A prediction of human visual target detection performance is important for many military and civilian applications. A previous paper by the authors in this journal develops an extensive numerical hyperbolic curve fitting routine for the Blackwell-McCready (BM) laboratory contrast threshold data set. The BM data set is extended to real world scenes by developing a new operational definition of ambient luminance. The process involves an empirically derived formula for computing the diurnal ambient luminance as a function of solar position, time of day and observer location on the earth. An analytical methodology predicts probability of detection Pd for various target areas, contrast levels and background ambient luminance. A subsequent analysis shows that Pd is a very sensitive function of target parameters during the twilight region before dawn and after dusk. Several numerical examples illustrate the utility of this formalism to various target detection applications involving aided and unaided optics.