Where did the naming of LED colors in "K" come from?

[leehljp]

I have been puzzled by the naming of the colors of LED lighting as 3000K, 4000k, 5000k etc. I know that "K" stands for Kelvin, but I thought that was a "temperature". There is no way that 3000k LEDs reach that temperature, or 6000K lights reach that temp.

So, why are they named in a manner that does not resemble an actual temperature?

I have wondered about this for a long while, so now I am asking the experts!

[LCHIEN]

The naming of spectral peak temperatures is related to what is called black body radiation.

If you heat a black body to some temperature like 3000K it will glow orangish, at higher temperatures like 5000K it will glow whiter and or bluer.
Think of a piece of metal heated very hot, first it gets orange, then it gets red hot, then white hot.

As the temperature measured in K is increased, the peak emission wavelength shifts to lower wavelengths.

Oddly, 3000K is considered a warm color and 5000K is considered cooler which is really opposite the corresponding black body temperature.

The spectral peak emission comes from the black body temperature.
To some extent the color of a tungsten filament heated to a temperature like 3300K gives light with a peak spectral emission we call 3300K.
Note, as you make it hotter, the output in the visible spectrum increases.

So in general the peak spectral output of most lighting is what matches the peak spectral output of the black body.
Regardless of the actual source.


The huge amount of spectral energy in the infrared wavelengths that we can't see is why incandescent lighting is so darn inefficient; infrared is literally heat and all the waste energy goes off as heat.

LED lighting has much narrower bandwidth so much less is wasted as heat. LEDs have actually very narrow UV bandwidths and most are used to illuminate phosphors which are excited and re-emit light over a visible spectrum according to the phosphor mix. That is how they get the peak spectrum at different wavelengths corresponding to what we call the K temperature scale. Note that the phosphors can be kind of peaky and not as smooth as the spectrum from black body radiation and control of those peaks gives rise to what is known as CRI which attempts to measure the spectral smoothness over very peaky lights which if the wavelength is missing can make the items being lit less attractive.
K color temperatures were of course first used to describe the output of tungsten filament incandescent bulbs - basically engineers can modify the thickness and length of the filament to achieve different temperatures at a given power level and affect the coloration of the light (as well as the lifetime - hotter burns out quicker with shorter life). As lighting changed from incandescent to fluorescent to LED the K coloration was kept to help match or compare peak spectral coloration of the light even though the spectrum was different.

[Jim Frye]

Wow! Just Wow. Thanks

[leehljp]

Thank you Loring! This explains a lot to me and fairly simple too.
"If you heat a black body to some temperature like 3000K it will glow orangish, at higher temperatures like 5000K it will glow whiter and or bluer.
Think of a piece of metal heated very hot, first it gets orange, then it gets red hot, then white hot."

To me, It is interesting for the amount of invisible light rays as compared to the visible (to the human eye) and how many/much colors that we cannot see with our eyes.

[leehljp]

I am not a guy that has ever used a forge, but growing up on a farm, I watched my dad and a couple of uncles use an acetylene torch to heat metal, mostly iron or steel, to different stages of color in order to bend or shape it. It wasn't often that I saw them make the metal reach white hot, but they did on occasion.

[cwsmith]

Way to go Loring! We can always count on you to come back with a science-based answer.

About the only thing I knew about light color was from my photography readings or some one I knew told me. Either source, I walked away with the thought that 5,000 Kelvin was basically the color of light taken at the Washington monument on some July 4th of the past. My thought was, "Really?" Okay, but that must have been some special weather, no pollution day, Right? (I wonder what year that was supposed to be and under what conditions and who did the science. Oh well I guesss I wasn't that curious and didn't explore further.

I know that in photography the light color is quite important, and I have some of the new lighting equipment where you can actually dial in the light temperature. Actually I use a small hand-held light from 'Aputure' to dial a light color in order to match an existing light, thus finding what color bulb that is. The Aputure MC will also produce color an with a small app on your smart phone, you can photograph an object and the app will set the color of the light to match. I haven't personally used that feature, but I imagine that would ge pretty neat if you have the need. The MC light is only 2-1/4 x 3-1/2 x 5/8 inches (approx) and is rechargeable. (https://www.aputure.com/products/mc/).

CWS