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

Started 3 months ago | Questions thread
alanr0 Senior Member • Posts: 2,484
Luminance rather than reflectance

Mandem wrote:

Again this all comes back to trying to understand why there were values greater than a 100% on an x-axis called "reflectance" In that LutCALC Graph. It just didn't make any sense since, again, you can't get a value bigger than 1 on the formulas Alan posted above. Which is why the idea that "reflectance" actually refers to LUMINANCE is more fitting. When it says 18% reflectance or 90% reflectance or 100% reflectance in that LutCALC graph I believe it's referring to the Luminance of an 18%,90% or 100% diffuse reflector. The fact that it is LUMINANCE is also the reason why, I believe, the values can go above a 100% because certain reflectors(specular or mixed) or even incident light sources due to their directionality can be significantly more intense than a 100% diffuse reflector. Hope this made sense.

Once you accept that the percentage scale represents luminance (or relative exposure), rather than reflectance, you no longer need assume that the entire scene is lit by the same source.

The reason more-or-less logarithmic tone curves are used is because the real world very often diverges from this ideal, and human vision is pretty good at coping with this.

The most likely cause of wide dynamic range in an image is non-uniform illumination.  Either there are light sources in or near to the frame, or parts of the scene are in deep shadow.  A common example is where one wishes to show both the interior of a building and capture an exterior sunlit view through a window or doorway.

Specular reflections contribute too, but in practice we rarely need to reproduce their relative luminance with high accuracy, provided they appear sufficiently brighter than their surroundings.

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Alan Robinson

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