How is the brightness of lamps measured?
Light output, or radiant power – i.e. the brightness of a light bulb – is measured using a so-called integrating globe. To do this, the light bulb is positioned in the centre of the globe. The light output emitted by the lamp is then reflected by the inner surface of the globe. The sum of the directly emitted and the reflected light output then gives the total light output. This is then measured using a photometer and expressed in lumens.
This measurement method has the advantage that the beam pattern of a lamp – i.e. where the lamp emits more or less light – is irrelevant: the indirect light output is distributed evenly, thus producing the same illuminance everywhere. This prevents measurement errors caused by varying beam patterns.
What causes a decline in luminous flux?
The lifespan of an LED depends on many factors – more than just operating time:
- LED light bulbs and LED chips are composed of so-called semiconductor crystals. Despite careful manufacturing processes, it is technically impossible to produce completely pure crystals, meaning impurities occur in some areas. As the LED lamp is used over time, higher temperatures are generated, which these areas cannot easily withstand. This marks the start of the inevitable degradation of the LEDs.
- Although LEDs are very efficient, they generate a certain amount of waste heat despite their low energy consumption – particularly in the area of the semiconductor chip. The higher the operating temperature, the more negatively thermal processes affect reliability and light quality. If this heat is not adequately dissipated, the decline in light output accelerates significantly.
The lifespan of LEDs can be influenced
The effects of thermal, electrical and environmental factors on LED degradation are measurable – and, with proper operation, can also be controlled under the following conditions:
- Thermal management: Aluminium bodies or special cooling fins help to dissipate the generated heat efficiently. Both the design of the lamp and the light fixture material play a role here.
- Ambient temperature: In warm or poorly ventilated environments, heat dissipation deteriorates, which accelerates LED degradation.
- Maintenance factors: These describe the extent to which a lighting system requires maintenance or the need to readjust light output over the years to ensure specific illuminance levels are maintained in the long term.
- Role of the driver: A high-quality driver ensures a constant current flow and protects the LED from voltage spikes. Cheap or unsuitable drivers, on the other hand, can lead to failures or accelerate the degradation process. Particularly with dimmable lamps, compatibility between the dimmer and the driver is crucial for stable operation.
