I don't know tech stuff enough to know how to answer all of that. I bought the led fixture that i linked to (i noticed the original link i made is broke, this is the same fixture elsewhere):
http://www.ebay.com/itm/10W-Underwa...561?pt=LH_DefaultDomain_0&hash=item5656ca7a19
awhile back and realized that the lens was the perfect size, it snapped into the fixture i was making perfectly, and worked well as far as ability to adjust beam of light from wide to narrow. I only needed to move light source a few inches to go from one to the other, which means the fixture can be short. There were no specs listed for the lens. I couldn't find them off the shelf. Lots of companies at alibaba will make them for you, but it was looking really expensive to do it that way. As mentioned, i need fifty of them, possibly more at a later date.
Material: i don't know enough about the options to know what is best, i only know the lens in the fixture i linked to works perfectly, as does this one (which is the wrong size):
http://dx.com/p/50mm-18mm-2-80-degree-glass-optics-12834#.Uu8kHbRqf24 . I assume they are glass, but could be some other material like Borosilicate.
Effective focal length: They're flat on one side, curved on the other (plano convex i think?). I positioned each so a light six feet or so away hit the flat side and moved the glass until the image of the bulb projected was sharp, then measured distance from flat edge of glass to projection surface. They measure 2.5 (the dx.com one) and 3 inches. All things considered, shorter FL is better because it means the fixtures will be smaller, i don't know if there is an advantage to having longer focal lengths.
Center thickness: I don't know what the dis/advantages are for this. Less thickness is better as far as keeping fixture small, but being able to move light a small distance to get from narrow beam to spot is important.
Coatings: I don't know a lot about the different coatings, but do know that some coatings help reduce color fringing by reacting differently to different wavelengths of light, i'm not sure yet how effective they'll be, i'm more interested in lower cost for now until i see. The lights will be useful regardless, but i'll buy coated versions later if it seems a problem. Clear aperture, radii, conic, i don't know what these mean, or if they're important as long as the other specs work.
As far as price, the lenses i've been using (about 9-10mm too short) have been $3-$4 each (less in bulk).
Thanks again,
Rob