Looks vs. function?

I have reviewed many threads on lighting and bulbs, etc...
I have a power compact light fixture with 4 bulbs. Two of the bulbs are blue actinic ones. My other two are different. One is 12000K daylight bulb and the other is 6400K bulb. My question is why does the 6400K bulb look more pleasing with and without my blue bulb added than the 12000k bulb? I thought the 12000k bulb was the prefered reef bulb? I have live rock with some coraline and polyps and I plan to add a few basic corals in the future. Which is more important , good looks with the 6400K bulb or better light spectrum for corals with the 12000K bulb? Or, am I all confused and messed up with my thinking?

 

12000K is close to the middle of the Kelvin scale, with 10,000K probably being the most, "white". Lower on the scale is more yellow, higher on the scale (20,000K being the highest) is blue. I think 6400k is fairly close to natural daylight. I am running 3, 6500K cfl bulbs on my tank right now. While it might not bring out the most intense colors, I do think it is what it would look like if you went snorkeling on the reefs.

The very yellow end of the scale will promote algal growth (if parameters allow it), while blue light is, generally, better for coloring and fluorescence of zooxanthellae in our coral.

 

The color spectrum of the 6400K is closer to the midday color spectrum which is 5100K at the equator. The main reason higher color spectrum bulbs are preferred is that lower spectrum bulbs can cause the corals to start looking brownish. Higher spectrum temperatures bring out color. The lower the temperature the faster the growth but the less pretty the color. The higher the temperature the more colorful the coral but less growth. 12000K gives a nice combination of both growth and color. Using as much of a mix of temperatures can give you the best of all. If you can find 460nm/420nm and 6500K/12000K combo bulbs.

 

What you two are saying is exactly what I understood to be true also. But,,,, what I am saying is that my 6400 bulb looks brighter and whiter than my 12000 bulb. Both are same brand of bulb. That is why I am leaning toward going with 2 6400 bulbs when I re-order replacements. But I am concerned about algae growth if I go that way. I would have thought the 12000 bulb would lokk whiter and brighter?:-/

 

The eye sees the same amount of yellow-green light, 20x brighter (or more) than the same amount of blue light, even though it's the same amount of light. When we measure lights for ourselves, homes ect... we therefore use LUX, which is a measure of light, adjusted for the sensitivity of the human eye. PAR is the same light, just not adjusted; all visible light is equal.

6700K lights will contain more yellow-green, so it will appear brighter. It also, will tend to waste less light in the UV range, as the range of output is shifted, so, it will have more PAR.

In nature the sun averages around 5500K around the equator, but what gets through the water, depends how deep you are. Red light is filtered out relatively quickly, then yellow then green. Whether or not it will provide better coral growth depends on the coral. Many corals will be more happy with more blue light (12000K-20000K), as this penetrates better and they are better adapted too it. However, there may be plenty of red-yellow light in the shallow reef flats and photosynthetic responses can vary by organism.

Most of the corals we keep will do perfectly fine with 120000K-20000K light though. Not many will need the extra-yellow-green of the 6700K light. Also, many pigments will be produced in response to blue light, and blue/violet/uv will make then fluoresce. Red/yellow/Green may bring out non-fluorescent red/yellow/green more for example though. With the T-5s, people are getting around this by using blue dominant light, with a bulb or so, with a red spike (i.e. ATI purple+). For LEDs, the radion is blue dominant, with red, and green LEDs for aesthetic purposes.


Of course, these light sources, produce lots of light and direct them down into the aquarium well. CF lights do not. So, with CF lights, there may be some argument to use lower K bulbs where there is more total 400-700nm light (PAR). However, for aesthetics, I would go with 10000K-12000K, I don't think the difference would be that big as far as photosynthesis is concerted. Some very shallow water organisms, such as clams, and maybe some SPS, may need lower K lighting in that situation, as the output is low to begin with, but for most corals, it probably won't mater at all.