Actualy it is the higher K rating chips of Cree that are producing the greater LUMN rating. It would be great for some of our mind if all the manufacturers listed there chips in PAR ratings but then in reality PAR is not a true breakdown of what our corals need it is just a little better than LUMs. Even with a par meter you can have a higher PAr from a chip strong in Green light that does little compared to to another chip with a higher light concentration in the 480 nm range. You need to look at specific spectrum points to realy compare.
I agree PAR (µmols) are not an ideal metric, but it's a decent indicator, and much better than lumens. Of course, one needs to look at the quality of the light via spectrograph, and quantity of said light via quantum meter (PAR). This will give you a rough idea of PUR, providing you know what each individual coral requites. Again not a magic number, but as close as we are going to get.
Yes I agree but I also disagree. Both single emitter and multi emiter do have there advantages in different situations. The shallower an individuals tank is the more advantage the single emiters are at the taller the tank the more the advantage goes to your multi emiters.
In that view I consider the tank height of roughly 24" as the point where it is basicly a toss up between the two and considering build cost I would still go with single emiters. I have run 250 + watts with single emiters on a 120 gallon tank without the need for cooling fans. Can you do that with multi chips? On this and some other threads I see some individuals spending more on chip cooling than the rest of there lighting system.
The application where a single emitter would be advantageous is a nano tank with a lid, as clearance is minimal for cooling and illumination angles. A 50-100w multichip would have to be run without a lens (instead, a flat glass shield to keep clean/dry) to cover the tank adequately without hard shadows. A grouping of 10w 3 channel multichips would avoid the pronounced colour banding you get with single emitters.
If you don't mind a giant heat sink, you can cool a 500w multichip without fans

The common limitations of coolers are fan noise and heat sink height. Price is not a problem if you are buying online.
Single emiters generaly have life expectancies over 20,000 hours when run at max power. Considering we only run the lights 12 hours per day max that means roughly 4.5 years. In 4.5 years will your multi emitters be absolute? The way LED lighting is making changes every day I would say a system 2 years old in either single or multi chip will be absolute.
I agree 100%. I have been working on a multichip fixture for over two years and I still haven't caught up with the newest innovations. When PFO was selling fixtures for $4000, you had to amortize the cost over ten years. Now that we are looking at $500 fixtures, 2-3 years is acceptable.
Most hobbyists spend $5000-$30000 on their complete reefs. When lighting was in the 4 digits, it was a careful decision, but now it matters little whether it is $500 or $750 per fixture, especially when your tank only needs one. Capital cost isn't a major concern outside of this DIY forum.
Efficiency was a big deal when we were using three 400w metal halides and another 440w of T5. Dropping down to three 100w multichips was a huge electrical savings even without looking at chillers, evaporation and bulb replacement. Now that many have switched over to LED, it matters little whether the fixtures are 100w or 150w each. We are talking about a few dollars a month in operating cost. Okay, so efficiency isn't a major consideration either.
Longevity is the other issue to look at. All of these lights we are discussing will be in the dump or our garages in 2-3 years, so whether it's 20,000 or 50,000hrs life span, it's moot. So let's take longevity off the list of parameters in selecting a good light fixture.
So what's left...
1) It has to be new
2) It has to look cool
3) The cost has to be low enough not to set off alarms with wife/girlfriend (or both)
4) The fixture has to involve the user in some way through building or programming
5) For most consumers it has to have the nod of acceptance form the forum members and reef bloggers