This is from Evil's Par38 post on NR.com:
http://www.nano-reef.com/forums/index.php?showtopic=233904&hl=
"The basics about the lamp:
21W power draw - This is including the losses in the driver and the power supply. It does not mean that the LEDs are running at 5.2W each. This is just silly.
Each LED is running at 950mA - This works out to 3.5W per LED. No, I'm not overdriving the whites. Yes, I am slightly overdriving the blue. Can they take it? Yes, provided that the temperature of the LED is under control, which it is. Even at the elevated currents, we keep our die temperatures much lower than the Cree spec, ensuring long life.
Cree XR-E LEDs - They're good. They work. They won't be changing any time soon. I've had numerous people ask me about why I'm not using XP-Gs. There aren't lens options for them just yet to justify the change. Well, unless you want 40 degrees or tighter.
Worldwide usage - These can be used anywhere in the world where you can find an E26/27 medium base lamp socket. They can operate on 90-240v AC at 50Hz or 60Hz.
LED replacement - Can you replace an LED if it's damaged or you want something different? Sure, but it's a lot more complicated than just swapping out a star pcb. All the LEDs are on a single large pcb, and you would have to use reflow soldering techniques to replace the LEDs. If you are going to do it, go for it. It will void any warranty though.
Lens replacement - Can you replace the lenses with something different? Sure. They are standard Cree lenses that everyone is familiar with, just without the individual lens holders. We offer them in 40, 60, and 80 degree angles, but there are 25 and 8 degree options on the market too. Do so at your own risk though. If you break off the dome on the LED when replacing the lenses, by-by warranty.
Operating characteristics:
This is where the majority of the heated debate comes from.
First, many of us have differing opinions about what usefull PAR actually is in terms of spectral output when it comes to LEDs. This is going to be a growth area for the technology as we learn more about them. All we can do right now is use the tools and the knowledge that we have right now to make educated guesses about the overall usefulness. We aren't going to be arguing about how to correctly measure PAR here. The test numbers are just to be used for reference, not taken as gospel.
Peak PAR on these lamps is high (at least what we know as PAR, and according to the tools we have). Remember, this is the highest measureable PAR values. This is in a very small area. PAR values drop considerably the further off center you get. This is the nature of a spotlight. Typical metal halide setups are flood lights. They create a very large, even are of illumination. Some people are claiming that these lamps can replace 150W/250W/400W MH setups easily. It's not that simple. A 150W MH with a decent reflector can cover a much larger total area than a PAR38 (24"x24 on average, versus about a 14" practical circle with the PAR38s). The PAR38 in the right configuration can achieve much high peak PAR numbers, but the average PAR over the entire coverage area is much lower. To get similar average PAR levels over the same area, many more lamps would have to be used to match a 150W MH. As a result, the power consumption starts getting closer to what a MH would be.
A single PAR38 cannot and will not be a 1:1 replacement for a 150W MH. I'm sure some of you are now thinking that I must have been leading people down the wrong path suggesting that a single PAR38 can light the likes of a BC14. It can do just that, but you have to understand the limitation that you are working with. Using these lamps is a very different way of thinking about lighting a tank. The coverage area of these lamps is rather small in comparison to other lighting technologies, but you can use that to your advantage. For a long time, low, medium and high light has been relegated to bottom, middle, and top of the tank respectively. All you have to do now is change that to center, near center, and far center of the lamp. You can still obtain the different light levels for different corals, but in a different way. Some have claimed that I have told them that you can replace large metal halide setups with these lamps. I have never said that. I have stated that you can create PAR levels similar to metal halide, but have always qualified the statement by saying that you will need to use multiple lamps to get good coverage and still achieve that level of intensity.
Please refer to the PAR plots that I have posted for the different lens angles at different distances. This will give you a better understanding as to what to expect from the lamp.
http://www.nano-reef.com/forums/index.php?showtopic=221433
Coral coloration - This is something that is going to be a talking point for a long time to come. This is an issue more to do with LEDs as a whole and not just the PAR38 lamps. What I will say is that there is a lot more to color coloration that just light. Water parameters, flow, feeding, and proper acclimation are all important to keeping corals vibrant and colorful. I won't argue that there is something missing that is causing some of the corals to lose their vibrance. Unfortunately, it's going to take a while to find out what that is.
Added 11/23/10- I wanted to add another section about light control and spillage. I'm getting a lot of questions about what lenses should be used to reduce the amount of light spill when used over a tank. This is not a good approach to start with. Because these lamps are not currently dimmable, you need to use height to set the appropriate intensity. As a result, you will/may end up with some light spill regardless of the optics used. Trying to control spill with different lens angles will only lead to situations where your PAR levels are too high for the livestock you are illuminating. If spill is still a concern, then there are other mechanical methods that can be used to help situations without changing intensity like you would by changing lenses. One option is to use clip on barn-doors (similar to what is used on stage lighting) to limit where the light goes. W.A.C. Lighting makes accessories like this that you should be able to clip onto the front of the lamp. At that point, you can adjust to your hearts content. Please, set the lamp(s) up with the lens, distance, and height required to get the job done properly so your livestock is comfortable first, and then control spill with mechanical methods.
Hopefully this clears up many of the falicies that I have been seeing around the forums recently. If there are any other point you would like me to address, please tell me. I will update this post with any new information I can provide."