Lichtstroom

Lichtstroom (Dutch for luminous flux) is the total amount of visible light emitted by a light source per unit time. It is a photometric quantity expressed in lumens (lm) and is weighted by the human eye’s sensitivity through the luminous efficiency function V(λ). This means lichtstroom reflects perceived brightness rather than the purely radiometric power of the emitted light.

Lichtstroom differs from related quantities. Luminous intensity describes the light emitted in a particular direction (measured in candelas, cd). Illuminance describes how much light reaches a surface (measured in lux, lm/m²). The same lumen value can produce different illumination depending on beam distribution and distance.

Measurement and calculation: In practice, lichtstroom is determined with photometric equipment, typically an integrating sphere or a goniophotometer, and standardized methods. The luminous flux is the angular integral of the source’s luminous intensity distribution: Φv = ∮ I(θ,φ) dΩ, or, for spectrally non-uniform sources, Φv = ∫ φ(λ) V(λ) dλ. Since V(λ) emphasizes wavelengths to which the human eye is most sensitive (around 555 nm), the same radiant power can yield different lichtstroom values for different spectra.

Applications and considerations: Lichtstroom is a central specification for rating light sources and luminaires. Higher lumen output generally implies more perceived brightness, but does not by itself indicate efficiency or beam spread. Luminous efficacy (lumens per watt) combines lichtstroom with electrical power. In practice, manufacturers also report lumen depreciation and lumen maintenance over time as LEDs and other sources age.

Typical values: A modern LED bulb might deliver 800–1600 lm, while a traditional incandescent bulb around 800 lm for a 60-watt equivalent. In Dutch-language contexts, lichtstroom is the standard term for this photometric quantity.