L&M's claims are realistic, though. They say their 13.5 watt light gives out 675 lumens, which is 50 lumens per watt.

That's a realistic performance, given the uncontroversial performance of HID lights measured in a variety of applications. The larger industrial HID lamps can do 80 lumens per watt or more. Some of the high end lamps put out more than 100 lumens per watt. HID do decline somewhat in lumen-watt efficiency as the lamp gets smaller, so smaller bike lamps would be expected to put out approximately what L&M say.

And yes, lumens themselves are a somewhat imprecise measure, since lumens, as standardized or as adjusted in various ways, are supposed to be relative to human vision. Still, they're not so imprecise as to make them meaningless given responsible measures.

Furthermore, if you see the L&M light, and you compare it to a 13 watt halogen lamp that you know puts out about 25 lumens per watt, you'd see that L&M's estimate is about right, too.

To return to the larger topic, though, HID put out more lumens per watt than halogen, whether halogen are overvolted or not. Don't take my word for it. There is ample information easily available to verify this. Here's a power company's comparison of various lighting types (offered as a recommendation of compact flourescent lights over HID, by the way):

http://www.bchydro.com/business/inve...igate3668.html

Notice that the Metal Halide lamps (HID) are rated at more than double the lumens per watt of halogen lamps. Now, there's no separate entry for "overvolted" halogen lamps, but overvolting halogen bulbs doesn't increase lumen-watt efficiency by more than about 20% or so. It certainly doesn't increase efficiency by more than 100%!

Again, though, halogen have an important advantage over HID (and any other lighting system now available, too). They're cheap. They're the best way to make lumens per dollar, by far.