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. 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). It is given by:
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. 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 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
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. 
- Riccò A. (1877). Ann. Ottalmol., 6, 373.
- Schwartz, Steven H. (2004). Visual Perception: A Clinical Orientation (3 ed.). McGraw-Hill Professional. pp. 46–47. ISBN 0-07-141187-9.
- Hood, D. C., & Finkelstein, M. A. (1986). Sensitivity to light. In K. R. Boff, L. Kaufman & J. P. Thomas (Eds.), Handbook of perception and human performance (Vol. I: Sensory processes and perception, pp. 5-1 - 5-66). New York: John Wiley.
- Crumey, A. (2014). Human contrast threshold and astronomical visibility. MNRAS 442, 2600–2619.
- Rose A. (1948) J. Opt. Soc. Am., 38, 196.
- Crumey, op. cit., Sections 1.5, 2.1 and 2.3
- From coefficients r4 or from coefficients a3 and a5.