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

Started 2 months ago | Questions thread
Re: 100% of what?

alanr0 wrote:

Mandem wrote:

spider-mario wrote:

J A C S wrote:

It might have something to do with diffuse vs. directed (specular) reflections.

Indeed. Those reference levels are for 18% and 100% diffuse reflections (from Lambertian surfaces). With such surfaces, as the name “diffuse” suggests, not all the light that lands on a given point from a given source is reflected in the direction of the camera. With specular reflections, it can be. See section 1.3 of BT.2390-9 for some discussion on this. This image from the first link is also a great illustration.

So when we look at the graph where we start dealing with above 100% reflectance, is that just specular reflectance getting translated into it's equivalent in terms of diffuse reflectance. Which, in other words, means that 100% spectral reflectance is the true 100% reflectance of all the light, X, that was incident on the object whereas 100% diffuse reflectance is not?

It does not need to be specular reflectance. It could just be to handle parts of the scene which are more strongly illuminated than others.

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.

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