Photometry
What is photometry?
Photometry deals with the measurement and evaluation of visible light. Unlike the purely physical measurement of electromagnetic radiation, it takes into account the spectral sensitivity of the human eye.
Photometric quantities can be used to describe light quantitatively. In the case of lights, for example, this allows us to investigate how much visible light is emitted, in which directions it is distributed and what illuminance is achieved on a surface.
Which quantities are used in photometry?
The most important photometric quantities include:
- Luminous flux describes the total amount of visible light emitted. Its unit is the lumen (lm).
- Luminous intensity describes how strongly light is emitted in a specific direction. Its unit is the candela (cd).
- Illuminance describes how much luminous flux falls on a surface. Its unit is the lux (lx)].
- Luminance describes the light emitted by a surface in a specific direction and is expressed in candelas per square metre (cd/m²).
These quantities describe different properties of light and must therefore not be equated with one another.
Photometry for spotlights
A single photometric value is often insufficient for assessing a spotlight. For example, a high luminous flux does not automatically mean that a particular working area is illuminated particularly well.
It is also crucial how the headlamp’s optics distribute the available light. Two work work lights with a similar luminous flux can therefore produce different beam patterns.
The resulting illuminance at various positions can be shown in an lighting diagram. This makes it possible to assess how wide and how far a work area is actually illuminated.
Photometry and human perception
Photometric quantities already take the spectral sensitivity of the human eye into account when evaluating light. However, this does not mean that subjectively perceived brightness can be fully described by a single measured value.
Contrast, ambient light, the eye’s adaptation and the distribution of light also influence perception.
In addition to photometric quantities, other characteristics such as colour temperature may therefore be relevant to the practical perception of lighting.
Why is photometry important for vehicle lighting?
Photometric measurements make it possible to objectively describe the characteristics of vehicle lighting and to compare different lighting solutions with one another.
In the case of work lights, for example, the measured values help to select the appropriate light distribution for a specific working area. In the case of headlights, photometric measurements also serve to verify compliance with prescribed photometric requirements.
Photometry thus forms the metrological basis for assessing light not merely as ‘bright’ or ‘dark’, but for describing its properties using defined quantities.