Considering the Surround

When viewing images, the relative luminance of the surround has a profound impact on the apparent contrast of the image. For this reason, photographic transparencies intended for projection in a darkened room are produced with an objective contrast substantially higher than that necessary for optimum reproduction as prints viewed in an illuminated surround. The dark surround causes the image elements to appear lighter and this effect is stronger for darker colors resulting in a loss in perceived contrast. This effect is also of great importance in device-independent color imaging since resultant images might be observed in a wide variety of media and viewing conditions. Research on psychophysical scaling of brightness and lightness and the effects of background and surround relative luminance on lightness and chroma is reviewed. The importance of this research for device-independent color imaging systems is described along with the prediction of these effects using the RLAB color-appearance model. Finally, experiments testing the use of RLAB and other color-appearance models in cross-media color reproduction applications are described. INTRODUCTION If the perception of lightness were a function of a single physical variable such as relative luminance, then the tone-scale reproduction of images in various media would be greatly simplified. One would only need to reproduce the relative luminances of the original on the reproduction to assure that the perceived lightness was accurately reproduced. Instead, the perception of lightness depends on several physical variables including the absolute and relative luminances of the stimulus, the background, and the surround (as well as several others). These interactions can be described more simply by stating that lightness depends on the viewing conditions as well as the relative luminance of the stimulus. Thus it becomes necessary to attempt to derive psychophysical relationships between the perception of lightness and this array of physical variables. Such relationships become of critical importance when one attempts to reproduce the lightness scale of an image (or original scene) in a disparate set of viewing conditions. As an example, consider the comparison of a photographic print viewed in typical illuminated surroundings and a photographic transparency projected in a completely darkened room. If the transparency were produced such that it had the same relative luminances as the print for each image element, then it would appear to be of substantially lower contrast than the print. This is because the dark surround of the projected transparency produces a decrease in perceived contrast. …