D65 and 6500K, what is the difference?

[lcaillo]

There continues to be confusion on this issue, and it becomes clear every time I see someone use the term D6500. This may just be a personality flaw, but it is like fingernails on a chalkboard to me each time I see the term used. It perpetuates the confusion between precise specification of white point and correlated color temperature. I have been accused in the past of "going all Poynton" (a reference to his frustration with the sloppy use of luminance to mean luma) over this rather technical difference, but frankly, that seems like a compliment.

So let me start by saying:

NOWHERE, in any standard for video that I can find , is there a reference to D6500. Period.

So why is it such a big deal? Well, when we talk about the desired white point for video, it is the basis for much of what we do. In most systems that will be D65 for our purposes. The problem is that lots of people have become accustomed to using color temperature to refer to the desired color of white. Color temperature, or more precisely Correlated Color Temperature (aka CCT) is an ambiguous value. If we specify a color temperature that is along the black body curve (Plankian locus) we get to one point, but not precisely D65 (D65 is not exactly on the black body curve) or other illuminant specifications. If we don't add that qualifier or just look at color temperature measures, we could end up with a white that is clearly lacking red or clearly too red. Note the lines for each color temp across the Plankian locus in the chart here:

http://en.wikipedia.org/wiki/Image:Planckian-locus.png

Any of the coordinates along these lines have the same CCT. Many of them are not even close to the white that we want. When we calibrate or evaluate the color of white, we nearly always use the specific coordinates of the color that we want or observe. CCT is commonly used to describe where we are in general, but you have to assume that you are on or near the black body curve for it to be useful. Where it is dangerous is if you try to assume only from CCT that you have the color that you want. It just is not precise.

So where did this term D6500 come from? Well, I think some started using it out of confusion, and since it closely approximates both terms D65 and a CCT of 6500K, many just carelessly began using it to refer to the color that we usually want for white. The unfortunate part is that it keeps people confused on the matter, or at least uninformed. What is worse is that many of the most experienced calibrators, writers, and trainers in the industry have become sloppy and used this term D6500, which I think is best described as video slang.

The difference, to answer the question in the title concisely, is that D65 is the specification for the standard illuminant that we use in many current video specifications these days, and 6500K is a CCT that can represent many sets of coordinates. One is precise, one is not. D65 actually has a CCT of closer to 6504K. This difference is due to a change in the precision of Plank's constant, a change that came after the specification for D65 by CIE.

Is all this really important? Well, it is if you are calibrating a display. It is if you want to know that your white point is correct.

[Alan Brown]

I think 'video slang' is an apt appraisal of the misnomer. Many people learned about Lord Kelvin and the Kelvin scale in Middle School or perhaps later. When Joe Kane and the ISF started reaching out to consumers and the home theater press, they started discussing the importance of "color temperature" and related terms for picture quality in consumer televisions. Initially, they were trying to use terminology that would be more readily understood by the majority of consumers. I remember seeing plenty of references to "6500 degrees Kelvin," which is incorrect as well. Then there was a shift to "6500 Kelvins," for color temperature. Only rarely was the CIE "D" point mentioned. As the home theater community grew, and online technical discussion forums formed, more precise and thorough understanding of what constitutes video performance increased among consumers. The term "6500K" is commonly used in the lighting industry to loosely refer to lamps near CCT 6500K.

Communication is an art. Sometimes technical issues are more easily understood by the masses if less precise terms are used at first. Education is a progressive process that has to start somewhere. "D6500K" appears to be a hybrid term, or shorthand, used by industry professionals, in an attempt to make the reference white point of CIE D65 more readily comprehended by more people. That's my observation and analysis over the last couple of decades.

Best regards and beautiful pictures,
Alan Brown, President
CinemaQuest, Inc.
A Lion AV Consultants Affiliate

"Advancing the art and science of electronic imaging"