I'm no physicist, but I'll tell you about my personal experience.....Peter Guy:
First, I'm not a cave diver, though that's something I have been interested in since before learning to dive, but I've been into some "caverns" and swimthroughs that technically I should not have entered
These caverns and swimthroughs are basically mini-caves with exits that are easy to find using some "uncommon sense," even if some bozo screws up the viz. What folks say about dark is true ~ there is no dark like the dark of a cave.... dry or wet! Unlike dry caves, the water actually seems to help with the dispersing of the light beam. My 18W HID is focusable, which means I can make it a smaller beam for signalling or a wider beam for covering more area. What I've found in dark, dark places is that the wide beam is awesome for lighting up the structure and formations of caves. It's way cooler to see these in context than in little snippets caught in a narrow beam. It's a fascination that is impossible to describe to someone who has not seen it or is not interested in such things. If you look at the few "Chandelier Cave" pics I have in my Gallery, you can see how well my HID lit things up even for pics with my little point~n~shoot digital camera. The little built-in flash would never be able to light things up that much in the underwater shots (the ones that say "chamber' were actually not underwater, but the rest were). It's only good for up to about 12-16", depending on viz.
So.... point is, if the diver is interested in the stuff to be offered via a cave dive, an 18W HID will illuminate plenty more than a narrow beam of view. Also, the allure of cave diving to me is much more than that which is to be seen. Kind of like tech diving for me... Going there to see the structure and/or life is primary, but part of the allure is the planning and preparing and talking about and executing the actual dive with a favorite "buddy."
