Contrast
Radiation propagation in the atmosphere

When we observe an object, either visually or with an optical instrument, we perceive it by its apparent luminance. However, this object is never present against a black background, so it stands out against this background, which also has a perceptible apparent luminance.

Given two sources of luminances L ₁ and L ₂ , we can define two quantities characterizing the contrast of source 1 with respect to source 2 : $C=\frac{L_1-L_2}{L_2}\text{et}{C}^{\prime }=\frac{L_1-L_2}{L_1+L_2}$