# Ricco's law

Several laws describe a human's ability to visually detect targets on a uniform background. One such law is Riccò's law, discovered by astronomer Annibale Riccò.[1] This law explains the visual relationship between a target angular area A and target contrast C required for detection when that target is unresolved (that is, is too small in the field of view to make out different parts of it).[2] It is given by:

${\displaystyle \mathrm {C} ={\frac {K}{\mathrm {A} }}}$

where K is a constant (for a given background, see below).

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-minute in the center (the fovea) 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 (or the number of photons per second) required to lead to the target being detected is summed over the area and is thus proportional to the luminance and to the area.[3] 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.

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[4] that for unconstrained vision (that is, observers could either look directly or at the target or avert their gaze) an accurate empirical formula for K is

${\displaystyle K=(c_{1}B^{-1/4}+c_{2})^{2}}$

where c1, c2 are constants taking different values for scotopic and photopic vision. For low B this approximates to the De Vries-Rose Law[5] for threshold contrast C

${\displaystyle C\equiv {\frac {\Delta B}{B}}\propto {\frac {1}{A{\sqrt {B}}}}.}$

However, at very low background luminance (less than 10−5 candela per square metre) the threshold value for the illuminance

${\displaystyle \Delta I=A\Delta B}$

is a constant (around 10−9 lux) and does not depend on B.[6] In that case

${\displaystyle C={\frac {\Delta B}{B}}={\frac {\Delta I}{AB}}}$

or

${\displaystyle K={\frac {\Delta I}{B}}.}$

At high B such as the daylight sky, Crumey's formula approaches an asymptotic value for K of 5.1×10−9 or 5.4×10−9 lux per nit.[7]

## 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. [1]