A "6500K" light source is supposed to be roughly equivalent to sunlight
illumination (there are other conditions that go along with that, but that
will do for now)
I do beg to differ, since sun's surface temperature and color temp.
of direct sunlight outside Earth's atmosphere is more like 5800 K.
Overcast days appear to me to be less bluish than 6500K lamps - I have
liking to consider "typical overcast conditions" to be 6000K even.
5500 is supposed to be some sort of typical of direct sunlight plus "to
relevant extent" light reflected towards illuminmation of
sunlight-iluminated photography subjects by "blue sky" and clouds.
I do sense some mention of to some extent using a "UV/Haze" filter to
attenuate UV to an extent such that spectral response of "daylight color
slide film" after modification by such a filter to result in a spectral
response closer to that of human vision.
and is fairly close to the "equal energy" white (a light
source that's flat in terms of energy across the visible spectrum).
I do find "equal energy/power per unit wavelength version-of-white"
to have "correlated color temperature" of 5455 K or by some accounts
closer to 5400, maybe around 5420 or possibly as low as in the upper
5300's.
Anything lower than that is basically biasing the white more toward the
yellow, and eventually red, end of the spectrum, and will appear
"warmer" (more yellow/orange/red) to the eye. Much above 6500K,
and the light source is turning distinctly blue, and so appears "colder."
Your eye adapts to anything that's reasonably "white," though, if it's the
only (or primary) source in the field of view, so after a while you don't
really notice that anything is wrong (unless you have some other source
to compare colors to).
What about 4100 or for that matter even if that needs 4300-4400 K
("mildly overheated" "cool white" fluorescent lamp") appearing to be
white? And "direct midday sunlight" in Philadelphia and nearby suburbs
appears to me to achieve color temperature mostly 4400-4800 K, but
"sometimes gets as high as 5100" as I see things here. I consider 5200 K
to be an extreme of Washington DC on a favorably clear-air day close to
"high noon"and close to "summer solstice", and 5400 K to be high side of
"direct sunlight" from sun-at-zenith at lower altitude/elevation with
low-side existence of whatever causes haze.
As a result, I expect illumination onto a planar surface that direct
sunlight is perpendicularly illuminating without obstructions to "light
from ther sky" to average at 5500 K, maybe closer to 5775-5800 K in
more-ideal situations of lack of cloud presence with sun at least
30-45-or-whatever degrees above horizon, or-similar...
And "idealized 5500K daylight" still appears to me to be
best-photographed in when the film is "color slide film" or "color movie
film" uf a UV-attenuating filter is deployed in order to make the "roughly
below-440-nm-spectral-response" of the film plus photographinh optics
including filters more like that of human vision. That part does require
attenuation of wavelengths near 400 nm and in the upper and mid-upper
300's of nm (including 350-360 and 390-390's).