Quantitative relation between detectability and noise power

The threshold contrasts of disc-shaped signals of a wide range of diameters have been determined. The signals were projected optically onto a set of backgrounds consisting of visible random fluctuations in luminance (random noise). The power of the noise was either the maximum available, or reduced to a quarter, a sixteenth or zero by reducing the amplitude of the fluctuations in luminance. A constant mean luminance of 7 cd m-2 was maintained for each background by the addition of an appropriate level of uniform luminance. The power of the full noise background has been measured and for a given signal the threshold contrast has been shown to be proportional to the standard deviation of the noise in the background, making an allowance for noise arising within the visual system. Substituting measured values of noise power into a modified version of an existing model of visual detection, the threshold contrasts have been predicted and have been shown to agree closely with those observed.