Lux
What is a lux?
A lux (lx) is the SI unit of illuminance. Illuminance describes how much luminous flux strikes a given area. It is one of the fundamental quantities in photometry.
One lux is equivalent to one lumen per square metre:
1 lx = 1 lm/m²
Whilst lumen describes the total luminous flux of a light source, lux therefore indicates how much of this reaches a surface.
What does ‘lux’ mean in the context of a spotlight?
With lamps, illuminance is of particular interest because it describes the actual illumination of an area. A worklight, for example, can produce high illuminance in the immediate vicinity. However, as the distance increases, the illuminance decreases.
The light pattern also plays a crucial role here. A wide-beam spotlight distributes its luminous flux over a larger area. A more concentrated spotlight, on the other hand, can achieve higher illuminance levels within a smaller area and over greater distances. The lumen rating alone is therefore not sufficient to compare the practical illumination provided by different spotlights.
Why does illuminance decrease with distance?
As the distance increases, the light is distributed over a larger area. Under idealised conditions, the illuminance from a point light source decreases in proportion to the square of the distance. If the distance is doubled, the illuminance on a surface perpendicular to the direction of the light is therefore only a quarter of the original value.
In practice, however, the light pattern, the headlamp’s optics, the mounting height and the angle of incidence of the light all influence the measured illuminance.
The relationship between lux and candela
Luminous intensity in candela describes how strongly light is emitted in a particular direction. Lux, on the other hand, describes the illuminance achieved on a surface. For a point light source and a measuring surface perpendicular to the direction of the light, the relationship can be expressed in simplified form as:
E = I / r²
Here, E represents the illuminance in lux, I represents the luminous intensity in candela and r represents the distance in metres.
The illuminance of a spotlight must therefore always be considered in relation to the distance and the specific measurement position.
Lux in the illumination pattern
The light distribution of a headlamp can be illustrated using a lighting diagram. In this diagram, lux values or areas of equal illuminance show how the light is distributed in front of the vehicle. This makes it possible to see which areas are illuminated particularly brightly and how far the useful illumination extends. Such a diagram therefore provides significantly more information about practical illumination than the lumen value alone.