Luminous flux is the photometric power of a light source measured in lumens (lm). This implies total flux emitted in all directions but since lamps, light emitting diodes,... do not typically emit light in parallel beams this can be a problem for planar detectors. A measurement geometry that enables all of the luminous flux to be acquired independently of spatial distribution is required. Light collecting integrating spheres or goniophotometers which can sum light output over 360° are often used. The luminous flux from a 100 W incandescent bulb is about 1380 lm, while that from a 20 W compact fluorescent lamp is about 1200 lm.
At 555 nm, where the human eye has its maximum sensitivity, a radiant power of 1 W corresponds to a luminous flux of 683 lm.
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