The results of two types of visual detection experiments are presented and discussed. One set of experiments was done using single bar targets optically superimposed on a variety of backgrounds. The experiments were designed to determine functional relationships between the target contrast at the detection threshold and a number of variables. The variables include bar angular width, bar angular length, background noise level, and viewing distance. The second set of experiments was done using radiographs of aluminum wires. The radiographs were produced using a conventional neuro-angiography system with a 0.38 mm focus. The aluminum wire diameter at the detection threshold was determined as a function of geometric magnification for several screen-film combinations. The aim of the experiments is to determine how to calculate visual signal-to-noise ratios. Some progress has been made toward this goal.