I've been lurking in this thread since day one and finally have a few questions. I'm nowhere near building a LED rig myself, but the concept is still fascinating to me.
These questions are nothing new and more of a gut-check to make sure I understand some of the concepts involved:
<a href=showthread.php?s=&postid=14835792#post14835792 target=_blank>Originally posted</a> by kcress
dogstar74; That heat sink is SOOO over kill!! LOL You could cover it, packed with stars.
Clearly his sink is overkill, and there are probably examples at the other end of the spectrum floating around, too. This raises a question - how do we know how much heatsink we need? I've seen references in threads to the brightness and life of LEDs depending on junction temperature. Can we measure that easily in mocked-up designs to experimentally arrive at a good choice for a heatsink?
I'm guessing that when constructing a "traditional" light fixture, such as done by the OP in this thread, overkill is probably the easiest way to provide enough heatsink, and it's arguably not that expensive or cumbersome to do. But one of the things that attracts me to LEDs is that the LED itself is so small, it creates the possibility of all sorts of creative lighting designs. When exploring these creative designs, how do we know if we're moving heat away from the chip fast enough?
I'm also struggling to understand how one would decide how many LEDs to put over a tank. I suppose one method is to just copy the commercial fixtures, in terms of lumen-density (i.e. if a commercial filter has 10 LEDs per square foot and they're 100 lumens each, then we need 1,000 lumens/square foot). Back a few pages, stugray posted this:
Lets say I have ~1000 Watts of 15k Metal Halide lighting ( nice round number )
From what I can tell online, numbers for "effective lumens per watt" for MH is ~70 lumens/watt.
This assumes 80% reflection efficiency.
So my tank of 1000 watts should have ~70,000 lumens
Now for a CREE XR-E Q5 driven at 0.75A outputs ~220 lumens with no "reflection factor".
These are 3.0 Watt devices being run at 2.25 Watts.
So that is ~98 lumens/watt.
So to get 70,000 lumens I need:
70,000 / 98 = 714.
Here, he determined that 714w of Cree XR-E Q5 LED is roughly the same lumens as 1000w of 15k MH. I can follow the math, but I'm guessing this ignores some potentially important factors? How do you account for adding optics to the LEDs into the mix? Comparing different reflectors for the MH? Other things I'm not thinking about?
On a related note, it seems that many of these LEDs are spec'd with different outputs at different currents. For example, the white Cree XR-E Q5 is ~220 lumens at 750mw, and 107 lumens at 350mw. If you look at the curves these numbers create, there's a point of maximum efficiency (i.e. highest lumens per watt). It seems that the trend is to drive these LEDs near their maximum current, rather than their point of maximum efficiency. Is this simply because it's perceived to be cheaper to buy fewer LEDs and drive them higher, vs buying more LEDs and driving them more efficiently? How do you choose the current to run a particular LED at?
To apply this with stugray's math above, should one buy 300 XR-E's and run them at full steam, or 500 and run them at maximum efficiency? Is there anything to be considered here other than just weighing the economics of one option over the other?
And, while the examples of small tanks are great, are there any lurkers in this thread that have done this on a large scale (i.e. more than 10 or 12 square feet of tank surface?) The examples posted so far typically seem to be smaller than the OP's fixture.
i was looking at the hollow aluminum square tubing & it was kinda pricey for this lil project & then i stumbled across the U-tubing...... 
