Lysstrøm

Lysstrøm is the Danish term for luminous flux, a photometric quantity used to describe the total perceived power of light emitted by a source. The unit is the lumen (lm). Luminous flux differs from radiant flux (measured in watts) because it weights light by the human eye’s sensitivity, providing a measure of how bright the light appears rather than how much energy it carries.

Definition and calculation: The luminous flux Φv is obtained by integrating the spectral radiant flux across wavelengths, weighted by the photopic luminous efficiency function V(λ). In practical terms, a luminous power of 1 watt at the peak eye sensitivity wavelength (555 nm) corresponds to 683 lumens, the standard constant Km = 683 lm/W used in the definition of the lumen.

Dependence on spectrum: Two light sources can have the same radiant flux but different lysstrøm if their spectral distributions differ, because the eye responds differently to various wavelengths.

Related quantities: Luminous intensity (candela) is the luminous flux per unit solid angle, while illuminance (lux) is the luminous flux per unit area. Luminous flux is commonly specified for lamps and luminaires to indicate brightness.

Measurement: Lysstrøm is measured with photometric instruments, typically using an integrating sphere connected to a calibrated photometer or spectroradiometer.

Applications: It is used in lighting design, product specification, energy efficiency analysis, and marketing to quantify perceived brightness and compare light sources independent of direction.