These bulbs need a ballast, of course. You CAN overdrive HID lamps for more brightness, but it greatly shortens bulb life. The MR16-size bulbs I've seen are 10W rated (About 1000 lumens). LED beats HID for lumens up to about 15W still. In a small reflector you won't see the throw benefits of the small arc.
The MR16's are pricey and floody so I went to 35W car HID's and have never looked back. You need to source a reflector that takes vehicle shaped bulbs (e.g. H3) but the collimated throw/spot potential and at one third the price means there has to be a pretty good reason to stick with MR16.
Thanks for the replys guys. DimitrisV there was nothing showing up on that page.....:-( The lamp is for a dive torch with no space for ballast. At the moment I have a 50W mr16 halogen 9degree lamp but the burn time is only 45 mins. I want to keep the lumens but increase burn time. The led ones I have seen look too big to fit in the lamphead. It needs to be the exact size of a standard MR16 with the same intensity of a 50W halogen. Any recommendations???
There may be no space inside for a ballast but don't foget the outside.
Having an outside ballast in a dive torch achieves two things where heat is concerned. Firstly, 35W+ ballasts get hot so the surrounding water cools it. Secondly, it prevents everything inside becoming a heat-sink. You don't want your batteries getting any hotter than they already will with internal resistance.
If you still decide to go HID my earlier hint about not using MR16 HID's still stands. Check out the difference between two beams from two 35W HID's Both draw the same power but clearly one is more usable as a primary dive torch than the other...and a third of the price!;
This one has the ballast on the outside with nothing more holding it than thick wad of polyeurathane glue (Sikaflex Marine). The four leads are re-routed through the glue so the inside of the ballast stays waterproof, and hte flexibility of the glue allows for the dissimilar expansion and contraction during the dive.
The latest miniaturisation innovation for HID's has the igniter and ballast in one slimline rectangle. The point being you don't need to accommodate a separate igniter like you used to.
MR16 and HID don't work together ... you would never get enough space to fit these parts together....... and if you fit accidently together you can't use light power without much bigger parabola like Klem uses for instance
Again, the size kicks you in the pants. LEDs need a thermal path, and high-power ones are larger, requiring a bigger reflector to get the same throw. A Cree XM-L driven at full current can just about match an MR16 for output, and a throwy reflector could make a nice beam. But that'll require reworking your drive electronics, and ensuring a thermal path, and...
Retrofitting old lights is tough, and not usually the best use of your money.
You can quite easily build a 35W HID with an MR16 HID globe if you want to...but my point is why would you go to all that trouble when there are better and cheaper solutions than the MR16?
Here's an earlier MR16 35W HID torch built using plastic plumbing parts, a plastic pond light, and an Otterbox. I stopped diving with this light years ago given a vehicle HID with 100mm diameter collimated reflector eclipses the MR16 hands down (see beamshot comparison above).
In answer to your question about the sourcing a H3 reflector, you need to look around for the halogen torches. The two in the next photo are from a cheap $10 Chinese 6V incandescent torch that I found in a hardware store. I kept the reflector and everything else went in the bin. 100mm diameter and focussed to a spot. You don't want to go larger than 100mm because it displaces too much air and you need to start adding ridiculous amounts to lead to keep it negatively buoyant. I have tried 125 and 150mm diameter reflectors but they become way too unwieldy underwater, and too heavy above water.
Here are three examples of H3 auto HID globes in 35W rigs; a 100mm reflector on the left, and a 150mm reflector next to it. The 150mm needed a small dive belt to keep it down. The right hand photo is a 125mm car spotlight with no more than the seals gummed-up for waterproofing and the back of the reflector painted for rust prevention. Again, it displaces too much water...too buoyant. You can add lead to the inside of the housing but while that makes it negatively buoyant it still always wants to point to the surface, and it became all too hard.
100mm diameter reflector is a good trade-off between maximising light and unwanted buoyancy/bulk.
If you are replacing an incandescent globe and you want a spot (collimated) then you will need to play around with the seating of the HID globe to get the glow spot in exactly the right position for the refelctor. The H3 HID globe will likely be out by about 2mm. You also might want to use a 5mm+ synthetic lens like Perspex or Lexan with these large unsupported diameters. Convex tempered glass to the outside water pressure works, but only to a certain unknown depth and then...'lights-out!' At least with synthetic you have more peace of mind.
If you want a brighter light than a 100mm 35W then I recommend not to go a wider reflector, but a more powerful HID. But there is a point when it all gets too big and unwieldy.