An Empirical Formula for Visual Search

This paper presents an empirically derived formula for computing the threshold contrast C for visually detecting distant objects in search situations. The threshold contrast is the value of contrast that produces detection probability 1/2 on a single glimpse of about 1/3 to 1 second in duration. The formula allows one to compute C as a function of background luminance, target angular area A, and an empirically determined constant K. For small A, the formula reduces to Ricco's law, which says that the threshold constrast C, and Area A, are directly proportional for small values of A. The limiting threshold contrast is determined by K. Results of fitting this formula to a set of observational data are shown. The formula provides a useful approximation for determining detection capability for planning visual searches.