation 5.8 takes the form of Equation 5.2a,
that of contrast ratio.
By comparison, if, as in Figure 5.1c, the target luminance (1 fL) is
lower than the background luminance (L t < L b ), the calculated value for
contrast (Equation 5.1b) is:
C = (L b - L t) / L b = (20 - 1) / 20 = 19/20 = 0.95
and, the calculated value for contrast ratio (Equations 5.2b and 5.2c) is:
C r = L b / L t = L max / L min = 20/1 = 20.
Note: The equation for contrast ratio is defined always by the ratio of the
greater luminance to the lesser luminance.
Values for modulation contrast for the luminance patterns of Figure
5.1 generally are not used. However, consider the luminance pattern in
Figure 5.2. This pattern consists of a series of light and dark bars. While