Suppose that an achromatic target of
angular area A is viewed against a uniform background
luminance B (e.g. a disc of white light is projected on a white screen, or a nebula is seen through a telescope). For the target to be visible at all, there must be sufficient luminance
contrast; i.e. the target must be brighter (or darker) than the background by some amount \Delta B. If the target is at threshold (i.e. only just visible) then the threshold contrast is defined as C = \Delta B / B . Riccò's law states that for targets below a certain size, threshold contrast is inversely proportional to target area, i.e. CA = R for some constant R. Different values of background luminance B will yield different values of R. This can be seen in contrast threshold data for different levels of background luminance, plotted on a single graph as \log C versus \log A. In each case (i.e. for each background B), the threshold curve for small targets is a straight line of gradient −1, i.e. \log C = -\log A + \mathrm{constant} \log (CA) = \mathrm{constant} Targets for which the law holds are indistinguishable from point sources. Reading towards the right of each threshold curve, there is a target size at which the law begins to break down, i.e. the slope deviates from -1. This is called the "critical visual angle". It is the size at which targets may begin to be seen as visibly extended (bearing in mind that the threshold data are averaged from multiple observers, and individual performance may vary). Notice that for any background B, the threshold curve approaches a slope of zero for large target sizes; i.e. the curve is asymptotic at both ends. The "Ricco area" A_R is conventionally defined by the intersection of the asymptotes. The corresponding visual angle, 2\sqrt {A_R/\pi}, is larger than the critical visual angle, but better defined, and sufficiently useful as an approximation of the least size at which an object is expected to be seen as clearly extended, for a given background luminance. ==Physical origin==