2. Relative luminances, references, notion of white

In a room, by definition made up of walls and objects, the luminance pattern is perceived as a pattern of contrasts or brightness ratios. However, in our ordinary visual experience, these brightness ratios are interpreted as ratios of reflectance factors, largely independent of the average brightness level. The surface luminance ratios that can lead to such an interpretation are therefore necessarily limited.

When entering a room, the visual system chooses a luminance reference in relation to which to construct the visual space. This can be a source if it is not dazzling and if it occupies a sufficiently wide angular range in the visual field; a sowing of sources cannot be taken as a reference. The celestial vault or a luminous ceiling can be used as a reference. If a source can be considered as a reference, it's because the visual system has been able to attribute...