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Originally Posted by
xpatUSA
The site indeed is a very good source of information about light sources.
There are several pages, that can be accessed simply by clicking "next" and "previoius" arrows.
Much of the info is summary in one page, but can have better coverage in another, so for example the difference between light sources below 5000 K and from 5000 K and up are not specified in the page http://www.lrc.rpi.edu/programs/nlpi...eringIndex.asp although they are briefly mentioned:
"For CCTs less than 5000 K, the reference illuminants used in the CRI calculation procedure are the SPDs of blackbody radiators; for CCTs above 5000 K, imaginary SPDs calculated from a mathematical model of daylight are used. These reference sources were selected to approximate incandescent lamps and daylight, respectively."
(My boldfacing)
The difference between blackbody radiators and "daylight" is the spectral distribution, where the blackbody has a smooth curve that is high at low frequencies and falling toward high frequencies, while "daylight" has two "humps" close to each end, with a "valley" between. "Daylight" is considerably stronger in the blue to violet range than the blackbody and weaker in green. Different references are used for the respective colour temperatures. Below 5000 K the reference is blackbody, and from 5000 K and up, the reference is "daylight", an imaginary entity, a construct from a mathematical model. Lamp manufacturers still use not only calculation, but also subjective evaluation for assessing CRI. Test samples, maps of colour patches illuminated by the standard reference and the test source respectively, are observed and compared.
In the camera, as well as in software for RAW processing, the two different spectral curve models are used when setting WB as a K value.
No energy saving lamp, CFL lamp or LED lamp has the properties of the black body radiator. They usually have a CRI below 90. This is very evident if mixing CRI 99 incandescent lamps with FL or LED sources. However, when not mixing light sources, rendition of colours might be more appealing with modern energy saving lamps than with incandescent lamps in spite of their lower CRI, because their spectral distribution may be closer to that of daylight. Therefore, mixing light sources should be avoided. Using only one type mostly produces very good colour rendition in images when WB is set with a reference card.
The K scale can be trusted for WB setting with incandescent sources, but should not be used for other types of lamps.