Lumens, on the other hand, has its own problems. Lumen ratings across manufacturers can vary wildly across brands. Many manufacturers give a rating for emitter output, which is highly optimistic because you can factor in about 30% light loss due to the reflector and window. Others make up wild claims and numbers just to look good. Precious few, give accurate lumen ratings. HDS/Ra is known to give exact lumen ratings. Surefire is known to underrate their lights.
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In order to get a lumen rating for a light a special tool called an integrating sphere is needed. It is expensive and a precision instrument. But if you wanted to to subjectively compare the total output of one light versus another a good way is to go into a small, enclosed space with little to no light, such as a bathroom, and perform a "ceiling bounce test," which consists of pointing an activated flashlight at the ceiling and measuring (with your eyes) how brightly lit one of the walls are, repeating for each flashlight to be compared.
