Hi!
Any device that may create excessive heat or emmit inflamable gases is regarded as dangerous. There's a long list of simple household items that might cause troubles when on a plane.
Theory:
The IATA Dangerous Goods Regulation (DGR) has a section 2.3.3.2 where the handling of dive lights is described.
Dive lights have to be packed in the carry on and batteries (or, if not possible, the bulb) has to be removed to avoid switching on.
Lead-Acid batteries are strictly forbidden (danger of leaking acid - it could ruin the whole airplane). Also, items that react badly when humid or wet (condensation!) like LiIO batteries are a hazard.
Some flight operators require prior registration of such equipment.
Check details at the IATA website
The real world:
Check-in staff sometimes doesn't know these regulations very well. There should be a site officer at every check-in who is able to read and understand the DGR. Try this after 10pm at Mexico City airport ;-(
I have 3 strong lights and therefore lots of experience in this field.
It's wise to dismantle the lamp so you might pass without re-packing. It's also wise to keep all the stuff (bulb, batteries, tools to open the lamp etc.) in the carry-on so you're able to proof that it's real working torch when requested. Check-in staff sometimes has pretty funny ideas.
I usually call the airline and ask if they know about the regulations and if they care. If possible, I ask for a written statement on how they want the items to be packed.
This may be the hard way but it (sometimes) saves time and troubles when at the check-in counter. Most of the time the problems arise on the homebound flight. You're possibly in a country where you either can't speak the local language good enough, standards might be different and if you're in an area where divers are not that common, check-in staff doesn't recognize even the simplest items as harmless. I had to argue about the fact that a compass on my dive computer won't interfere with the on-board navigation system and that the device has to be carried at cabin pressure ...
Helge ;-)=)