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

Started 2 months ago | Questions thread
OP Mandem Junior Member • Posts: 28
Re: Words - why white is white except when it ain’t
1

alanr0 wrote:

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.

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.

So i'm guessing it is indeed "sloppy terminology" and a misnomer when they say "reflectance" . That would make sense as the formula for Reflectance=Reflected Light /Incident Light just doesn't work in any scenario as, like I said previously, it can't go past a 100% unless brighteners are involved.

The talk about diffuse reflectance and specular reflectance is starting to make more sense in the context of the intensity of light(what we'd call candelas/m2 or nits) hitting the sensor and that THIS IS what the "reflectance"(confusing misnomer) of the x-axis is actually referring to that gets converted into a digital code value(The ADC conversion process )

To make an example(and please correct me if I'm wrong. God I hope I'm not wrong... ), say we have a camera outside in an open field, at noon, facing head on a 100% diffuse reflective white paper and a mirror right next to it. Both the paper and mirror have incident light from the sun hitting them. Consequently, both are reflecting a 100% of the light incident on them. The fundamental difference arises in the fact that the diffuse reflective paper is scattering the light everywhere hence resulting in significantly less light intensity hitting the sensor as opposed to the mirror where the light rays are all directional towards the sensor thus a much stronger intensity of light is landing on the sensor. Which finally brings me to the formula:

"Reflectance" = Object Reflected or Incident Light Intensity / 100% diffuse reflected white paper Light Intensity.

Edit:perhaps instead of saying "Light Intensity" it would be more clarifying just to say Luminance (Candela/m2)

Thus, in cases where the light intensity is greater than 100% diffuse white reflector we get above 100% "reflectance" (again awful misnomer) values.

Am I correct? Even if I'm not. I'd still like to thank all of you for taking the time and effort for trying to teach me. It just means something isn't really clicking for me. But I'll still try get to the end of this. If anyone has some other way of looking at it or explaining it. I'm all ears. Thanks

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