Not sure exactly why it is called that, but Kelvin is also used to measure color temperature (and is what your digital camera's white balance also tries to compensate for). Ironically "cooler" colors have a higher Kelvin value than warmer colors. Daylight is something like 5500 Kelvin while flourescent lighting or a very cloudy day is usually somewhere in the 7000ish and tungsten bulbs in the 3000ish (just rough figures off the top of my head). This can be handy to know when photographing stuff. Usually your digicam will auto-white balance, but knowing the rough color temperature of the light you're photographing in also allows you to pre-program your camera, allowing for particular effects, or adjustments.
It doesn't matter so much today when you can adjust most things on a computer, but if your shooting with analogue film (it has its charms and uses
) it can be quite important.
Anyway: The diagram shown above with the 100% axis I belive represents the so called CRI or Ra index. The higher the more accurate color rendering on the lit object. As you can see the flourescent light has peaks where it renders very well and other areas where it doesn't do so well. Natural light is more evenly distributed. The interesting part about LED is that it claims a 92-95% CRI, which is also part of the reason why the technology is being looked upon with such great interest both within the medical sciences, art, or the graphic business. However I haven't had a chance to try it out myself yet (I'm on a very tight budget these days), but without doubt it is the future. Furthermore LED technology allows you to custom remove any parts of the color spectrum you don't need, such as ultraviolet colors. Or, in the case of greenhouses: optimize the blue tones of light which stimulates photosynthesis.
