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

Started 3 months ago | Questions thread
alanr0 Senior Member • Posts: 2,484
Definitions, jargon and sloppy terminology
1

Mandem wrote:

alanr0 wrote:

Calculate using any of

  • reflected / incident luminous flux (lm)
  • reflected / incident luminous energy (lm.s)
  • ratio of reflected luminous excitance / incident irradiance (lm/m²).

But using the above 3 formulas we can never get a value higher than 1(100%) for reflectance.

Correct. That is how reflectance works in the real world.

Or are you saying calculate the Reflectance value for the diffuse reflective 100% paper and the reflectance value for whatever object and divide the 2 so:

Reflectance of Object / Reflectance of 100% diffuse reflective white paper

and if it's higher, like a mirror for example, we get supra 100% values? In the case of the mirror 47000(4700000%)

No.

That is not reflectance in the accepted metrological sense. https://www.shimadzu.com/an/service-support/technical-support/analysis-basics/fundamentals-uv/reflectance_measurements.html

If you wish to compare luminance values, divide by some reference level, and call that reflectance, then I can't stop you. However it is not what most scientists, engineers and architects understand by reflectance.

Conventions do vary between fields of endeavour. For instance throughout my career in telecommunications and photonics, it was accepted for "intensity" to refer to power density. It took me a while to appreciate that in radiometry this is called "irradiance", and radiometric intensity is measured in Watts per steradian, not Watts per square metre. The corresponding photometric terms are illuminance and luminous intensity.

In the case of your screen shot, I suspect it is sloppy usage, rather than accepted video terminology, though I could be wrong here. Perhaps spider-mario can comment.

It is worth noting that ITU report BT.2408-4 has lots of percentages, but a quick search found no reference to reflectance greater than 100%. The closest I encountered was a "superwhite" level of 109%, referenced to a 90% reflectance reference.

As far as I can tell, ITU prefer either absolute luminance values (cd/m²) or relative camera exposure, expressed as a percentage.

"Guidance for operational practices in HDR television production" ITU report BT.2408-4 (03-2021), Appendix 1, page 39.

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Alan Robinson

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