Originally Posted by steve-s
The higher the number the more blue/violet the light spectrum emitted.
Please compare the spectral output of the 6500K Iwasaki 250 watt Mercury Vapor bulb and the 20,000K Osram 250 watt Metal Halide Bulb. You will see that the Iwasaki bulb produces more light in the blue/violet spectrum than the Osram and yet the Kelvin color is far lower. This is because Kelvin is just the final color of all the mixed wavelengths of light.
Originally Posted by sipe
Bang Guy, I checked Reef Aquarium Vol 1 and Kelvin colour temperture is directly related to wavelength.
They are related but they are far from the same thing. Kelvin Color is dependent on the mix of all the different light wavelengths emitted by the bulb. Electromagnetic spectrum is measuten by the wavelength of the light in nanometers (NM). The color of the bulb (mix) is what's measured in Kelvin degrees.
A bulb ideally suited for growing algae would be a mix of blue spectrum (~450nm) and red spectrum (~670nm) light. These are the two photosynthetic peaks. Please forgive me if the actual numbers are slightly different, this is from memory but it will suffice to make the point. This mix would produce a greenish, yellowish light around 5000K on the Kelvin Scale but the amount of blue vs red could send the color almost anywhere on the scale from 2000K if mostly red up to 20,000K if it were mostly blue. A bulb emitting a single wavelength of light around say 570nm would would also be about 5500K on the Kelvin scale but it will be almost useless for growing algae.
So, spectrum is important, the Kelvin color really isn't. The Kelvin color is very important for asthetic reasons though. Nobody wants a yellow looking reef no matter how healthy it makes your corals.