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Several Laws describe a human's ability to visually detect targets on a background. One such law is Ricco's Law. As a generalization it states that for small areas of the retina the absolute threshold is inversely proportional to the area stimulated.
This law explains the visual relationship between a target area and target contrast required for detection when that target is unresolved. It is shown below
Ricco's Law is applicable for regions where the target being detected is unresolved. The resolution of the human eye is approximately one arc-second. Thus, Ricco's Law is applicable for targets of angular area less than this size. This region is variable based on the amount of background luminance. Ricco's Law is based on the fact that the 'Signal' of the target being detected is proportional to the threshold contrast multiplied by 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.
- ↑ Schwartz, Steven H. (2004). Visual Perception: A Clinical Orientation, 3, McGraw-Hill Professional.