While there are several factors to consider when choosing the right portable flashlight, total light output, expressed in Lumens, and a flashlight beam's apparent brightness, expressed in Candela (Candlepower) -- are two of the most important considerations. Following is a brief overview of what these measurements signify and how they are derived by Streamlight.

Lumens is a measure of a flashlight's overall visible light energy or total light output – which can include the total light from the bare bulb itself or the actual measured system output of the lighting device. Generally speaking, however, the lumens measurement is mostly a function of the light power of the flashlight bulb, whether of the incandescent or LED variety.

Lumens does not measure how bright a target will appear when the flashlight beam is trained on it –  the measurement of the brightest spot (or Peak Central Intensity) of a focused flashlight beam is expressed in candela. A function of how effective both the bulb and reflector are in producing a focused beam, candela is a good representation of both the tightness of the beam's focus and the apparent brightness of the light on a target at a distance.    

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Streamlight measures lumens using an integrating sphere, a special device that quantifies the total of the light radiated in all directions from a light source. A photometer, typically placed 100 feet awy from the light source, is used to measure candela.