- Recent studies on low-observable target acquisition relate clutter to traditional true- and false-detection performance parameters. This paper presents modified definitions of the target signature and clutter in the image plane and their relation to alternative metrics. Measured detection results for experienced observers in a controlled experiment strongly correlate with target signal-to-clutter ratios calculated for the various images. Empirical false-alarm rates were quite constant as a function of time during the typical observer search process; a sudden decrease to zero occurred at a typical "saturation time" for the standard observer. The rate of false alarms and the saturation time were invariant to the particular signal and clutter signature. The algorithms derived on the basis of the theory of signal detection suggest an effective decrease of the threshold signal with increased observation period and attribute the detection-probability behavior with time to this factor alone. Although the behavior with time differed from that predicted from random-glimpse statistics, an algorithm using that approach is also presented and serves as a numerical approximation. The signal and clutter metrics may be applied as tools to compare and rank target detectability and background complexity in various images.