Lord Kelvin “Never Saw the Light”

BY T. King, 2011

Color temperature is measured in degrees Kelvin (degrees K). but William Thompson Lord Kelvin did not create this term in reference to the chromaticity of light. Why is his name associated with the science of light? Read on.

First let’s introduce a new term (to some readers). It is a “black body”.

A black body is a theoretical object that absorbs 100% of the radiation that hits it. Therefore it reflects no radiation and appears perfectly black

As the term “theoretical’ indicates it is an object that can only be confirmed by scientific and mathematical processes. In scientific terms it is a “body” that only exists at a temperature of absolute 0. (That is -273.15 degrees centigrade.) This is the temperature at which all molecular motion ceases. Therefore any action placed upon this black body at absolute 0 is totally absorbed by the body. Enter Lord Kelvin since he is the person that identified the existence of absolute zero and therefore where the absolute “black body” exists. Since absolute 0 has never been reached (but close to it) a pure black body has never been “seen”.

However, by placing a current through metallic alloy of carbon and tungsten a correlation between the light produced and the Kelvin temperature scale can be created. (Lord Kelvin’s experiments did make use of carbon to find an absolute black body.) As the current passes through this alloy, it is heated and begins to glow. The light emitted at particular temperatures (heat) measured in degrees Kelvin follows a temperature scale. As the emitted light goes from no light to red in color through the spectrum to ranges of white (and into the ultra violet and beyond) the temperature of the alloy is measured (in degrees K), hence the term “Light Temperature”. If the alloy is at absolute 0 in temperature, no light would be emitted (remember no molecular movement at 0 degrees absolute) regardless of the electrical energy “pumped” into the alloy.

So even though Lord Kelvin had nothing to do with light measurement his temperature scale is used in light measurements. “He never saw the light” associated with his scientific efforts.

Now to Color temperature

The Kelvin scale is used in the measurement of the color temperature of light sources. Color temperature is based upon the principle that a black body radiator emits light whose color depends on the temperature of the radiator. (I think that I just said that above.)

Black bodies with temperatures below about 4000 K appear reddish whereas those above about 7500 K appear bluish. Color temperature is important in the fields of image projection and photography where a color temperature of approximately 5600 K is required to match “daylight” film emulsions. In astronomy, the stellar classification of stars and their place on the Hertzsprung-Russell diagram are based, in part, upon their surface temperature, known as effective temperature. The photosphere of the Sun, for instance, has an effective temperature of 5778 K. (Oh me now we are “way out”.)

In other words Color temperature is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, manufacturing, astrophysics, and other fields. The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of comparable hue to that of the light source. Color temperature is conventionally stated in the unit of absolute temperature, the kelvin, having the unit symbol K. (Gee, I must have had it correct above.)

Another way to describe it is: The color temperature of a light source is the temperature at which the heated black body matches the color (appearance) of the light source in question. (Did I repeat myself?)

Color temperatures over 5,000K are called cool colors (bluish white), while lower color temperatures (2,700–3,000 K) are called warm colors (yellowish white through red).

In case you still don’t get the picture here it is:

Planckian locus in the CIE 1931 chromaticity diagram.

(CIE is the International Commission on Illumination.)

The CIE 1931 x,y chromaticity space, also showing the chromaticities of black-body light sources of various temperatures (Planckian locus), and lines of constant correlated color temperature.

Planckian locus defined:

In physics and color science, the Planckian locus is the path or locus that the color of an incandescent black body would take in a particular chromaticity space as the blackbody temperature changes. It goes from deep red at low temperatures through orange, yellowish white, white, and finally bluish white at very high temperatures.

It is a curious thing that what we refer to as a “warm white” occurs at the lower temperatures while what we refer to as “cool white” occurs at the highest temperatures.

Are you thoroughly confused now? I rest my case.