Thoughtful, detailed coverage of the Mac, iPhone, and iPad, plus the best-selling Take Control ebooks.

Degrees Kelvin?

A few readers asked about the reference to "color temperatures" in last week's article "Old Monitor Makes Way" (in TidBITS #223) and the measurements given in degrees Kelvin. Some were concerned about such hot objects sitting on their desks!

I knew that the 5000, 6500, and 9300 degrees Kelvin measurements referred to the white level displayed by the monitor, but didn't know how or why. A little digging confirmed what little I did understand, and added the fact that the white level is described in terms of degrees Kelvin because you're describing the exact "shade" of white that's radiated by an object heated to that temperature!

A hypothetical "black body" (an object that reflects no electromagnetic radiation) looks black when it's cold because nearly all the energy emitted is in the infrared end of the spectrum. As it warms, it glows a dull red (the low part of the visible-light spectrum), then moves into the yellow and blue as it heats up. When it gets really hot, the peak is in the blue, but there's lots of yellow and red, too. The result is that your eyes, which register red, blue, and green, see white, since all of your receptors are firing at the same time.

Hotter objects appear to have a higher blue component, but are never quite blue because the red and yellow components never reduce. A cooler object (if objects so hot can be called "cool") appears to have a more reddish cast. Thus, the "white point" of a monitor, or the "temperature" setting, indicates the shade of white displayed by the monitor as a comparison to the temperature of a black body at which that shade of white will be emitted. If you'd like to know what shades of white are represented by the "color temperatures" of 5000, 6500, and 9300 degrees Kelvin, just get a real hot oven and heat some convenient "black body" to those temperatures.

Information from:
Chuck Bartosch --


READERS LIKE YOU! Support TidBITS by becoming a member today!
Check out the perks at <>
Special thanks to Joseph Worth, Jim Pokorchak, Eduardo Pantoja, and
Bart Smith for their generous support!