An assessment of human visual performance for detecting low contrast targets is essential in many army target acquisition applications. We examine the comprehensive Blackwell-McCready (BM) contrast threshold data set that was acquired under carefully controlled laboratory conditions. We also examine the applicability of this methodology for all levels of apparent contrast. A hyperbolic numerical curve fit is made on the measured minimal target/background contrast threshold data as a function of target area and ambient background luminance for a wide range of presentation times. An extensive parametric analysis shows that the largest variations in human performance occur during the time intervals before dawn and after dusk. The ABC law generalizes Ricco's and Blackwell's laws and gives some particularly simple relationships between the relative target/background contrast, target area and ambient luminance, which are useful for military and commercial applications.