What exactly does the K, Kelvin rating mean on light bulbs?

[hairtrigger]

I know the higher the kelvin is usually more desired. Eg... 10,000 to 12,000 K is the best.
However, I see some LFS' and online selling PC bulbs that are 6500, 7500 K. They are a little cheaper... but are sold alongside PC bulbs that are 10-12000 K.
Which is best for a reef, and why?
What exactly does the K mean, and why is the higher one desired more?
Thanks yall.
Yes, it is time for some new bulbs.

 

[amphiprion15]

Well Kelvin in reef lighting is talking about the color appearance of the bulb. like 5000k is natural daylight color at noon, 6500k a little whiter, 10000k a crisp white with a slight tinge of blue(depends on manufacture) and 20000k is like swimming pool blue. It all depends on how blue you want your tank. 6500k and 7500k bulbs are kinda on the yellowish side and I'd think you'd want to supplement with actinic, if not go with the 10000k which is a crisp white. 12000k is a little bluer, but not as blue as 20000k. Higher K is desired merely by appearance for the most part.

 

[thomas712]

Originally posted by HairTrigger
What exactly does the K mean, and why is the higher one desired more?
Color Rendering Index (CRI)
In general, CRI is an indication of a lamp’s ability to show individual colors relative to a standard. This value is derived from a comparison of the lamp’s spectral distribution compared to a standard (typically a black body) at the same color temperature.
The K rating, or CRI, gives you the ability to match the tank lighting to that of your animal's natural habitat. Without this important advances in the industry we could never hope to duplicate lighting conditions at DEPTH in our living rooms, and provide as near identical conditions for our corals and inverts as that in nature.
Wattage plays a part here, too. The higher the wattage, the deeper it will penetrate. With the right K rating though, a hobbyist could conceivably duplicate conditions at say 50 feet in an aquarium only 18" deep. It's a trade-off, higher wattage vs. proper K rating. Combine the two and you achieve reef lighting Nirvana! Why go there? SPS corals, for one.
The 5500k is the bottom end of acceptable CRI because frequencies below this promote algae blooms as these algae prefer less blue-white light, anything lower will have more red and yellow spectrum in it.
6500 is the good starter freequency and normally used in conjunction with the actinic blue bulbs.
I like and use the 10,000k bulbs made by URI. They are the upper end of the advanced or special need frequencies as they produce incredible amounts of deep penetrating blue-white light, they are great for deeper tanks.
As you have found there are 12000k bulbs and then up to the upper end of the spectrum a 20,000k bulb which is simply the upper end of the deep water spectrum.
Then there is your Actinic blue bulbs. Corals and inverts absorb light at this blue end of the spectrum. These bulbs should be used when using k ratings under 10,000k as a suppliment for the lack of blue spectrum in the lower K ratings. Its this blue light that really makes a difference in the photosynthesis of your corals and more is better.
I like the VHO 10000K 50/50 URI, and the 420 nanometer Actinic URI's. They only last about 8 months but have been very good bulbs for me.
Thomas

 

[hairtrigger]

Ok, so let me ask you this. I am about to buy two new PC bulbs. I was going to buy a daylight, 10000K white one, and a 50/50 10000k white/ half actinic.
What would you say is more important...
getting one fully actinic and one daylight... or the way I have thought about doing it.
I already have actinic flourescents above my PC's. That is a factor.

 

[thomas712]

If you already have 2 actinic bulbs (how many watts each) and your pc lights then those two actinic bulbs should be a great help. It is my opinion but I like the additional blue to the 50/50's and think it is of benifit to the corals.
Which one do I think is more important......The 50/50's
With the two actinic's you could very well do it the way that you are thinking with a 50/50 and the Daylight some people like that look. Both will be 10000k and be in that blue-white spectrum.
Your thoughts seem fine to me. Go for it.
Thomas

 

[hairtrigger]

Ok... let me ask you this:
What would let me see the best results for the coral?
Which would help them thrive the best?
2 10000K daylight PC/s and 2 50/50 PC's....
or 4 10000K daylight PCs and 2 four foot floursecent actinics?
or 2 10000K daylights, and 2 full actinic PC's...
You get what I am saying.... if you have the time... which would your ideal combo be, and why?
I have room for 6 55 watt PC bulbs, plus 2 48inch flourescent ballasts.
Throw me an idea... if I didnt confuse you enough.

 

[thomas712]

Alright my idea is this.
For overall coral growth as well as coralline growth. I would use the 2 48 inch Actinic bulbs with the 2 50/50's and 2 Daylight bulbs.
Why: The 2 48 inch Actinic bulbs are a great start for the photosynthetic loving corals, add to this the blue-white of the 50/50's and this would be a well rounded lighiting combo. Add to this the 2 Daylight bulbs and you will get the noon day bright sun look (thats more for your eyes and not the fish.) But the corals would not mind the extra 10000K spectrum.
You will get the blues you need from the Actinic and the 50/50's, the 50/50's will give you that mid morning and mid afternoon look, whereas the Daylight will give you that noon day sun look, more of a crystal or ice blue-white. Might also add to that rippling effect.
What I would do if I could is have the Actinic's come on first followed by the 50/50's, then between say 11am and 2 pm have the daylight bulbs be on. Then lights off in reverse order when you specify.
Hows that?
Thomas

 

[hairtrigger]

That is exactly how I'd thought it might work best too. Good good, that reassures me. I like the idea about putting on the daylights around noon. I will probably try it to see how it goes. I turn on the actinics first now, but the regular lights come on shortly after. Will the lack of daylight bulb hours take away from coral growth or thriving? Thanks a lot for the advice. I appreciate the effort.

 

[thomas712]

Nah I don't think the lack of daylight bulbs will harm anything at all. If it were me I would not want them on all the time anyway.
Good luck
Thomas

 

[scarybo]

Thomas,
If you were buying 4 VHO bulbs what would be best for corals

 

[thomas712]

In my opinion the 50/50 again and the Actinics both made by URI. I feel that is the best forourecent bulbs to go with.
What lenght or watts are you looking for.?
Thomas

 

[scarybo]

36 inch.
So you would go with :
2- URI 50/50
2- Atinic

 

[thomas712]

I not only would that is exactly the bulbs I have now in the 36 inch 380 watts.
Thomas