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

Started 3 months ago | Questions thread
alanr0 Senior Member • Posts: 2,484
Re: 100% of what?
1

Mandem wrote:

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.

If you want absolute reflectance of a surface, you measure incident illuminance, and reflected luminance under controlled conditions.

As far as I can tell, video folk appear to use reflectance as a relative measure of image brightness.  For wide dynamic range formats, the levels corresponding to nominal reflectances of 18% grey and 100% grey are both considerably less than the peak image brightness.

spider-mario quoted from one set of recommendations - specifically ITU report BT.2408-4.

If I understand correctly, the calibration is for a signal which produces an output luminance of 203 cd/m² from a wide dynamic range display. This is intended to represent 100% diffuse reflectance from a surface with "nominal" illumination in the region of interest. The actual luminance at the subject will depend on the illumination employed, the camera gain and exposure settings, and the intent of the photographer.

The same section refers to a display with nominal peak luminance of 1000 cd/m², while the introduction discusses "ideal peak luminance" of 10000 cd/m².

Table 1 on page 6 (8 in the pdf) specifies display luminance values for both HLG and PQ formats.

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Alan Robinson

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