Lumen

The lumen (lm) is the SI derived unit for light flux, which is the amount of light being emitted into 1 unit of solid angle (1/4π of the area of a sphere) by a light source with a luminous intensity of 1 candela. Therefore a source of 1 candela will be putting out a total of 4π lumens.

• Figure 1. Both bulbs emit 800 lumens, however the LED bulb uses less electrical power to do so.
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  Incandescent bulb: Requires 60 watts of power.
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  LED bulb: Requires 9 watts of power.

Light bulbs are often rated in lumens in order to communicate their brightness. For example, a 9 watt LED light bulb and a 60 watt incandescent light bulb have the same brightness of 800 lumens. The LED bulb accomplishes the same goal while using less power.

Lux

Figure 2. The difference between the Lumen, the Candela, and the Lux.

Lumens can be thought of as the "amount" of light within a beam. However, lumens do not explain how much light will arrive at any given distance from the source. The unit for this is lux, which is equal to 1 lumen per square meter. In Figure 2, the difference between the Lumen, the Candela, and the Lux is clearly represented.

A full moon on a clear night is tremendously bright (high lumen value), however, the lux received on the Earth is about 1/4 of a lumen per square meter. In comparison, a light bulb will have an illumination of approximately 50 lux, while direct sunlight can get up to 130,000 lux on the Earth's surface.

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