Originally Posted by Doug Blackburn
Quoting color temperature is essentially a worthless specification for bias lighting (or for video displays). Color temperature is primarily influenced by red and blue. You can subtract or add HUGE amounts of green and the color temp will barely change.
That is why you ALWAYS want to specify d65 as the bias lighting specification. 6500K (actually 6504K) measured color temperature can have a lot or very little green, there is (almost) nothing about 6500K that controls how much green is in the light (and visible to your eye). So your "6500K" lights from some source other than Cinemaquest/Ideal-Lume are a total crap-shoot as to how close they come to the d65 reference point.
6500K is an essentially worthless calibration target/spec because it doesn't much control the amount of green in the light. d65 is a single specific point in color space that can only be attained when red, green, and blue are all properly balanced.
d65 means that the light being produced/measured is roughly 72% green wavelengths, 21% red wavelengths, and 7% blue wavelengths. To the human vision sysem, this looks fairly white under most conditions (though our visual system can be fooled by quite a few things... if you surround d65 with light-bright-blue, d65 will look yellow because of the way our vision system works, for example). If you remove some green wavelengths, the light will still measure 6500K as long as you maintain the (aproximate) 21:7 ratio of red to blue light (3:1). So if you remove half of the green wavelengths so you measure only a total of 36% green light, as long as red and blue maintain the 3:1 ratio, the light will still measure 6500K. In this example, that would mean 36% green light, 38% red light, and 26% blue light (percentages rounded for illustration purposes). Obviously this second example isn't going to look white... it will be very magenta looking because there's not enough geen wavelengths combined with the red and blue -- but it will still measure 6500K. Anything labeled "6500K" by the seller, could range from being MUCH too green to MUCH too magenta and still meet the 6500K "spec".
That's the fallicy of buying "6500K" bias lighting from some "general" supplier who doesn't really understand (or care about) your application versus purchasing a product developed by and quality-controlled by a someone IN the home theater industry who IS concerned about the end result you achieve(like Cinemaquest/Ideal-Lume).
Doug brings up some important points regarding theory, but referring to color temperature has legitimate value in the consumer world, as opposed to the professional arena. Such terminology allows many consumers to quickly differentiate between very large differences in lighting color quality when sorting through all the options on a store shelf. It should also be kept in mind that there are two contexts in which the term D65/d65 can be used. The CIE D65 specification applied to daylight simulators in the lighting industry refers to a spectrum, not just a white point. That application has very broad tolerances when it comes to white point. In the motion imaging industry, D65 refers to a much more precise white point, with comparatively narrow tolerances, and less concern with the overall spectral power distribution. These issues are rather complex for the average consumer or video hobbyist to comprehend, and even many industry professionals still get confused about them. Most people have been taught a little about Lord Kelvin in school, and some may remember why the Kelvin scale refers to both temperature and color. Far fewer consumers have any familiarity whatsoever with CIE chromaticity and "D" points, let alone terms like "spectral power distribution" or "metamerism." It's for these reasons that the lighting industry, and to some degree the video industry, use color temperature terminology, and simplified tolerance qualifications like color rendering index.
In practice, the use of 6500K color temperature in consumer lighting specifications and marketing is even less precise than Doug describes. This is especially problematic in consumer LED product claims and literature. My company has sought a suitable LED replacement for fluorescent lamps for many years. We have seen references here in this forum and elsewhere in the market to LED products claiming to be 6500K, or D65, for years. In almost every case, we have purchased samples for testing to see if such products could be added to our store. We have yet to find any consumer LED solutions that meet the right color requirements for correct video bias lighting. In almost every case, LEDs sold as 6500K are far too blue. They typically start around 8000K or even higher. I have come to suspect that the Chinese lighting manufacturers just lump all blue-ish looking lighting into a general category they call "6500K."
My company has dedicated its efforts over the last 14 years to develop products and solutions that address the requirements of what Joe Kane refers to as "the ideal viewing environment" for correct video systems. Along the way we have learned many valuable things about color quality and human color perception. Our professional bias lighting product has become a world wide video industry reference, being used by experts such as: NIST, Technicolor, Deluxe, THX, ILM, ISF, Dreamworks Animation, Dolby Labs, etc . The consumer models we offer have been found suitable for many professional applications as well. We have seen repeat orders for the consumer products from the likes of Technicolor, Deluxe, Universal Studios, etc. Our goal was to simplify the process of locating, configuring, and implementing correct bias lighting for video systems that strive for reference image reproduction.
We are advocates, along with the founders of this forum, for imaging science principles and practical solutions. There is much confusion and misinformation in the consumer market regarding what composes video best practices and viewing quality. Viewing environment conditions are just as important to picture quality and the viewing experience as any other factor in program reproduction. Getting professional results justifies the extra effort and attention to detail.
Best regards and beautiful pictures,
G. Alan Brown, President
A Lion AV Consultants affiliate
"Advancing the art and science of electronic imaging"