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

Started 3 months ago | Questions thread
alanr0 Senior Member • Posts: 2,484
Words - why white is white except when it ain’t

Mandem wrote:

Mark Scott Abeln wrote:

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.

If exposure and metering theory assumes uniform light throughout the scene then the only thing I can understand causing more than a 100% reflectance is like you said previously the conversion of non-visible light into visible light through whatever subtle process. Where can I learn more specifically about this? As most sources online barely ever even mention scene linear reflectance when discussing about dynamic range.

Uniform illumination throughout the scene is a sometimes convenient fiction, which generally will not apply to log gamma encoding and wide dynamic range displays.

In the real world, surfaces with greater than 100% reflectance are uncommon. What does occur is non-Lambertian directional scattering (in particular specular reflection) and especially non-uniform illumination.

Whiter than white objects are possible, but rarely amplify by a factor of 5 or more, unless there is an awful lot of ultra-violet light present.

The various video tone curve standards specify how changes in recorded signal are converted to intensity by a display device. The numbers correspond to relative changes in luminance referred to the maximum output of the display. For reasons best known to themselves, the video community describe these brightness changes in terms of reflectance, even though most variation will arise from changes in subject illumination.

There is no mysterious physical process. It is simply a way to describe how the video signal represents different brightness levels.

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Alan Robinson

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