It's a wonderful site, but what you quote is one of its weakest parts because these "preferences" might just be based on marketing experience, sales figures, or regional standards.
Regardless, the link shows a
substantial demand for higher CCT lighting which currently just isn't being met by LED manufacturers. Like I said, I think they got so enamoured with being able to produce warm white which is reasonably close to incandescent that they thought their job was done (at least in regards to spectrum). Remember, there's a
huge market for 5000K to 6500K high-CRI fluorescents. Granted, some of this is gimmickry, as in so-called "full-spectrum" lighting to reduce SAD. A lot of though is due to the growing realization that people function best in light resembling the type of light they evolved under.
As the details of the setup and light sources matter, just remembering some isn't enough, sorry. (probably wasn't the light sources that got rated, but the illumination, btw.)
I might have the study on one of my hard drives, along with lots of others. I don't really have time to look for it right now. Funny though how people asking me for sources aren't willing to produce any of their own to contradict what I'm saying. Perhaps it's because such sources don't exist for the simple reason "warm-white" is more an acquired taste rather than something which could be explained in terms of physiology. The Kruithoff Curve is the closest thing I've ever seen to explaining lighting choices, but even here note that it's strictly based of what type of light people find pleasing, not what is best for seeing. Also note that there are huge variations in preferences. At low by today's standards indoor lighting levels of 40 lux the preference ranges from 3000K to 6000K, for example. At somewhat higher lighting levels the lower bound exceeds 3500K. The curve only brackets the typical incandescent CCT range of 2700K to 3000K at around 10 lux. Most indoor spaces are far more brightly than this (i.e. 10 lux is roughly 100 lumens in a 100 square foot room).
I guess we are familiar with your personal lighting tastes, and who would not welcome more choices in higher CCT lighting? I have no problem with your preferences or your extended personal experiences involving an unidentified sample of people and lamps. Anything beyond this, I find the kind of evidence provided of somewhat, um, questionable quality. What you point to, in the NREL link, goes back to this:
Franta, G.; Anstead, K. (1994). "Daylighting Offers Great Opportunities." Window & Door Specifier-Design Lab, Spring; pp. 40-43.
Have you actually read this? I admit I did not, and I intend to search for it as little as I read Anders H.'s posts.
No, I didn't read the paper you mentioned, but that's only one of many sources listed in the paper I linked to. It's not hard to find dozens of sources and also plenty of anecdotal evidence stating that most life forms which evolved in sunlight prefer it over other types of light. Unfortunately, for most of recorded history humans didn't have available to them a light source which could imitate sunlight. First we used fire, and then glowing metal. Neither has existed long enough for humans to adapt evolutionarily to them, particularly because until recently artificial lighting was very expensive, and hence used as sparingly as possible. It's only in the last 70 years, give or take, that we could make fairly inexpensive light sources which could reasonably approximate the CCT of natural daylighting (I'm not counting the arc lamps used possibly as far back as the ancient Egyptians because those were hardly ubiquitous, and the power sources, basically primitive batteries, were huge). Arguably, it's only in maybe the last 25 years or so that we could inexpensively make artificial lighting whose spectrum somewhat resembled sunlight. In short, until about a generation ago, the lighting options which resembled natural lighting either had deficiencies, or simply were too expensive/unwieldy for common use (i.e. arc lamps). For practical reasons then, people stuck with either candles, or starting in the early 20th century, incandescent lamps. While this wasn't enough time to biologically adapt to these sources, it's certainly more than enough time to get used to them, and to even consider them "normal". This I believe explains the reasoning behind the LED manufacturers (and CFL manufacturers also) for pursuing warm white.
As SemiMan said, in the final analysis 2700K to 3000K really doesn't have much of a place when you look at things from a reasoned, rather than an emotional, perspective. Yes, this is what lots of people are used to, but the irony here is this preference is likely only based on about 100 years of artificial lighting. People are very malleable. I'll bet good money most of the demand for "warm-white" comes from the over 40 or 50 crowd who grew up in an era when homes, and even a lot of schools, were lit with only incandescent. Those younger than that have lived under a much larger variety of artificial light sources, and therefore would be at least willing to give something different a try. What's at stake here is that we can save power equivalent to several power plants by just specifying warm white as little as possible. Perhaps in addition to making LED lighting in all common CCTs, there should also be a little information printed on the warm white varieties explaining the energy-use issue and also the biological reasons why cooler lighting might actually be superior. Again, SemiMan is on the right track-a lot more colors just pop under 4000K or 5000K, as opposed to being lost in a haze of yellow. People have to try it and see it for themselves before they actually get it. On a practical perspective, it also makes decorating easier. Colors basically look the same whether it's day or night. I know all too well from the days of incandescents that trying to have any type of decor where the colors look good day and night is next to impossible. You need to pick one or the other, and optimize for it.
Anyway, believe what you will. I'm merely trying to explain here why what the LED industry is doing makes no sense. I'm glad at least that the idea that you always need 90+ CRI is being questioned. I hope the next step is questioning the need for warm-white. The LED manufacturers seem to think sales would suffer if they didn't focus efforts there. I think people's tastes are mostly malleable enough that they wouldn't. The problem I think is too much weight was given to what lighting designers say. I've noticed far too many lighting designers still regard incandescent as the ultimate light source. We need to question and study that assumption very carefully.
Of course, you can use red LED plus phosphor white and get high CRI and efficiency, though not the deep reds.
