Candela
What ist candela?
The candela (cd) is the SI unit of luminous intensity. Luminous intensity describes how strongly a light source emits light in a particular direction. It is one of the fundamental quantities in photometry.
In contrast, luminous flux, expressed in lumens, describes the total amount of visible light emitted by a light source.
What does ‘candela’ mean in the context of a headlamp?
For a lamp, it is not just the total amount of light it emits that matters. Equally important is the direction in which this light is distributed. A narrowly focused headlamp can achieve a high luminous intensity in its main beam direction. If the same luminous flux is distributed over a wider area, the luminous intensity in individual directions is correspondingly lower. Two lights with a similar luminous flux can therefore produce significantly different beam patterns and ranges.
The relationship between candela and lumen
Luminous flux and luminous intensity are related via the solid angle. Put simply, luminous intensity describes how much luminous flux is emitted in a particular direction within a solid angle.
One candela is equivalent to one lumen per steradian:
1 cd = 1 lm/sr
The steradian (sr) is the SI unit of solid angle. It describes a solid angle and is therefore the three-dimensional counterpart to an angle in a plane.
Candela and lux are not the same thing
Whilst candela describes the luminous intensity in a specific direction, lux indicates the illuminance on a surface.
This distinction is important for the practical assessment of a work light. The candela describes a characteristic of the light emitted by the work light. The lux, on the other hand, describes how much light reaches a specific point on an illuminated surface. Illuminance therefore depends, amongst other things, on the luminous intensity, the distance from the light source and the angle at which the light strikes the surface.