The effect of the density of background elements on the conspicuity of objects

Engel's concept of conspicuity is formalised to define a conspicuous object as one that will, for a given background, be seen with certainty within a short observation time regardless of the location of the object in relation to the line of sight. Thus for a defined level of probability of seeing and given observation time, the angle of eccentricity at which the defined level of probability of seeing occurs becomes the measure of the conspicuity of the target. Using this measure the conspicuity of disc targets has been determined when the targets are presented in complex backgrounds made up of random arrays of discs. A first experiment determined thresholds for discrimination of size differences and luminance differences for two discs since this was regarded as the elementary task in the detection of the target within a complex background made up of elements of the same shape. In the main experiment the task was the detection of a target disc located in a background consisting of randomly arranged discs. The target disc differed from the background discs in either size or luminance or both. The background array had three densities of background element. When the target discs differed in luminance from the background discs an increase in background density made the target disc less conspicuous but changing background density had no effect on the conspicuity of target discs that differed in size from the background discs. It was found that the task of detecting a luminance difference when there were only two discs (first experiment) was easier than detecting a target disc in the random arrays. However, the detection of a disc of different size in the random arrays appears to be no more difficult than the elementary task of comparing size differences between two discs. This suggests that the visual processes for detecting a target in a complex background by size difference are different from those for detecting the target by luminance difference.