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OK so I'll make an attempt to get at what Sk8r was originally posting about...generally useful information for folks new to LED's or the hobby to know. Well, actually, I'll start with a list of criteria that I believe are useful to consider and compare, some of which matter to coral health and appearance, some are just a personal preference of the hobbiest, and some are of little use or are actually unhelpful and misleading. Lets assume we are dealing with a reef system, not FO or FOWLR.
I'll start with what I consider the the most important criterion...
1. PUR. At the end of the day, what seems to me to matter most, is how well the light fixture is providing the radiation/light/energy (however you want to describe it), necessary for healthy coral. If you do not know what PUR is, go read some more. The point here is that what matters is what specific frequencies of light are produced.
2. PAR. This is a widely used criteria but is of limited value IMO because it measures light indiscriminately, whether the light is useful, desirable, or not. High PAR values that are driven up by green and yellow light over-represent the "amount" of light that that is useful for coral health.
3. Cost of fixture
4. operating cost ( electricity )
5. failure rate, probability of failure ( often due to fan failure)
6. Vendor customer service
7. Personal preference for the appearance of tank...overall “color” as interpreted by the human eye.
8. your ability and willingness to change emitters that come on cheaper LEDs to remove and replace undesirable colors and even if this is possible with a particular fixture.
9. Mounting ease for your particular situation... what is possible
10. Driver electonics... PWM or Current reduction technology. This is a pretty big deal but will take a seperate discussion to explain the difference and implications.
11. Programability, Controlability, including number of channels, dimming, ramp-up down times, etc.
12. Programing software, wireless control, control via phone, etc
13. aesthetics... Physical appearance of the fixture
14. Connectivity to popular control systems such as APEX
15. Coverage, depth penetration, spread, optics, the ability to change optics.
16. Heat generated, lost. Rememberthe more input energy that gets lost in heat, the less energy available to produce light.
17. And OK, here we go..... watts. This is a widey used criterion that I believe is generally misguided because many do not understand what a watt is. Simply put, Watts is a measure of power (a function of voltage and current) that is consumed in the operation of the fixture or emitter. Watts is not a measure of light output. A 3W emmiter does not, by definition, produce more light energy output than a 2W emitter. A 3W emitter simply consumes more energy than a 2W emitter. The wattage of a device tells you how much the device will cost to operate base don electicity usage. It is a measure of input not a measure or representation of light output. While it may be true that a higher watt emitter happens to produce more output than a 2W emmitter, this is far more a function of the efficiency of the emmitter than the power it takes to run it. I disagree with those that say you should strive for a certain watt rating of emmitter. Instead, I suggest that you strive for the most efficient emmitter, the emmitter that creates the gratest input/output ration. In the end, your coral do not care how much power it takes to produce the light they want. The discussion surrounding watts is really about power consumption, not light output.
And I've run out of time so I will have to leave it at that. I'm sure there are more that I forgot at the moment and others can offer to add to the list.
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