LED and Lifetime Calculations
The actual lifetime of a LED luminaire is important for cost savings
For some of our products, the lumen degradation is better than the industry standard. After 50000 hours of life, not only 70% of the life remains, but 80% or even more. The LLMF is then raised to 0,8 or above and the lighting designer does not need to over-dimension the lighting installation as much. With this upgrade, up to 25% of the luminaires may be left out, depending on the ceiling type. That means lower installation costs and a lower energy bill. At the same time, the lux levels are always meeting the lighting norm requirements.
New standards on LED lifetime
New international standards on how lifetime should be declared on LED luminaires have now been published. The standards are IEC 62717 LED-modules for general lighting – Performance requirements and IEC 62722-2-1 Particular requirements for LED luminaires.
What do the standards say?
IEC 62722 states both test method and minimum required time for testing LED lifetimes. The minimum test time is 6000 hours where the luminous flux is recorded every 1000 hours. These values are extrapolated using a method stated in IES TM21.
The lifetime for the LED module and the driver should be declared separately. If the lifetime for the driver is shorter than that of the module, a driver replacement may be necessary before the luminaire’s lifecycle is completed. This means that there cannot be a single figure declaring the total luminaire lifetime.
A helpful metric for “median useful life” has been introduced in IEC 62717. This is the time elapsed until 50% of the LED-luminaires in use reaches the stated light output, for example L80.
How manufacturers lists lifetime for LED luminaires
The lifetime of a LED module inside a luminaire is related to the luminous flux depreciation at a given ambient temperature. L70, L80 or L90 indicates how many lumens (in percentage related to the initial lumens) that remains after end-of-life. The L-value may be explained B- and C-values.
• The median useful life (IEC 62717) LxB50 at Ta 25 degrees. Lx may vary from 70 to 90 depending on the product and the corresponding lifetime goes up to 100.000 hours.
• Lamp lumen maintenance factor (LLMF) 50000 h, B50, Ta 25 deg. This is a factor from 0 to 1,0.
For industrial luminaires, we also list:
• The median useful life (IEC 62717) LxB50 at maximum Ta. Lx may vary from 70 to 90 depending on the product and the corresponding lifetime goes up to 100.000 hours.
The failure fraction for By expresses only the gradual light output degradation as a percentage y of a number of LED modules of the same type that at their rated life designates the percentage (fraction) of failures. The value B50 indicates that the declared L-value will be achieved by minimum 50 % of the LED modules and that the remaining 50% may have a lower lumen value. The value B10 means that minimum 90 % of the LED modules will meet the declared L-value and only 10% will have a lower flux level. A B10 value is therefore more conservative than B50. In practical terms it means that B10 is reached at an earlier stage in the life time on a set of LED modules compared to B50.
The failure fraction for Cy expresses only the abrupt light output degradation as a percentage y of a number of LED modules of the same type that at their rated life designates the percentage (fraction) of failures. A catastrophic failure is when the LED module does not emit any light.
The figure shows the two factors that affect LED luminaire lifetime – gradual degradation of the lumen output or abrupt failure of the luminaire. Source: ZVEI.
The F value depicts the combined failure fraction. This is the combination of both gradual (B) and total failures (C).
The LED driver can be compared to other electronic components such as HF-ballasts for T5 lamps. The expected life time depends on the design itself, the used components and the temperature of these components. The drivers are marked with a reference temperature point, Tc, where the stated temperature never should be exceeded. Often the maximum ambient temperature (Ta) of the luminaire relates to the maximum value on the Tc-point. At this Ta we declare a lifetime of minimum 50.000h with an estimated failure rate of 10%.
Why is the L-value so important?
The L-value is directly related to the maintenance factor (MF) used when making a light calculation. MF includes the following parameters:
MF=LLMF x LSF x LMF x RSMF
- LLMF = Lamp Lumen Maintenance Factor
- LSF = Lamp Survival Factor. The LSF factor can be ignored (set to 1,0) if the luminaires are replaced immediately after failure.
- LMF = Luminaire Maintenance Factor. Depends on type of luminaire, how clean is the environment and finally the cleaning interval of the luminaire. Normal values for interior use are between 0,93–0,98 for clean environments.
- RSMF = Room Surface Maintenance Factor. Depends on reflectance factors in the room, how clean is the environment and finally the cleaning interval of the room. Normal values for interior use are between 0,95–0,97 for clean environments.
LLMF is basically the same as the L-value. L80 corresponds to a LLMF of 0,8. In order to find the correct L value you must first define the correct required lifetime and the correct required ambient temperature (Ta).
LMF and RSMF values are defined in CIE 97 and most PC based light calculation programs have tables that help you chose the correct value.
Recessed LED luminaire used in an office landscape. This luminaire has a lifetime of L90 = 50000 hours at Ta 25 degrees. Environment clean, cleaning interval twice a year, reflection factors 70, 50, 20.
MF = LLMF x LSF x LMF x RSMF = 0,9*1*0,96*0,96 = 0,83
Recessed LED downlight used in a corridor. This luminaire has a lifetime of L80 = 50000 hours at Ta 25 degrees. Environment clean, cleaning interval twice a year, reflection factors 70, 50, 20.
MF = LLMF x LSF x LMF x RSMF = 0,8*1*0,96*0,96 = 0,74
1 For products designed for enclosed installation, the lifetime will be lower than that for open installations. For example, a downlight may have a lifetime of L70 100 000 hours at 25 deg and 75 000 hours at 35 deg (max Ta). An enclosed installation would call for a lifetime of 75 000 hours.