Anything related to the *luminous flux* emitted by a light source. The luminous flux is a physical quantity used in photometry to measure the amount of light power radiated from a light source that can be perceived by the human eye. Its unit of measurement in the SI is the *lumen* (lm).

Anything related to the luminous flux emitted by a light source. The luminous flux is a physical quantity used in photometry to measure the amount of light power radiated from a light source that can be perceived by the human eye. Its unit of measurement in the SI is the lumen (lm).

Note that luminous flux, also called luminous power, is a different quantity from the electromagnetic power emitted by the light source (which is measured in watts, as usual for any power). In fact luminous flux takes into account only the light components that fall in the visible spectrum and also accounts for the different sensitivity of the human eye to the different wavelengths.

In lighting engineering luminous flux is a definitely more useful quantity than the radiated power, since it neglects all the spectral components that don't contribute to the actual human vision and thus aren't relevant in lighting design.

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