Atelier Art Classes, Brisbane.
In case anyone reading is from Brisbane, just thought I should mention that Atelier Art Classes is flying me up in April for an intensive weekend colour theory workshop on the 16th-17th - details are on their website. In any case I strongly recommend that you check out the classes at their new studio. The teachers, including Ashtons wunderkind Ryan Daffurn, are all excellent, and they regularly conduct workshops with visiting artists. It's really great to see a school specializing in serious art instruction is now established in my home town.
Non-Linear Brightness and Luminance
As of now I still can't quiet understand one thing: The Conversion You did in Your two tables (for inclination angle and point source distance) from radiance to non-linear brightness.
How did You convert them?
But first let me see if I understood the meanings right now.
Brightness is a human perceptual value and non-linear, compared to and being the "percieved equivalent" of Luminance, which stands for the physical value of radiance. Two different words are used, to clearify that we see slightly different from what we would expect to see from the physical values. A grey surface with 18% radiance for example, appears mid-tone grey to us, although logic would tell us it should be darker according to a value from 0% - 100%. So Brightness is non-linear in relation to Luminance and it's physical radiance values.
Then You said Photoshop works with non-linear Brightness, in other words, the values from dark to light, dim to bright are graded according to human perception.
Then I found one formula You mention on Your site for converting from non-linear Brightness to linear Brightness (non-linear Brightness = linear Brightness * 0.45).
Does that mean linear Brightness = Luminance?
If not, could You please clarify the difference between these three terms?
And how to convert from Radiance to non-linear Brightness?
I'm a little bit confused now about Lightness too. I thought Lightness was the one perceptual equivalent of Brightness in the beginning. Since it's comparing to a white surface. Now I notice You say they are both perceptual values.
Luminance is the physical value of radiance, which can be physically, scientifically measured. (Okay, the others can to, but all basically in relation to this.)
So, Brightness is the perceptual value of light going from dim to bright. The perceptual value of luminance.
And Lightness is the perceptual value (the perception) of how bright any color seems to appear in relation to each other, including grey; generally compared to the "Lightness" of a white surface.