I found this article:
"Spectrum -The Color of Light
Corals look best and grow best under light with a more blue coloration, in my experience, and that of many successful reef aquarists. So, I advise having more blue spectrum lamps on your reef aquarium.
Light color that a lamp produces is measured in degrees Kelvin, abbreviated K
. Kelvin ratings do not relate to light intensity directly, a higher K
number does not represent more intensity, in other words a 20,000K lamp is not more intense than a 10,000K lamp. This is something that a lot of folks get confused about, I will get calls all the time from folks that say they just added more intense 20,000K lamps to their tank. Wrong, they added more blue light to their tank with the 20,000K lamp and not more intensity.
Lamps for aquariums will have K
ratings listed on them and this relates to the color temperature, the lower the K
number the more yellow-red the light will be from that lamp and the higher the number the more blue the color of the light from that lamp. For example, a 6500K lamp will be very yellow in comparison to a 20,000K lamp that is very blue. Actually, intensity tends to decrease as the K
rating increases. So, a 6500K lamp will produce more light than a 10,000K lamp and it will produce more light than a 20,000K lamp at the same wattage."
There is a lot more info in the article.
(Friend of Bill W., One day at a time)