Michelson Contrast
Description
Michelson contrast (also known as the visibility) is commonly used for patterns where both bright and dark features are equivalent and take up similar fractions of the area (e.g. sine-wave gratings). It measures the relation between the spread and the sum of the two luminances. This method does not care which color is text or background.
\[
\frac{L_{max} - L_{min}}{L_{max} + L_{min}}
\]