Surfaces with low reflective values require more light than those with high reflective qualities if the luminance is to be equal. Therefore, calculational procedures based upon the light reaching a surface will give little indication of how well a task can be seen. The eye receives bundles of light from objects of different luminance in space and must accommodate to these luminance differences by an averaging process. Comfort and visibility are dependent upon the luminance patterns within the visual field. Visibility is affected by the adaptation state, quality of luminance patterns, and clutter within the visual field.
Clutter can create an overload of the visual system due to excessive luminance ratios.
Contrast Contrast is the basic seeing mechanism in vision. Contrast threshold is a measure of the ability of an observer to distinguish a minimum difference in luminance between two areas a given percentage of the time. The ability to see surface contrast is affected by physical contrast and the surrounding luminance, L s . L t , the task contrast will be lost and the target will go into silhouette. L t , the task contrast may not be affected by excessive luminance but may create discomfort, and hence a reduction in visibility.