Lumens, Lux and Burn Time - Caveat Emptor
Various LED dive light suppliers use different methods of determining light output, plus expressing them in different forms of measurement. What matters is the visible light from the LED that bounces off the reflector and passes through the lens. The quality of the reflector and lens, as well as the actual amount of power delivered by the battery have a major affect on the amount of light that reaches the target. Dive lights would be better described by the system lumens (sometimes called out-the-front or OTF lumens), which is the amount of visible light emitted by the entire system of battery, bulb, reflector and lens. Describing any LED dive light in terms of the radiant lumens specification of the entire light output for a raw LED module (including IR and UV wavelengths we can't see) at maximum power under laboratory conditions can be very misleading.
We must rely upon our supplier published specifications for our descriptions, but we suggest you take all of them with a grain of salt. Like the fuel economy ratings on automobiles, your results for light output and burn times may vary considerably from those published by the dive light manufacturers. In our opinion, using lux measured at the target, and not just initially but across the entire burn time, is the best way to describe the brightness of dive lights. The difference is that the lux takes into account the area over which the light beam is spread. The lux is the visible light measured at the target distance, not the bulb. A light of 1000 lumens, concentrated into an area of one square meter, is said to illuminate that square meter with an brightness of 1,000 lux. However, the same 1000 lumens, spread out over ten square meters, produces a much lower illuminance of only 100 lux.