B0wz3r, your posts are the most informative I've read about CRI and color temp on CPF. very interesting stuff. Thanks.
Again, my pleasure.
Here are a couple of articles I found with a quick search that look like good treatments of the subject.
This one is very extensive, and has a section specifically how CRI is defined and measured. It also talks about the origin of and distinctions between 'warm' and 'cool' tints. While he doesn't specifically address the issue of what is 'neutral', it is clear to me from my reading of the relevant sections, that there is an in-between area that would technically be considered 'neutral'. Interestingly, green falls into this in-between area. Note also, that the only credit I can find on this article is that the author is "Bruce McEvoy", and there is nothing about his credentials. That said, it is an extensive treatment, and of the parts of the article that I am particularly knowledgeable about, they are correct as far as I know, so I am assuming the other things he discusses that I am less informed about are also valid.
http://www.handprint.com/HP/WCL/color12.html
Here is a nice little introductory article about color temperature; not very in depth, but covers all the essentials and is easy to understand.
http://www.colorbasics.com/ColorTemperature/
Note about the McEvoy article (first link); one of the things he mentions is the relationship of depth perception to color perception, and he discusses it at some length. The idea here is that many people tend to perceive 'cool' colors as more distant, and 'warm' colors as closer, so artists have used this to add to the illusion of depth in paintings, by contrasting cool and warm objects. However, he entirely misses an important point about color temperature differences and distance perception, which is related to my mention above about the scattering effect of light in the atmosphere.
Since objects that are farther away have their reflected light traveling through more of the atmosphere, there is a greater scattering effect and since we have to look straight through the atmosphere to see them, they tend to appear more blue (think, "purple mountains majesty"), and darker as well. But for closer objects, there is less scattering, so we tend to see them as warmer, because there is less scattering of the short wavelengths.
Now, this might seem to contradict what I said above about scattering at sunrise/sunset, but this is where the function of the visual system comes into play. Our perception of the color of light occurs through what is called additive color mixing. So if you take three monochromatic lights, one blue, one green, and one red, that are each tuned to the wavelengths that each of the three receptor types are most sensitive to, and then combine them, we will see a white light. This is in contrast to subtractive color mixing, which is where more pigments absorb more and more wavelengths from light, and reflect fewer and fewer wavelengths. So when you mix many different colors of paint, you eventually get black, because the paint is reflecting fewer and fewer different wavelengths.
Back to scattering and perception: Now when we see a given color under 'white' light, what happens is the light we are receiving has a differential wavelength composition, there are more of some wavelengths than others. So when we see a red object under white light, it is absorbing most all other wavelengths except for those that correspond to red. Our receptors respond accordingly; the red cones are strongly stimulated, and the green and blue cones less so, and the result is we see the color 'red'.
Now with respect to things being 'blue' at a distance, there is more scattering of the shorter, blue, wavelengths of light by the atmosphere, but what this does is saturate the atmosphere with that scattered blue light, which makes it the dominant wavelength in that light, so we see it as blue, not as red or green, etc. You can see this in the sunset/sunrise situation; at the horizon, there will be a thin layer of very dark atmosphere from the horizon up to where the sun is, at which point the intensity of the light is enough that the scattering effect of the blue is overcome and we can still see the warmer colors. Directly above this, there is usually a much lighter, warmer band of light extending across the horizon. As you look upward from the horizon, the colors we see will get progressively cooler because of the combination of the scattering/filtering effect of the blue, and the decreasing intensity of the light, which means their is less power in the warmer wavelengths, and so the scattering effect again overpowers the penetration of the longer wavelengths through the atmosphere.
In other words, what is happening, is that effect of subtractive color mixing is having more of an effect on the light than the additive mixing taking place in the light itself, so we see less of the warm colors and more of the blue colors for objects, such as mountains, at a distance.
Anyway, I'm sort of rambling at this point... much of what I've related here is stuff that I put in some of my lectures on color perception in the class on Sensory Perception that I teach. In fact, I will make a couple of PDF's of my slides and notes for those lectures, post them on my Photobucket account, and post the links to them here for you guys to read at your convenience.
I also have a ton of things to do today (don't we all?), so I won't be able to check back in again until later today at the earliest, maybe later tonight, not sure yet.
Regardless, I'm glad you guys are finding my posts on this topic informative and interesting; that's the payoff for us professor types... we're sort of attention junkies... we like it when people like when we're talking about the stuff that interests us personally... makes us feel all warm and fuzzy inside.
I'll post those lectures for you guys in a bit. And thanks for the positive feedback.
Oh, one more thing... UnknownVT here, (Vincent) has done some of the best reviews and posts on this issue of color perception and tint that I've seen here, so look up some of his posts. I haven't seen him post in quite a while, so I don't know if he's still active on the forum or not. (I'd love to see him and Selfbuilt do some collaborative flashlight reviews... that would be the best of both worlds, the technical and perceptual characteristics of a flashlight.)
