How many LED's are needed?

[djfrankn]

Hi to everyone. I am new to the reef centraland this is my second post, by the way. I have a 55 gallon tank (48" L, 12" W, 24" H). and I've been thinking of going on the DIY route for my tank lighting. I currently have a Current USA Sunpaq 48" dual 10,000k/blue actinic. I have to replace the light bulbs but I was thinking to build a LED system for my fish tank. I've been reading about DIY guides all over the web but I haven't found anyone who can tell me how to calculate how many I need.

I have some polyps, a duncan coral and some fish. Thanks for any response.

[fastpcuser]

Don't know but here is bump for ya ....

will follow interested in LED's

[Ben34105]

I'd be interested in knowing how to figure it out also. It's coming up on relamp time for me and I've been kicking around the idea of going to LEDs. I have a 150g tall (30" deep) so I'd need some heavy duty LEDs to reach to the bottom of the tank.

[ludnix]

Probably could get away with 48 XR-E leds.

[TheFishMan65]

48 would be good or maybe 36 if you don't have light loving corals.

[der_wille_zur_macht]

I'd be interested in knowing how to figure it out also.

Wouldn't we all? :D As with any form of tank lighting, there are many variables, and many similar-looking solutions that aren't really similar. How often do you hear people make sweeping statements about how a 250w MH can cover X amount of tank area for X type of corals. Meanwhile, the output from different lamp/ballast/reflector combinations can vary by hundreds of percent! Not picking on MH, just making a point that rules of thumb provide general guides, at best, for well-established forms of lighting. Something new like LEDs is going to be even more rough when trying to come up with these formulas.

That said, there have been enough successful LED-lit aquariums that we can at least approximate a good starting point with some degree of certainty.

There are really a few major variables:

1) What model of LEDs and what drive current(s)? This basically determines how much light comes out of each of each point in your array.
2) How many LEDs will you use? This question can also be phrased as "what spacing will you use?" Or "How dense will your array be?" This establishes how many points of light you'll have.
3) What optics will you use? This determines where the light from each point will end up.

To answer the first question, we can first develop a criteria for choosing the LEDs used. Personally, I focus on the following:

1) Highest efficiency possible. This is measured in luminous flux (lumens/watt) for white LEDs and some colors, and radiant flux (mW) for other colors.
2) Reasonable spectra. We want light in a spectrum that allows for growth, coloration, and pleasing appearance.
3) LEDs from a manufacturer that knows what they're doing.

Given those criteria, the Cree XP-G, XP-E, and XR-E are near the top. Luxeon Rebels are close too, but less popular. Next, we have to pick which colors of LEDs we will use. This ventures deep into the realm of personal taste, as you'd get with other forms of lighting - some people like 20kk MH, others like 10kk with actinic supplementation, and so on. MOST people who have built LED arrays use a mix of cool white and royal blue, though some people have experimented with plain blue, neutral white, and warm white. For cool white LEDs, the Cree XP-G is the best choice based on the above criteria. For royal blue, the XR-E and XP-E are more or less tied with some slight differences. Many people find that a ratio of 60:40 royal blue XR-E or XP-E to cool white XP-G give a pleasing result, though the best policy is to experiment to find the ratios you like.

To choose drive current, 700mA - 1000mA is safe for any of these LEDs and gives a good balance of outright power and efficiency.

Moving on to question number two above: How many LEDs do you use? I prefer to answer this in terms of square inches of surface per LED, because that makes the math easy and gives a good "visual" of how much territory each LED will have to cover. Looking at successful LED arrays for typical mixed reef aquariums, most people are in the range of 10 - 20 square inches per LED. So, for instance, if you had a typical 4' x 2' x 2' 120g aquarium, you have 1152 square inches of surface area. So, you might want to be between 70 and 100 LEDs for a typical mixed reef. Above 100 if you want the whole tank suitable for high-light stuff, or below 70 for a lower-light reef.

On to question number three: optics. Optics are marketed in terms of FWHM angle. We can simplify this by thinking of it as how "tight" the beam will be - smaller numbers are tighter. Many people try to relate optics to a particular intensity of MH, but IMHO that results in other issues. Optics don't change how MUCH light there is, they just alter where that light goes. So, use optics to control where your light goes! Do you need a lot of coverage for a low-density array on a fish only tank? Don't use optics. Do you have a dense array over a deep tank and you want good penetration to the bottom? Use medium-tight optics. Do you have an array mounted in the ceiling 5 feet above a deep tank? Use super-tight optics. And so on. Optics give a LOT of flexibility and really allow an LED array to be designed for a wide range of installations, many of which would be difficult or impossible to solve with typical reef lighting.

Hope that helps those who are looking for some general guides on sizing LED arrays. Of course there is still a lot of room for growth and refinement of these guidelines, but at the least they will provide an entry point. As I stated above, IMHO, the best policy is to get some LEDs and experiment yourself.