- The contrast of the target is very important in determining its detectability for a human observer. Traditionally, contrast can be defined as the absolute difference of the mean of the target minus the mean of the background, divided by the mean of the background. However, measuring the contrast is troublesome, for often the target is blurry or diffused in its background in a way which makes locating the target's boundaries very difficult. Even if the target's boundaries are known, there is still a distribution of intensities inside the target rather than one intensity value which can be defined as "the target's intensity." In , the maximal intensity of the target is chosen to represent intensity, but internal features which contribute to its visibility are not considered. In , the standard deviation of the luminance values is chosen as a measure for contrast, but no separation between target and background is made.