Brightness
What does ‘brightness’ mean?
The term ‘brightness’ generally describes how bright a light, a surface or an environment is perceived to be. In lighting technology, however, brightness is not a clearly defined physical quantity with a specific unit.
Instead, various photometric quantities are used to describe different properties of light in measurable terms. These include, in particular, luminous flux, luminous intensity and illuminance.
Lumen, candela or lux?
Which quantity is relevant depends on which property of light is to be considered:
- The lumen (lm) is the unit of luminous flux [INTERN: Luminous flux]. It describes the total visible light output emitted by a light source.
- Candela (cd) is the unit of luminous intensity. It describes how strongly light is emitted in a specific direction.
- Lux (lx) is the unit of illuminance. It describes how much luminous flux strikes a specific surface.
The statement that one headlight is ‘brighter’ than another is therefore ambiguous without further details.
What determines perceived brightness?
How bright an illuminated environment is perceived to be does not depend solely on a spotlight’s luminous flux. The light distribution, the distance, the illuminated surfaces and the ambient conditions also play a role.
Two work lights with the same luminous flux may therefore appear to have different levels of brightness in practical use. A floodlight with a wide beam angle distributes its light differently to a floodlight with a highly concentrated beam.
When comparing floodlights, it is therefore more useful to consider specific photometric parameters and the respective light pattern, rather than simply referring to ‘brightness’.