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Lumen Measurement: The real story.

The luminous flux (lumens) is the total light emitted by a lamp regardless of the direction of emission.

There are two methods being used in the marketplace to state light output:
1. The calculation of the theoretical light output of a lighting system based on the LED-manufacturer's stated maximum light output
2. The measurement of the actual light output of a lighting system using a calibrated integrating sphere

Audiophiles face a similar issue when comparing the power output of sound system amplifiers. Cheaper components are often advertised with "Peak Power Output" (or "PMPO") watt values, indicating the maximum power that can be briefly delivered before sonic degradation or system failure. This is useless for describing the real-world power of an amp, high-quality components' power output is customarily expressed as "RMS continuous Power" derived from a sine-wave input. So a 5-watt (RMS) amp can become a 480-watt (PMPO) with the stroke of a copywriter's pen.

In most cases the lumen values given by other light manufacturers are based on the first method.

Sample measurements (performed on Lupine Germany's Intergrating Sphere)
Light System
# of Cree XM-L LED emitters
Theoretical Output
2000 Lumens
4000 Lumens
7000 Lumens
Measured Output
750 Lumens
1500 Lumens
2600 Lumens

The integrating sphere in action:

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