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Luminous flux: light output measured in lumens

Depiction of luminous flux as a measure of luminous output.

Luminous flux is a measure of the light output of a light source

Luminous flux (Φ), measured in lumens (lm), describes the total light output emitted by a light source in all directions. The luminous flux is measured with special measuring instruments or by calculation. This total light output is calculated from the radiant flux by evaluation with the spectral luminous efficiency of the human eye.

Overview on the topic luminous flux

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What does luminous flux tell us?

Luminous flux: depiction of the spectral brightness sensitivity of the human eye for day and night vision

Luminous flux as a photometric quantity indicates the total light output emitted by a light source. This specification is used as a measure of the total brightness of a light source as perceived by the human eye V (λ).
In principle, this radiant flux could be specified as emitted energy in the unit watt (W). However, the optical effect of a light source is not adequately described because the emitted radiation is recorded in an indiscriminate way over the entire frequency range and the different spectral sensitivity of the eye is therefore not taken into account.

How is luminous flux measured?

Luminous flux is measured in laboratories with a spectrometer in combination with the so-called integrating sphere or Ulbricht sphere (integral measurement). The luminous flux is the basis for further calculations, e.g. calculating the radiant flux. Determining the luminous flux of a light source (lamp or luminaire) is technically quite a complex measuring procedure, which is why luminaire manufacturers usually specify the luminous flux values in product documents.

In which unit is luminous flux measured?

Lumen (lm) is the standardised unit for luminous flux. Colloquially we talk about the amount of light, and express how much light a light source emits.

Can the light output be calculated?

The light output can also be calculated. The formula:

luminous flux (lm) = illuminance (lx) × surface (m²)

What should be considered in lighting design?

In the context of lighting design it is advisable to look at the luminous flux of the luminaires irrespective of the lamp luminous flux, i.e. the freely radiating luminous flux of a lamp. The design of the luminaire already results in losses here (e.g. due to shielding or light deflection), which must be taken into account. For this reason the luminaire luminous flux is measured outside the luminaire, while the lamp luminous flux is measured separately from the luminaire.