Why We Use Temperature to Describe Light Sources

Photographs are impacted by the lighting conditions that exist at the time the photo is taken and photographers need to consider this impact when shooting pictures.

Modern digital cameras provide options for compensating for different lighting conditions.  Usually the options include multiple presets described based on the type of lighting present.  These settings include daylight, incandescent and fluorescent with other options present on many cameras.  Some cameras offer the ability to manually set the camera's color correction.  (Described as setting the white balance since the photographer is telling the camera that a specific color should be rendered as white.)

Determining what setting to use does require knowing a little about the quality of light involved.


Describing Light

Light is described using a temperature scale.  There is an historic reason for this that dates back over 100 years to the work of Lord Kelvin.

The Kelvin temperature scale is used when describing the color or quality of light produced by a particular light source.  The Kelvin scale is the same as the Celsius scale, but adjusted so starting point is absolute zero instead of the freezing point of water.  Subtract 273 from the Celsius temperature to get the Kelvin equivalent.

Why Temperature?

Credit Lord Kelvin for this.  He is the one that developed a system of describing light sources based on temperature.  Lord Kelvin started by heating a piece of carbon.  Carbon glows when it is heated and the color produced depends on the temperature to which the carbon is heated.  It starts at a dark red, shifts to a brighter red, then orange, followed by yellow, white and then shifts to blue.

Lord Kelvin then assigned light sources a temperature number based on the temperature to which carbon had to be heated to produce a similar light.

The quality of light produced by a particular source is described using temperature based on this work by Lord Kelvin.  (The modern value is based on theoretical physics and a "black body" instead of carbon.)   Both natural and artificial lighting conditions can be described using this method.

Examples of Temperature Values

Knowing the value assigned to the lighting conditions present when taking photographs can be useful when using a camera with a limited number of lighting presets.  The lighting conditions present may be similar to one of the lighting conditions covered by one of the preset options.  Using this setting may result in better results than the camera's automatic white balance results.  (Try the overcast/cloudy sky setting when shooting in heavily shaded areas.)

Some values:

Incandescent light bulb: 2800K
Halogen light:                3000K
Direct Sunlight:             4800K
Cloudy Sky:                 6000K
Blue Sky:                   10,000K (and up)

Fluorescent lights can manufactured to mimic incandescent light or daylight.  Warm white fluorescent lights are rated at 3000K, Cool white or daylight at 5000K.

LED lights can produce a variety of light.  Daylight LED lights produce light in the 5500 to 6000K range.  LED lights that mimic incandescent lights are available as well.

Note on Fluorescent Light:
Fluorescent lights usually produce light that falls outside the black body temperature chart.  Fluorescent lights don't produce light across the entire spectrum.  These missing wavelengths result in a color of light that does not directly correspond to any present in the temperature scale.

The same is true to a certain extent to LED lights.