I can measure lumens within 5%. I recently ordered a Nite IZ 74 lumen bulb and installed it in a vintage flashlight. I measured 60 lumens in front of the flashlight. Near enough. The manufacturer specification is usually measured at the output of the LED itself. Normally the front lens of a flashlight emits a little less light due to reflector inefficiency etc. Thought I'd get a little brighter with FALANFA (rated 180 lumens). FALANFA should be at least twice as bright as Nite IZ. When I finally installed the LED bulb, I was surprised at how weak it was (I measured 20 lumens). It's either buggy or some kind of false advertising. I made sure the bulb said "3V" on the side (to make sure I got the correct voltage LED bulb).
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