Up to now brightness was considered only as function of luminance. As luminance is the product of incoming illumination intensity and reflectance (for non-emitting surfaces) the same luminance can be obtained from surfaces having different reflectances by changing the illumination. Franck [Fran94] showed that reflectance influences perceived brightness as well. This is valid only for complex scenes, and his conclusion is based on an experiment. There is still no quantitative data or suggested formulas, but he claims that a white surface will always appear brighter then a black surface of the same luminance when cues to the reflectances are present, as is frequently true in digital image synthesis. This fact is often neglected, but obviously should be taken into account by tone mapping functions.