How can you have a scene linear reflectance greater than 100%?

Started 3 months ago | Questions thread
Mark Scott Abeln
Mark Scott Abeln Forum Pro • Posts: 18,137
White is white except when it ain’t

Mandem wrote:

Does this not mean that there is some standard against which it is being measured? Perhaps the average illuminance of the scene? Say we have 2 areas in a frame, one significantly more illuminated than the other and both of these areas have an 18% middle gray card in them. Of course the 18% gray card in the strongly illuminated area will reflect in ABSOLUTE terms much more relative to the 18% gray card in the less illuminated area but proportionally they're both reflecting equal amounts. So how would we go about deciding which 1 is actually an 18% reflectance. This is getting quite messy and confusing the more I think about it.

There is a human physiological or psychological phenomenon called “lightness constancy”, which assigns an unvarying tone to our mental models of objects. A white object, in our mind’s eye, is always white and we always expect it to look white, and it almost always appears to be white, and are very surprised when it doesn’t.

An 18% gray card reflects 18% of the visible light falling on it, but how much light is falling on it? That varies, wildly. Direct sunlight might be a thousand times brighter than dim living room illumination at night.

Exposure and metering theory and standards typically *assumes* that the light falling on the scene is completely uniform, even if it isn’t.

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