Lumens? How are they measure?

[Kgp]

I am a noob, bare with me...

I have a question about how a manufacturer measures there lumens? I'm talking out the front lumens. If two of the same flashlights had different reflectors. One for throw, and one for spill. Would the lumen rating be different? Being the one with the throw advantage put more light in one spot, and the spill divids the light over a large area?

 

[Marduke]

Out the front lumens are measured with an integrating sphere. However, many manufacturers use emitter lumens which do not take optical and heat losses into account. Still other manufacturers just make up a number out of thin air.

 

[kramer5150]

Intrgration Sphere...
http://en.wikipedia.org/wiki/Integrating_sphere

 

[Yoda4561]

Lumens are a measurement of the total light emitted from a bulb/led whatever. Beam shape and intensity, floody, spot, high or low candlepower, etc, have no direct bearing on lumen measurements. A calibrated integrating sphere is the only way to accurately measure this, most don't have them (they're VERY expensive, even for manufacturers). Most use the bulb/led figures from a spec sheet, and estimate output based on their drive current and voltage. While not nearly accurate enough for official numbers, many members here have made their own shade-tree IS, using a lux meter, various white enclosures and a large sample of lights with known output. Graphing those numbers and using an appropriate correction value can get a fair estimate.