Bright light from one region, against a bright background, will have a retinal effect lowered by the bright context. Its final effect might be like its effect if the illumination level fell, so it was now providing less light, but so was its background. 26.Gelb , using concealed illumination, showed that the introduction of a bright patch , revealing the actual strength of the concealed illumination in part of a field, allows us to adjust outr perception of the lightness of other patches in the field. Some argue this bright patch acted as a cue, allowing better inferences about the other patches. Some argue the ratios operate mechanically, at a low level in the perceptual process, before inferences are possible.
Colour constancy is maintained by adaptation to the overall bias in a colour-tinged illumination.
Also, ratios of wavelengths in adjoining regions in a scene are likely computed by the visual system. Some contend familar colour can affect perception.