Riccò's law, discovered by astronomer Annibale Riccò, is one of several laws that describe a human's ability to visually detect targets on a uniform background. This law explains the visual relationship between a target angular areaA and target luminance increment required for detection when that target is unresolved. The law is given by: where is a constant. For constant background luminance, the equation can be restated as with a different constant. The fraction is referred to as Weber contrastC. Riccò's law is applicable for regions where the target being detected is unresolved. The resolution of the human eye is approximately one arc-minute in the center but the size 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. Riccò's law is based on the fact that within a receptive field, the light energy required to lead to the target being detected is summed over the area and is thus proportional to the luminance and to the area. Therefore, the contrast threshold required for detection is proportional to the signal-to-noise ratiomultiplied by the noise divided by the area. This leads to the above equation. The "constant" K is actually a function of the background luminance B to which the eye is assumed to be adapted. It has been shown by Andrew Crumey that for unconstrained vision an accurate empirical formula for K is where c1, c2 are constants taking different values for scotopic and photopic vision. For low B this approximates to the De Vries-Rose Law for threshold contrast C However, at very low background luminance the threshold value for the illuminance is a constant and does not depend on B. In that case or At high B such as the daylight sky, Crumey's formula approaches an asymptotic value for K of or lux per nit.
Ricco's Areas
Ricco’s Areas are any areas of the retina where cells can detect a visual stimulus at threshold. The areas will change in location and size depending on the type of cells, the light conditions and the type of stimulus.
