Lumen
Lumen (lm) is the unit of measurement for luminous flux. Put simply, luminous flux describes the total amount of visible light emitted by a light source. Unlike electrical power in watts, the photometric quantity of luminous flux takes into account the human eye’s varying sensitivity to different wavelengths.
In the case of vehicle headlamps, the lumen value therefore provides information about the total amount of light emitted. However, it does not indicate how this light is distributed or how much light reaches a specific area.
Why are lumens not sufficient for assessing a headlamp?
A high luminous flux does not automatically mean that a headlamp illuminates a particular working area more effectively. How the light is distributed by reflectors, lenses and other optical components is also crucial.
Two work lights with the same luminous flux can therefore produce very different light patterns. For example, one work light may distribute its light over a wide area at close range. Another may concentrate a larger proportion of the light, thereby illuminating a narrower area at a greater distance.
When assessing a work light, therefore, light distribution and illuminance should be taken into account alongside the luminous flux.
Effective and theoretical lumens
With LED headlights, a distinction is made between theoretical and effective lumens.
Theoretical lumens are calculated based on the luminous flux specifications of the installed LEDs. This calculation does not fully account for losses within the finished headlamp. Effective lumens, on the other hand, refer to the luminous flux actually emitted by the finished headlamp. This makes them better suited for a realistic comparison of different headlamps.
J.Führ specifies the effective luminous flux for its products
Lumen, candela and lux
Lumen describes the luminous flux of a light source. For the practical assessment of a vehicle headlight, other photometric quantities are important:
- Lumen (lm): How much light does the light source emit in total?
- Candela (cd): What is the luminous intensity in a specific direction?
- Lux (lx): What is the illuminance on a specific surface?
It is only the interplay of these quantities and the light distribution that enables a meaningful assessment of the lighting.