Several laws describe a human's ability to visually detect targets on a uniform background. One such law is Riccò's law. This law explains the visual relationship between a target area and target contrast required for detection when that target is unresolved. It is given by


Riccò's law is applicable for regions where the target being detected is unresolved. The resolution of the human eye (the receptive field size) is approximately one arc-second in the center (the fovea) and increases in peripheral vision. Riccò's law is applicable for targets of angular area less than the size of the receptive field. This region is variable based on the amount of background luminance

Luminance

Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...

. Riccò's law is based on the fact that within a receptive field, the light energy (the number of photons) required to lead to the target being detected is summed over the area and is thus proportional to the area. Therefore, the contrast threshold required for detection is proportional to the signal-to-noise ratio multiplied by the noise divided by the area. This leads to the above equation.