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)
