Nancy B. Goodman

Human sensitivity to within-page color uniformity

The human visual system is finely tuned to be able to detect moving objects or patterned stationary objects. For a printed page, this translates into an ability to discern both the intended information content and any other spatial variations. Therefore, human sensitivity to spatial variations is an important consideration in determining the image quality of a document. The open literature describes the visual response to neutral lightness variations as a function of spatial frequency, to color variations on a neutral base color and to color differences between solid patches. A complete representation of human sensitivity to spatial color variation is very complex, yet must reduce to these special cases. This paper explores the more general case of human sensitivity to variation about a non-neutral base color, both on intended uniform areas and on real customer images. There is a peak in our sensitivity to lightness variation at about 2 - 4 cycles/degree (about 0.4 - 0.8 cycles/mm at a normal reading distance of 30 cm) for any base color, but the dependence on spatial frequency varies between neutral and non-neutral base colors. The more structure there is in the image, the less sensitive people are to color non-uniformity within the page. Large areas of halftoned low chroma colors are especially stressful because they require uniform printing of small dots in each of several colors and also because people are most sensitive to color shifts in that regime. Several of these effects are illustrated.