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

Started 3 months ago | Questions thread
Jack Hogan Veteran Member • Posts: 7,873
Clouds, Snow and Mixed Reflectance

alanr0 wrote:

Mandem wrote:

I' m gonna try and explain this conundrum with an example. Imagine we have a frame where we are OUTSIDE and we can see:

-The sun at noon(Incident)


-A caucasian person(reflects)

-White Paper(reflects)

-18% middle gray card(reflects)

Now in many measurements the clouds(especially ones close to the sun) will have a reflectance % significantly above 100%, whereas white paper will be somewhere 90-100%

What makes you say that the clouds will have reflectance greater than 100%? As you point out, the total light scattered cannot exceed the total light incident.

In practice reflectance comes in three flavors: diffuse, specular and mixed - mixed being a combination of the other two.

'Properly' capturing mixed reflectance is one of the reasons why I think Ansel Adams put mid-gray in the middle of Zone V (L*50 in L*ab, out of L*100 with 100 representing maximum diffuse white), leaving 4 full zones above that before saturation in Zone X. Otherwise 2 Zones of headroom to the top of Zone VII would have sufficed to capture all diffuse reflections (L*50 is centered around 18% of max diffuse white linearly, 2.5 stops below L*100).

I like clouds, I think those with texture often add value to a composition and I strive to capture the detail in them (within reason). I often find that clouds in my alpine landscapes contain some mixed reflectance above 100% diffuse white, as do distant fields of snow for instance.

The reason is what Alan alludes to: more or less crystallized droplets of water which intersperse specular reflectance in an otherwise diffuse surface, interacting with it. In those situations I typically have to dial in about a stop of negative Exposure Compensation to capture the amount of detail I like. Adams was generous adding a margin of 2 stops beyond diffuse white to get to full saturation in Zone X. Then again film's non-linear response was inexpensive in the highlights.


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