Lighting-What Kelvin to use

[handrew]

I have an question that I hope someone can answer for me. What Kelvin rating should I (or anyone) use? I have searched before and found some answers but I had an idea the other day while in physics. We just finished learning about light and how light energy affects objects. I also just finished studying about photosynthesis in my bacterial physiology class. Putting the two concepts together made me start to think. If plant chlorophyll(CHLa) is the primary acceptor of light energy and uses a wavelength of 700nm, wouldn't you want to match that wavelength? I also know they have caretonoids that use a wavelength around 580nm (I think-didn't pull the book to look up the exact value). Now in physics, we learned how to convert K into nm for light. If you convert 700nm, you get 4142K. 580nm gives you 5000K. It seems to me that you would want a bulb that had a spectrum near this range to be the most effective for plant growth. I understand that water will absorb some of this as depth increases but it seems the more I read that most hobbyist aquariums are not deep enough for this to be problematic. I read some people say that 10,000K is needed but that converts to 290nm. This is UV light and is getting close to the wavelength that causes damage to DNA, at least in humans and bacteria. I understand that some people prefer the color that certain spectrums give off and that different applications may dictate others as well. Does anyone have any thoughts on the matter? I guess I am interested in mainly planted tanks so this may not be the correct forum. Thanks though for any thoughts you all can give.

[Wild Turkey]

Plenty to learn about planted tanks around here.

I use 6700k in all my planted tanks, works brilliantly.

[DrNic]

I've found that 6500K, and 7500K bulbs will work great in most tanks. Currently though I'm using 10,000 in almost all my tanks, without any problems. The spectrum (i.e. wavelengths of light) the bulb puts out might not exactly match the conversion you have however, due mainly to the specific gas content in the bulb.

Although you're conversions are probably correct (I'd like to know how you did it) a bulb emitting only at 700nm would emit red light, while at 580 it would be yellowish. (See link here ([Only Registered Users Can See Links.])) Because the light is white (or at least almost white) coming out of the bulb it is actually emitting multiple wavelengths of light. Some bulbs will actually show the full spectrum on the box, although not all do.

To answer your question though, I would definitely stick with something 6500K or higher for any tank with live plants. I definitely prefer the plant growth I see with my 10,000K bulbs now.

[Taurus]

So, a fixture or strip with two tubes, one 6700k and one, 10000k would be ok for a given set up with plants? :22:

[Wild Turkey]

So, a fixture or strip with two tubes, one 6700k and one, 10000k would be ok for a given set up with plants? :22:

Thats fine





I use 6700k in all my planted tanks, works brilliantly.

That was incorrect i use 5500k in two of them, just spacing today

[handrew]

Here is the formula that I am using to get the conversions: wavelength(nm)=(2.9*10^6)/Temp(kelvin). You can swap wavelength and kelvin rating to convert back the other way. I do realize that the light emitted by the light bulbs is many different wavelengths but the kelvin rating is supposed to be the portion of the spectrum that is primarily produced. I remember someone saying once that they did not like a particular bulb because of the color that the tank looked. I pulled the physiology book and found the preferred wavelengths, 680 and 700nm for CHLa and 400-600nm for carotenoids with a peak at 580nm. I seems to me that for optimum growth that using a bulb around the wavelengths preferred would yield the best results. I know that the bulbs in the 5000-6000K range are doing great but I am wondering if maybe something in the range that hits those wavelengths might work "better". I am using better as a relative term as other conditions may influence a choice in bulbs. The reason I am even thinking about this is because I need to order some new bulbs and am considering what to get.

One last thing, the formula for the conversion came out of my textbook, College Physics Chapter 25 or 26. Thanks for the insight so far.

[DrNic]

I'll have to look into that formula more, I wasn't aware that there was a direct correlation between K and nm.

I talked to a botanist that works in my building (I'm a research scientist). He told me that although there are 'general' wavelengths given in books not all chloroplasts are the same and can have different light requirements. He also said that as water moves from your filter or airstone, it can bend the light like a prism and change the wavelength (i.e. separate the light) which could change the effectiveness of the bulb.

From the conversation we had, he seemed to agree that the wider spectrum the bulb the better the plants will typically grow. Although they do have a peak wavelength for their energy cycle, most plants prefer to also have exposure to many wavelengths.

Either way I think you're setup will be fine for your plants. thumbs2:

[handrew]

I appreciate the help so far. The conversion is also known as Wien's law. It gives the relationship between wavelength and the blackbody temperature of an object. In this case, the accepted value given by the bulb. The equation that I gave has already been adjusted to give the wavelength in nanometers instead of meters. The original equation is wavelength (m)= b/T where b is a constant of proportionality and has a value of 2.897768*10^-3 m. The value of 2.9*10^6 is rounded slightly and is the accepted value for most calculations (according to my professor). I imagine that regardless of what temperature bulb is used, a wide range of light is going to be emitted. I am just wondering if bulbs closer to the 700nm and 680nm wavelengths would produce better growth. I imagine I will go with a combination of bulbs to cover as wide as possible. I have four 55W power compact fluorescent bulbs to replace and was thinking about 2 5500K and 2 6700K bulbs placing one of each at each end of the tank. Sometimes having ADHD is a problem because it makes me think way to much while I am bored in class.

[handrew]

Just happened to come across this article and it seems to make some sense. [Only Registered Users Can See Links.] From what I read it seems that a 6500K bulb gives about the "best" results. It seems that it will cover the spectrum best for both CHL and carotenoids. Thanks for every bodies input so far.

[Taurus]

Carl has put a lot of work into his website. The entire website is well worth reading.

[dragoonwoman]

Thanks for the link to the article - very interesting reading.

I am in the process of setting back up an old 5 gallon tank, with a hood that used a 15w incandescent bulb. I am looking at using a CF bulb instead, because it will fit the fixture and give TONS more light. I want to plant the tank, so I need to find a CF bulb that will be the right temperature and spectrum for the plants. Now I know just what to look for!

I'm very inpressed with this forum - I've only been a member for about a week, and I've already learned all kinds of stuff that was taking a lot longer to find digging around on the Web.

Keep up the good work, folks!! thumbs2:

[Wild Turkey]

Thanks for the link to the article - very interesting reading.

I am in the process of setting back up an old 5 gallon tank, with a hood that used a 15w incandescent bulb. I am looking at using a CF bulb instead, because it will fit the fixture and give TONS more light. I want to plant the tank, so I need to find a CF bulb that will be the right temperature and spectrum for the plants. Now I know just what to look for!

I'm very inpressed with this forum - I've only been a member for about a week, and I've already learned all kinds of stuff that was taking a lot longer to find digging around on the Web.

Keep up the good work, folks!! thumbs2:

Thanks for the compliments! we try!(blush) :hmm3grin2orange:

Here ya go;
[Only Registered Users Can See Links.] I get these at Home depot for about $2 ea now; 5500k or 6500k is fine

[dragoonwoman]

Wouldn't 60w over a 5 gallon tank be "overkill"? I'd be afraid of frying my plants! Or does this bulb come in lower wattages?

I found Home Depot has a similar bulb, in 40w, for about $4. It's called Bright Effects, and the temp is 6500.

Forty watts is still a lot (8wpg), but the tank looks very good. Can't tell about plant growth yet - set it up just last night.

[rich311k]

Is that the actual wattage or the equivalent amount of incandescent light it puts out? the color is fine.

[dragoonwoman]

That's the equivalent light output (lumens?). Actual wattage is 9.

SO - that would be less than 2wpg. Confusing, isn't it?

[rich311k]

Yes, you need to go by the 9 watts number. The bigger bulb will do just fine.

[Wild Turkey]

Yes, you need to go by the 9 watts number. The bigger bulb will do just fine.

Agreed! Rich has it right, thats just a bs number they post on the package, go by the actual wattage. I use 2x14w(60) so 2.8/g cfl