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

Figure 5.3. Typical gamma curve.
Optical Performance 109
between the number of shades of grey and the contrast ratio. This
relationship is:
Number of SOG = [log(C r)/ log(�2)] + 1 Equation 5.9
The addition of the 1 takes into account the first luminance level (grey
shade). This can be illustrated by considering the number of SOG in a
scene which is of uniform luminance, i.e., the minimum and maximum
luminances are the same. For this special case, the contrast ratio is 1/1 or
1, and using Equation 5.9:
Number of SOG = log(C r)/ log(�2) + 1
= log(1)/0.1505 + 1
= 0/0.1505 + 1 = 0 + 1
= 1,
which means that a scene of uniform luminance has one grey shade. Table
5.3 shows SOG and corresponding contrast ratios.