Usually Joe posts very good information but that wasn't some of it.
Thanks Ryan, I'll take that as a complement!
To begin with I don't want to start anything major 'cuz I'd like to save that for after I try to self publish my own data for a price, BUT I have a little research to back what I am saying.
Bottom Line:
Ryan is 100% correct it's all about the reflectors, and T5s are simply better . .
BUT and this is a big but for Alex who posed the original question. If you compare "apples to apples" and are simply switching the end caps and going for a 1 for 1 exchange you will lose PAR by switching from a T12 bulb to a T5. Inside the lamps T5s and T12s are the exact same technology. The only difference is size which causes the T5 to run on lower energy which in turn reduces heating which in turn makes the lamp more efficient.
As you can see I have my own 6 lamp ATI fixture, which by the way I think is simply the best light you can provide for corals period even beating my venerable Radiums which I use on my display, but the truth is the Hamilton 4 lamp VHO unit sitting next to it provides essentially equal PAR around 200 umol/m2/sec in an aquarium at a distance of 6 inches above the water and a depth of 24 inches to the bottom using UVL 75/25 lamps in both fixtures and an Apogee meter for measurement and corrected readings using a proprietary formula I developed from data given to me by Jacob Bingham an engineer involved in the development of Apogee meters.
Again Ryan is 100% correct that T12s will be gone soon enough, but the fact is and was that when we all switched over from T12 to T5, we all got just a little hoodwinked by the manufacturers who got to increase their lamp sales by a factor of 50%. T5s are more efficient. T5s come with nice parabolic reflectors, BUT given equally efficient reflector design it takes three T5s for every two VHOs to equal the same output, and assuming a 5 year cost analysis it works out to nearly identical total cost of ownership and use. Aquarists are not and will not save money by converting to T5 technology because unlike a warehouse application, we as coral keepers have specific needs over a defined area so the choice is 4 x T12 or 6 x T5 for equal output and the added cost of buying additional lamps negates the energy savings of the T5s (and that doesn't even factor in the cost of production). Of course, one could go 4 x T5 and then the savings are real, but you're giving up PAR because four T5s do NOT put out equal PAR to four VHOs even with the awesome reflector design of an ATI fixture.
Lastly, as I mentioned above, for anyone who doesn't believe that T12s can make colorful corals just call Pacific East and ask the venerable Dr. Mac what lights they used for years and years on their tanks.
Clearly I am :deadhorse: beating the proverbial dead horse, and I promise to drop this line of argument with this post, and folks can hate me or disagree with what I am saying. In the end it won't matter since T12s will be gone, but again for Alex I say the answer is:
Stick with your T12s or buy a new fixture.
Your Reefin' Buddy,
Joe