In USMA message #53611, Stan Jakuba wrote, "On the positive side, I applaud the use of the kelvin, both as a spelled name and as K. On the other hand, I am at a loss as to the usefulness of this information to the average buyer." (Sorry, I've already erased it so I can't "reply" to it.)
Traditionally, humans progressed from fire to electrically heated filaments as a means of generating artificial light. These closely approximate black body radiators whose spectral distribution is well approximated by absolute temperature and Planck's law. Household bulbs run about 2700 - 2900 K, some photofloods to 3200 K, and direct sunlight about 5500 K. This color temperature affects how we view the light, as "warm" or cold and harsh. (Note the higher temperatures seem "cold" to us due to more blue content. CFL and LED lightbulbs do not emit a continuous spectrum of light but discrete lines. They may or may not appear natural. This is described by two characteristics, color temperature and color rendition index (CRI). Think of the color temperature as a best fit of the discrete line spectra to a black body radiator characteristic at the specified temperature, and color rendition index as the deviations in that fit (CRI = 100 is ideal). Both are important to human perception of the bulb. I accidentally bought some "daylight" CFLs, rated 6500 K. While characteristic of the sun on an overcast day, indoors they look horrible, because indoors we are used to lighting around 2800 K. I put them in the garage and porch fixtures because I couldn't stand them indoors. Memo to self: Always read the label.
