Can someone explain what the "K" means when talking about lights?

Natural unfiltered sunlight is around 5000k.

The whole topic is complicated by the way water affects light. As the depth of the water increases more of the visible light spectrum is lost. Reds first, and blues last, which is why "deep ocean" type bulbs are more bluish. Plants live in shallower water, and generally do better at color temps (K) closer to the 5000 of natural sunlight.

Intensity, or brightness, is a partially subjective phenomenon. It depends on what your human eyes are designed to "see". Some bulbs that appear brighter actually have less intensity. Intensity, to oversimplfy, is measured in lumens. Standard "cool white " flourescents are designed for human eyes, and have lower than optimal color temps for plants and corals. They also tend to distort colors. You get the "truest" colors around sunlight color temperatures, 5000 to 5500 K. This true color measure is usually expressed as "CRI", or color rendering index.

A good bulb for fw plants and general FW viewing will be around 5000k to 6700k. The lower end of this range tends to appear a bit "yellow" (like the sun), so many people prefer a slightly less yellow, and therefore "bluer" bulb, with a higher K. I use a 6700 K over my planted fw tank. It looks natural, and the plants thrive.

Corals, having evolved at some depth, do a little better with bluer light. In general, the importance of fine distinctions in color temps is exaggerated. Intensity drives photosynthesis, and is very important for corals and plants. Color temps (K) are important for your own esthetic preference, and also for the health of the aquarium inhabitants. In general, if you have adequate intensity, you can't go wrong with a color temp between 5 and 7 thousand for most fw applications.
 
OK, first let me say I have the "wrong" bulbs for my planted tank, but that is neither here nor there.

I have read here a couple times that Higher K is bluer and lower K is redder. At what level does this kick in?

I know low K like the 2700K screw-ins I have on my 10 gallon are yellowish. Here are a couple pictures of my 28 gallon. The first is with a 4100K bulb, it looks yellower, the second is 8500K - dull but distinctly red coloured. (Together the 2 look natural white.) So why is my 8500K red?

q_light2.jpg

q_light3.jpg
 
Color spectrum and color temp issues can be really confusing, for two reasons (at least). One reason is that most bulbs are not "full spectrum". They are combinations of various phosphors the give off specific bands within the spectrum. If you combine red and blue, you get purple, which is what some so-called plant bulbs do. They combine the spectrum bands that plants utilize most in photosynthesis, and leave out much of the rest of the spectrum most visible to human eyes. That's why some "plant " bulbs have a dull pinkish to purplish cast. The same with "color intensifying" bulbs. The concentrate spectrums in selected areas to enhance certain colors by using specific phosphors. In general, these bulbs are not a great idea.

Another issue is that color temps are determined by several factors and technical measurements, and bulbs with very similar color temps can look very different. It depends on which phosphors are utilized. The numerical transition up and down the K scale is not a smooth seamless transition from one shade to another. Actinic blue uses just one phosphor, and generally measures at a K of 7100. The light looks nothing like other bulbs with a color temp only a few numbers away.

Full spectrum bulbs try to include all of the visible, and often some of the invisible spectrum. The proportions, determined by the phosphors within the bulb, vary widely, and that is why various full spectrum bulbs can look very different. This is a complex technical area, not able to be explained simply. In connection with some of the indices, think of a pound of feathers compared to a pound of lead. Both weigh one pound, but their substance and natures are different. This is not a very good analogy, but it's the best I can come up with on the spur of the moment. CRI can be very useful, because it measures the effect of light as you really see it.

Remember also that a lot of what you read is manufacturer's hype, pure sales pitches, in a very competitive market. Advertising specialists come up with all kinds of attractive descriptions for their bulbs. I have only a limited amount of direct experience, but I have found a few bulb types to be very useful. Full spectra between 5500 and 6700 from any manufacturer almost always work well on fw tanks. GE's full spectrum and Fresh/Salt bulbs seem to work well, and are not expensive. Triton bulbs are expensive, but I have had great results. UV Resources bulbs are excellent, and their VHO Actinic Day is hard to beat for SW applications.

Unless the bulbs have one built in, the use of good reflectors is extremely important for maximum intensity reaching the aquarium.
 
Hey Kveeti,
Nice tank! Cooler and warmer may be more useful adjectives than bluer and redder. The second tank is actually bluer not redder (more violet?): its a problem with descriptions. You'll definitely agree to cooler? Most folks seem to put the pivot (white) somewhere around 6000k or so.

Plants like light in the blue and especially in the red parts of the spectrum: they have very little use for green, which is why they reflect back green light. Since we're very sensitive to green, a plant bulb could have spikes in the red and blue areas of the spectrum and still appear relatively dim to us. This is what makes lumens a less than great guage for plant lights. It also bangs up the Kelvin thing a bit, which is why CRI is also something to look at, as agilis mentioned. Being around the right Kelvin temps with a higher CRI is probably the most practical way to go.

Tom Barr weighed in on this not too long ago in Plants. It seemed to me his recommendation was warmer, maybe 5000 to 4000ish, don't really remember. I remember being a little surprised that it was a little lower than the 6000s. (I think I'm using 6500s). He also said that plants will grow under less than perfect light (10000k) just not as efficiently, and that he was always surprised by how people would shell out for lamps and then cut their efficiency by using the wrong bulbs. At the end of the day, if you're happy with the growth and happy with the look, you're happy with your tank. :) Happy is good.
 
A good place to start is to read the front end section on fluorescent tubes on the manufacturers website.

Philips has (had?) an excellent discussion explaining CRI, K, Lumens, Watts, efficiency etc.

From that you should be able to better understand these terms and not be at the mercy of the LFS etc.

Gumby
 
I had two of those 18K bulbs over a 40 long and the plants grew pretty well.

In most cases, intensity is much more important than spectrum. Plants will grow equally well from around 4000K- 8000K.

Plants are naturally terrestrial and will do better with lower K values. Algaes are from the ocean and will do better with higher K values.


Actinic bulbs are "blue" and do not have a spectrum value. Marketing has alot more to do with the reported K value of a bulb rather than testing. Two 6500K bulbs can look very different.
 
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