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

Started 3 months ago | Questions thread
Jack Hogan Veteran Member • Posts: 7,873
Diffuse = max 100%, Mixed and Specular > 100%, Linearly

Mandem wrote:

alanr0 wrote:

MrBrightSide wrote:

Those numbers from 0 to 720 look like mV values for the luminance channel. Is that what you’re asking about?

Anything is possible, but the OP is convinced they represent percentage reflectance values - possibly because that is how they are labelled on the screen shot.

The choice of 18% and 90% reflectance values seems significant. These numbers crop up in the ITU documents that spider-mario linked, including ITU report BT.2408-4, where they appear as widely used grey scale reflectance values. See for instance the figure here.

You could check out the gitbub repository for LUTCALC which I believe is the source of the graphic. My guess is careless terminology for relative exposure values, but I have not dug deeper to find out.

Quite the contrary. After our discussion, I am fully convinced the "reflectance" values on Lutcalc are indeed luminance values. The idea of an 18% or 90% is in reference to the luminance of an 18% reflectance gray card or 90% reflectance card. Like you yourself said, you can't have a reflectance greater than 100%.

If you are a practitioner and you've followed posts in this thread, you should by now understand that reflectance comes in three flavors: diffuse, specular and mixed diffuse and specular.

Maximum diffuse white reflection in photography is considered to be 100%. From the ITU link above:

"Reference Level: HDR Reference White (100%) also diffuse white and Graphics White"

However, there is often detail above 100% diffuse white which one may want to capture, as explained in the earlier post on Ansel Adams, clouds and snow. Such detail is typically the result of mixed reflections which live above 100%. In some extreme cases one may actually want to capture the illuminant source itself (say the sun), in which case we would be capturing a signal several thousand % above maximum diffuse white. All this linearly, thanks to the physics of radiometry.

Photography normally works in relative units, that's why many people don't know at what absolute exposure their cameras clip in lx-s - but are interested in where an 18% gray card fits on the histogram in order to choose the appropriate relative aperture and exposure time to capture all the detail they are interested in.

Absolute values of reflected Radiance or Luminance in photography are mainly relevant when perceptual phenomena need to be taken into consideration, say for instance in for adaptation and HDR applications.


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