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

Started 3 months ago | Questions thread
Re: Diffuse = max 100%, Mixed and Specular > 100%, Linearly
1

Mandem wrote:

Jack Hogan wrote:

Mandem wrote:

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.

In certain lighting conditions, independently of absolute radio/photometric levels, the whitest diffuse white possible shows up at 10000 DN in the raw data with the given camera settings. If an 18% gray card is inserted in the scene it will be around 1800 DN. Mixed clouds perhaps at 15000 DN.

So if max diffuse white is considered 100% diffuse reflectance at 10000 DN, it follows linearly that the clouds can be said to provide a mixed response equivalent to 150% of max diffuse reflectance.

I'll try my best to explain it from my pov. Bear with me.

If both the Max Diffuse White, Clouds and the Mirror are lighted by the same light source, say the sun, which is providing an X amount of light, how can their reflectance ever be greater than a 100% as

Reflectance= Reflected Light / Incident Light.

If it was greater than a 100% that would mean somehow, magically, more Lumens are coming out of the objects than are incident on them. Since these are only reflective objects and not light sources, they can only give out at a max amount of light of as much as they get hit with. So they can only reflect a 100% of light.

And yet looking at these 3 objects each of them have different intensities of light hitting the viewers eye. Despite the fact that they're all reflecting all the light that's hitting them. The 100% Max Diffuse is visibly white, the clouds white but certain spots even brighter than the Diffuse reflector, and the mirror blindlingly white and bright still. And yet they can all reflect only the maximum of what's incident on them otherwise that would mean they're magically producing extra light out of nowhere(we're not considering optical brighteners here and remember these are only reflective objects).

So what's happening? They can all reflect a maximum of a 100% light, yet the mirror is blindingly white but the 100% diffuse reflector is much less brighter but still white.Answer is Directionality of the light rays. While the 100% diffuse reflector is indeed reflecting all the light, it's all scattered everywhere, so not all the light rays incident on it are ACTUALLY reaching the sensor. Moving onto the clouds. A huge amount of it is diffusely reflective but a small portion of it due to the arrangement of water molecules is specularly reflecting much more of the light rays incident on it back at the camera sensor(which is why it looks more brighter than the diffuse reflector and the surrounding diffusely reflective clouds and yet any given portion of the cloud can only reflect a 100% of the light incident on it) Finally, the mirror. Positioned correctly, the camera will see the brightest light it will ever see in its lifetime in the mirror,THE SUN. And yet again, the mirror can only reflect a 100% of what is incident on it(just like the diffuse reflector) but this time the light doesn't get scattered everywhere, every single ray that hit the mirror reflected right to the camera. This is why the term luminance makes more sense instead of reflectance as you can have multiple things with a 100% reflectance(never over according to Alans formulas and intuition in general) yet some of them will be a visibly more intense/bright white.

Got it, I should have read your exchange with Alan more carefully.

Yes, of course, in photography there is an explicit assumption that the terms refer to what the eye or the camera see, that's where the steradians come into radio/photometric units.

That's why for instance we never consider the overall amount of energy that the sun generates at any given time to estimate Exposure Values - but only the infinitesimal proportion that reaches our scene.

And the much tinier amount that is reflected into our eyes and cameras - everything is relative to that. Hence the 100% and 150% example above.

Jack

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