OK so my planning is getting somewhere. Here's what I have so far:
Emitters and optics:
4 x Cree R2 (WG bin) focussed with aspheric lenses. Driving them at 1050mA, the R2 emits about 270lm according to this post. Using the 30mm ones that ahorton is selling should allow me to collect a large amount of light from each LED, and focus it very tightly.
Using the following formula for the inner bezel diameter requirement:
D = d * sqrt(2) + d
(where d = the diameter of each individual lens)
Gives me a bezel inner diameter requirement of 72mm. Assuming a 4mm thickness (should be able to do better, but worst case) that gives me an 80mm head. Not small, but not too large I think (comments on this welcome).
If I were to trim off the flange from the lenses (2mm) then the lens dia goes down to 26mm, which would mean that the inner bezel size would be reduced to about 63mm, allowing an outer size of about 70mm. A bit better.
The LEDs will be wired up using a single 1050mA AMC7135 driver board from DX using the nifty circuit shown on this page. I will remove the diode from the driver to increase efficiency.
There is no need for multimode, if I ever need less light for close up work I'll put a diffuser over the front.
The battery canister will contain a string of batteries in xP4S configuration. The x is because I haven't decided how many parallel batteries to put. I have also not decided what batteries to use.
I could use 3P4S x 18650 batteries, giving a can about 350mm long, 45mm in diameter and holding 7200mAh. That would be a very narrow can, so I could do 4P4S increasing the diameter to about 50mm and increasing the capacity to 9600mAh. 4P4S seems OK to me, but I've never used a can light before so I don't know if 350mm x 50mm are reasonable dimensions. Packhorse? Any advice?
Another possible arrangement would be a 7P4S arrangement of 14500 cells, which would give me a 50mm dia can roughly 250mm long holding about 6300mAh. 7P4S of 16350 (RCR123A) batteries gives a can 200mm long and 55mm in diameter, holding about 6000mAh.
The dimensions for the 7P4S 14500 and 16340 cells seem more pleasing on paper, but the energy density is obviously far lower. Bear in mind, though, that I will be drawing 1A from the pack, so even these packs would provide well over 4 hours of runtime, which would be enough for two or perhaps even three dives between charges. The other advantage of the RCR123A pack is that batteries are cheap and easy to come by anywhere. That being said, I'd prefer the larger capacity, if those dimensions are still within acceptable size for a light can.
If anyone has any advice, tips or general opinions on what I'm planning here, it would be greatly appreciated. Thanks in advance guys and I await your words of wisdom.