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

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

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.

I am having difficulty understanding the issue you are exposing.

Say that in certain lighting conditions, independently of absolute radio/photometric levels, the brightest diffuse white possible in nature shows up at 10000 DN in the raw data with the given camera settings. If an 18% mid-gray card is inserted into 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.

Most subjects that photographers wish to retain detail in are diffuse reflectors, so below 100% reflectance.  Perhaps it would be a good idea then to choose Exposure and camera settings that place max diffuse white at 100% of full scale

Yet Ansel Adam's teaches us that it is wise to maintain some headroom for non-idealities and subjects with mixed reflectance. Which is why no camera I know has metering that puts middle gray at 18% of full scale in the raw data, most put it below the teens. And even then.


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