Zundanc3:
Watts, Lumens, Kelvins, Amps, Li-ion, NiMH, Lead-acid.. what/which?
Lumens is the best way to measure light output. Good luck finding a subjective site that rates all the lights you may want to compare.
Battery technology is another big mess. The three basic options for rechargeable batteries are LI-ion, NiMH, and NiCAD. Li-ion will produce the longest usage in the smallest package, but at the highest price, and Li-ion is not available from most manufactures.
NiMH is memory free, and is likely the best compromise between price, density, and weight/size. Most manufactures are now offering NiMH batteries.
NiCAD batteries have memory issues, and one must be careful to keep them charged. If they sit at half charge for too long, the battery will no longer *remember* that it can take more change, hence the word memory. NiCAD batteries can be reconditioned with a deep discharge, but many chargers do not offer conditioning charge cycles. NiCAD batteries have been popular for a long time, and there is nothing wrong with using them. The debate on the *best* battery is subjective with both battery styles having strengths and weaknesses.
HID lights are have a high color temp of 6000K (think whiter) and produce a large output for the wattage used. A 10W HID light is roughly equal to a 50W Halogen bulb. HID bulbs are fragile, and expensive to replace. They are also hot, and many can not be used topside. They have to be babied a bit as one can not generally just turn them off, then on, then off again in quick succession.
LED lights are very durable, and burn forever. The light output is generally not as focused, and the color temp is around 5000K. They are cool, and very effective for a lot of applications.
Halogen bulbs are the ones found in the majority of flash lights on the consumer market. The are hotter than LED lights, but cooler than HID. They have a lower color temp (3400K, think browner), but are inexpensive to replace, and widely available.
You really need to provide a lot more information about budget, size constraints (if any) the type of light you want, and so on if you want to get good facts as to what to purchase. You will get a lot of people who will tell you just to go buy light X, and be done with it. But there are other factors, and carrying around a 21W Salvo HID may not be the best option for the diving you are doing even if it's a great light.
There are also some conditions where a focused light is more desirable vs. a wider beam and VersaVisa! One requirement I have for a light is that if I spend a fortune, I want to be able to use it as a light source for photography. HID lights tend to have a hot spot in the center of the beam which means that using an HID lightsource for video or photo applications requires a way to diffuse that hot spot.
Unfortunately lighting is more technical than most people realize. How a light head is designed (beam angle), what bulb technology is used, and what battery choice along with many other factors all impact how it performs, how big it is, how long it will burn, and how durable it maybe.
