Unlike lifetime based on luminaire maintenance, lifetime of the luminaire has to do with the reliability of the components of a LED luminaire as system.
Factors
Affecting System Performance - LEDs
Factors
Affecting System Performance - Optical
LEDs
are directional light sources, giving the lamp designer new challenges in
comparison to existing lamp technology.
The
use of reflectors, lenses and diffusers allows a designer to direct light in
many different ways.
The
efficiency and depreciation of the optical system must be considered and
factored into the overall efficiency of the luminaire.
Factors
Affecting System Performance- Mechanical
The
mechanical integrity of a luminaire is important in several different areas
including: IP rating to suit the application, heat sinking that will not become
compromised with time, lack of maintenance vibration resistance so the heat
sink does not become detached from the PCB, bonding mechanisms that are
suitable for the life of the luminaire.
Many
LEDs and specifically phosphor can react to different chemicals. Some gaskets
can out-gas chemicals which can affect the performance of some LEDs.
Factors
Affecting System Performance- Cooling System
The
performance of a LED is dependant on its temperature during operation.
The
design of the luminaire will influence its operating temperature and hence
published characteristics.
The
paint finish or fixture colour may affect the heat dissipation from the
luminaire.
A
PCB is the interface between a LED and the heat sink and has a thermal
resistance value.
The higher the resistance the less efficient the system is at
conducting away heat from the LED.
This may well impact on the LED lumen output
performance and ultimately its life.
Factors
Affecting System Performance- Electronic
Electrical
overstress is a cause of catastrophic failure of LEDs.
Some
LEDs contain an on board Transient Voltage Suppression
chip (TVS), which provides some
level of protection.
A
well designed lamp will feature the necessary design or protection in order to
minimise damage at installation or power-up.
Failure
rate of the external control gear shall be included in the overall assessment
of total life/failure rate.
System
Reliability
The
entire system only lasts as long as the critical component with the shortest
life, whether that critical component is a weather seal, an optical element, a
LED, an electronic control gear circuit or something else.
LED
light sources are simply one critical component among many, although they are
often the most reliable component in the whole system.
How
To Verify Lifetime Claims
In
most cases, the reliability of LED luminaires is longer than the practical test
times, verification of manufacturer’s life time claims need to be made in a
sufficiently confident way.
In
order to validate a lifetime claim, an extrapolation of test data is needed.
There is no validated way to translate the lumen maintenance curve of an
individual LED light source into a curve for the LED module or LED luminaire.
Beyond
the 10,000 hours testing recommended by LM-80, statistical predictions are
made. If LED light source data is available for 6000 hours, LED modules and LED
luminaires may have life testing carried out for just 2000 hours!
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