Degrees Kelvin

[Rob C]

Just when you thought it was safe to think of yourself as cutting edge, in touch with the very edge of the envelope, it turns out to have been another simple case of classical self-deception.

I had opportunity today to peruse my wife´s chemical dictionary (Oxford Dictionary of Chemistry) and, as one is inclined to do in idle moments, I turned to things photographic.

To my surprise and considerable alarm I discovered that our ´modern´ tools such as monitors, digital cameras etc. are, in fact, very old hat, obsolete even. I quote:

Kelvin. The former name ´degree kelvin´ became obsolete by international agreement in 1967. The unit is named after Lord Kelvin (1824 - 1907).

Hmmm... makes you think from a fresh perspective, non?

Ciao - Rob C

[digitaldog]

Photographers have been taught for years that tungsten film has a color temperature of 3400K (Kelvin; a unit of temperature). Lower Kelvin values appear more red, and as the Kelvin values get higher, the color becomes more blue. In actuality, a color temperature is a range of colors correlated to the temperature of a theoretical object known as a blackbody radiator. The blackbody reflects no light and emits energy in shorter wavelengths as it is being heated. Imagine a black cast iron pan on a very hot stove. As it is heated, it begins to glow dark red. As the temperature increases, shorter wavelengths are emitted, causing the color of the light emitted by the skillet to appear orange, then yellow-white, then blue-white. The tungsten filament of a light bulb behaves similarly to the blackbody and radiates energy in the form of light because its temperature is so great (around 3200K).

It's somewhat dangerous to use color temperature to define what you want because the reality is if all light sources were true blackbodies a particular color temperature would produce the same color of light. Because natural materials are not theoretical blackbodies, heating them to a specific temperature creates deviates from the theoretical color from magenta to green. It's really much safer to use the term correlated color temperature (CCT) because many colors of white may correlate to the same blackbody color temperature. Different illuminants can have the same correlated color temperature.

 This is one reason why the CIE defined the Standard Illuminants. These illuminants are defined spectrally meaning a certain amount of energy at each wavelength across the spectrum. This is an exact and non-ambiguous description of color. D65 is an exact color, it is not a range of colors. If you have a color meter that reports color temperature of a light source many light sources that appear different could read the same, that's kind of a problem!

[Rob C]

Andrew

I remember the blackbodied radiator well from schooldays - it gave rise to numerous jokes which I have mercifully forgotten; aren´t you all lucky people!

Ciao - Rob C

[Martin Ocando]

If I remember well, Kelvin is not longer used to determine temperature of liquids or gases, is just Fahrenheit or Celsius. But for color temperature is the unit of measure.

Don't know why, really.

[Steve Stayton]

Kelvin is not an obsolete measurement of temperature. In fact it is one of the seven base units in the well established worldwide system of measurement: the SI System.

For a good summary of the SI see:
http://physics.nist.gov/cuu/Units/units.html

Kelvin is used as a logical measure of the color of a blackbody radiation source (the "color temperature") because it is a unit of absolute temperture where zero degrees represents the minimal state of thermal activity.

I would agree with Andrew Rodney that Correlated Color Temp. is a much better measurement for the color of a typical light source since there are very few true blackbody radiators around. The sun is close to being a blackbody source over the visible spectrum but as photographers know, light from the sun is modified drastically by the earth's atmosphere into all sorts of constantly changing interesting colors that are not accurately described by color temperature.

[bjanes]

Kelvin is not an obsolete measurement of temperature. In fact it is one of the seven base units in the well established worldwide system of measurement: the SI System.

For a good summary of the SI see:
http://physics.nist.gov/cuu/Units/units.html

Kelvin is used as a logical measure of the color of a blackbody radiation source (the "color temperature") because it is a unit of absolute temperture where zero degrees represents the minimal state of thermal activity.

I would agree with Andrew Rodney that Correlated Color Temp. is a much better measurement for the color of a typical light source since there are very few true blackbody radiators around. The sun is close to being a blackbody source over the visible spectrum but as photographers know, light from the sun is modified drastically by the earth's atmosphere into all sorts of constantly changing interesting colors that are not accurately described by color temperature.
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Of course, correlated color temperature is also expressed in Kelvins, and is not that accurate as you move away further from the Planckian locus of a black body. The standard way to express illuminants is with a spectral power distribution as Andrew pointed out.

This [a href=\"http://casa.colorado.edu/~ajsh/colour/Tspectrum.html]link[/url] shows the spectrum of D65, a black body at 5780 K (the surface temperature of the sun), and the actual spectrum of the sun as viewed from space.

[Colorwave]

One thing that I learned in my other hobby, growing live coral in a saltwater reef aquarium, is that any color temperature measurement is merely an average.   Coral is highly discriminating in the spectrum it finds useful to grow, so it is critical to provide the right light source for it's energy.  Two artificial light sources can read the same color temperature in various systems, but have radically different wavelength distribution between them and look quite different to the eye.  A spike in an extremely narrow band of the spectrum can influence the overall color temperature numbers without influencing the majority of the visible spectrum our eyes see, something that artificial light sources are notorious for.  The spectral distribution of artificial light sources is at least as varied and wide ranging as 3-D plots show our ICC profiles to be.
 
Sometimes, the more you learn, the more you realize how simplistic and limited our means of quantifying something tend to be, and how sophisticated and adaptive our sensory organs are.  No matter what it is reading, one number cannot ever tell the full story, just as you would not assign a single number to describe the characteristics of a specific paper, output or capture device.  Everything has a complex, fingerprint-like signature.

-Ron H.

[DaveCurtis]

Imaging stars with your camera is a fun way to see the color of stars/gas.

You can defocus your lens slightly which helps see the color better as you are then not just looking at a point source but a blurred disk.

Here are some stars which range from red to blue-white (cool to hot):

Betelgeuse 3500K reddish
Aldebaran 4000K
Sirius 10000K
Rigel 11000K
Saiph 26000K bluish

[RogerW]

As a Physics teacher (now retired) I wonder what all this is about.  Yes, Kelvin is the SI unit for temperature - so what?  On with taking photographs.

[Jonathan Wienke]

As a Physics teacher (now retired) I wonder what all this is about.  Yes, Kelvin is the SI unit for temperature - so what?  On with taking photographs.

Perhaps the discussion would make more sense if you actually read the first and second posts. Kelvin temp is how white balance is described in most RAW converters...

[Rob C]

Perhaps the discussion would make more sense if you actually read the first and second posts. Kelvin temp is how white balance is described in most RAW converters...
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Thank you, Jonathan, for the general refocus! This little thread was started as a wee joke about my wife´s chemical dictionary and its definition of degrees kelvin, no more no less; oh well, back to pixels then...

Rob C

[CatOne]

Yes, "degrees kelvin" is obsolete; the word now is Kelvin.  Degrees is not included as it's a well recognized scale.

I guess I'm too young.  I got my BS in Physics in 1992 (so, gasp, I'm under 40!).  This means I never knew it by any other name -- I always recognized (or was taught, I guess, at some point) that the specifier "degrees" was inappropriate to me.  And it sounds weird.  Like hearing an LA person talk refer to a freeway as "the 5" or "the 10."  

[Robert Roaldi]

The "degrees" part is only obsolete as regards the use of the unit in scientific discussions and in publications that adhere to the usage of SI units (pretty much all of them). In everyday parlance, it's a temperature scale and using "degrees" is perfectly ok. I still occasionally use ASA instead of ISO with no harm done to anyone.

[Rob C]

Two things: ASA is always going to be my name for it - ISO seems ever so much part of what´s politically correct, in some strange way, designed to offend on-one, change for change´s sake (to me).

The other thing is that, whilst it may feel strange for Angelinos to refer to their roads with the use of a definite article, in other, less - here we go again - newspeak countries, it is pretty well de rigueur to use language in that manner. It might not sound quite right in song, but out of that context of suspended thought it is different. One wouldn´t say I´m going north today on M1, it would always be the M1. That´s not to say that it would, of itself, be a brilliant idea. You would run into over-congestion and many dreaded white-van men. In fact, you could argue that the entire concept of going northwards is  not a really thrilling idea, perhaps why I decided to live much further south.

But really, being under forty is no crime - to tell you the truth, it would be much nicer going back there than going north.

Rob C