How can 1000 K be warmer than 5000 K ?

[Nikkor Shooter]

The scale used to measure colour temperature — or more precisely
energy levels — is the Kelvin scale. OTH, the scale used to appreciate
colours tones is artistic, read cultural, at human scale.

When art was describing colour tones, there were no measuring scales
to use and lord Kelvin was not yet born. In art, warmer tones refer to
the human experience associating the "warmer" colours of fire and the
heat radiating from it and blue to cold. So red is artistically warmer than
blue… technically, it is the other way around.

In the visual spectrum, from red to violet, the warmer tones are seen
at the lower part of it and the colder ones like violet at the higher end.





The same visual spectrum is an integrated part of the Kelvin scale and
is a very narrow band in it.





Reds are measured as cold in K temperature in the visible spectrum,
they extend to so called infra red and are felt as hot by the skin. The
higher end — from blue to violet — are warmer on that scale though
artistidally described as cooler.


Photography apps have a problem: "How to set up the WB slider direction?"
Measure (in K) and expression (in art) are going in different directions.

The dilemma was solved with using the Kelvin scale — from left cold to right
warm — for precise references — and the art definition or description of what
warmer tones are — from cold to warm. Confusing.

[Ilkka Nissilä]

The color temperature of a light source in Kelvins refers to the temperature of a black body that emits that spectrum of light.

https://en.wikipedia.org/wiki/Black-body_radiation

Light consists of photons, which are elementary particles with energy E =h*c/lambda, where h = Planck's constant, c = speed of light, and lambda = the wavelength of light.
It turns out that light that we perceive as blue has low wavelength and hence higher energy. To radiate this kind of light the black body has to be hotter (higher K temperature). High temperature means the particles inside the black body move faster and are more energetic, and so is the emitted electromagnetic radiation.

I'm not sure why red light is perceived as "warmer". Since the sun is around 5800K in temperature, its output corresponds approximately to the spectrum of black-body radiation of 5800K. Such a spectrum contains very little blue light in comparison to the red light. Thus if we filter the light so that only blue is left, we don't feel much warmth. 

Another explanation is that the blue light is scattered from the surface of the skin (or if it does enter the cells, it is absorbed quickly near the surface of the skin) whereas the red and near-infrared light travels longer distances, deeper into the tissue. This means that more heat is absorbed by the interior of the tissue where the temperature increase can be sensed by cells that monitor the body temperature and transfer the information to the brain which then means we "feel" it.

[Nikkor Shooter]

I'm not sure why red light is perceived as "warmer".

I wanted to keep my explanation at a level most can understand as I wanted
to make a point between WB in K terms and the different felt artistic terms.

[Asle F]

I'm not sure why red light is perceived as "warmer". Since the sun is around 5800K in temperature, its output corresponds approximately to the spectrum of black-body radiation of 5800K. Such a spectrum contains very little blue light in comparison to the red light. Thus if we filter the light so that only blue is left, we don't feel much warmth. 

I have no problem understanding the blue light as perceived cold. In my part of the world, in this part of the year. Blue light is the light from the cold, sunless sky. And the red light is the light from any hot surface, particular the fire place.
The human brain has no natural experience with lightsources with temperature above the sun, so that part of the spectrum isn't assosiated with the light emmiting from a black body. It is just the color of the sky 2 hours after sun set…

[Bill De Jager]

I'm not sure why red light is perceived as "warmer".

It's due to the vagaries of how light interacts with the earth and the realities of human bodies living on earth.  Direct sunlight (~5000K) when the sun is in an elevated position is white, and usually feels warm (though atmospheric conditions may interfere with the perception of warmth at times).  However, at times white can be cold, as in snow or a winter sun lacking warm.  So white doesn't create a consistent impression of warmth in our experiences.

Meanwhile, fire (very roughly 900-1400K) feels far hotter than the sun due to the inverse square law.  So the red, orange, and yellow prominently given off by fire are associated with warmth.

In contrast, skylight can range from 10,000K to 30,000K but usually involves cooler perceived temperatures on the skin than being in direct sunlight.  Water tends to look blue and usually has a cooling effect.  The green of vegetation is more likely to be associated with coolness than warmth, whether by providing shade or through evaporative cooling.

So it's quite reasonable for a person experiencing the world through their senses to feel that red, orange, and yellow are warm while green and blue are cold, quite contrary to the emissions behavior of a black body.

[Frank Fremerey]

it is not "degrees Kelvin". It is "Kelvin".

[Nikkor Shooter]

it is not "degrees Kelvin". It is "Kelvin".

I know Frank. My daughter typed text and I told her to correct
that. She did but forgot the title… thank you!

[JohnMM]

it is not "degrees Kelvin". It is "Kelvin".

"kelvin" rather than "Kelvin". Lots of sources.

[Ann]

It is all related to the heating of black bodies:

http://www.birket.com/reading-room/technical-library/lord-kelvin-never-saw-the-light-understanding-color-temperature-tom-king-2012/

Kelvin temperatures were originally stated in Degrees °K but that was changed to the more realistically named 'Kelvin Scale' in the 1950s

[Bill Mellen]

Seems to be an incongruence between perception and physically measured values 

[Nikkor Shooter]

Seems to be an incongruence between perception and physically measured values 

Artistic licence of artistic perception.

[Bill Mellen]

Makes you wonder if what we see is what we get? 

[Bill De Jager]

I have no problem understanding the blue light as perceived cold. In my part of the world, in this part of the year. Blue light is the light from the cold, sunless sky. And the red light is the light from any hot surface, particular the fire place.
The human brain has no natural experience with lightsources with temperature above the sun, so that part of the spectrum isn't assosiated with the light emmiting from a black body. It is just the color of the sky 2 hours after sun set…

Looks like I somehow missed Asle's post which essentially said the same thing as mine.

[Asle F]

Seems to be an incongruence between perception and physically measured values 

Hot/cold and high/low temperature are not the same…

[mxbianco]

"kelvin" rather than "Kelvin". Lots of sources.

Not really sure about this, AFAIK when a unit name is derived from the surname of a scientist (Ampere, Joule, Volt, Watt, Pascal, Curie, Ångstrom, Kelvin...) it keeps  the capital. Other uits of measurement do not necessarily have a capital letter (meter, gram, second, hour, ...)

Ciao from Massimo