LED and lifetime
The light output from all lightsources including LED, halogen, metal halide and fluorescent, decreases over time. The amount of light from the lightsource at a specific time in the future is referred to as the lamp lumen maintenance factor, or LLMF. The lifetime of a LED module is defined as the time it takes until its light output, or lumen maintenance, reaches 70% of the initial output (L70). In other words, the module does not die instantly as many conventional lightsources do, but rather slowly dims down. The luminaire industry has standardized LED lifetime L70 to a minimum of 50,000 hours. This corresponds to an LLMF of 0.7 as long as the lifetime of the lighting installation is set to the same amount of hours.
Lighting planners take this effect into account when they dimension the lighting installation. As they want the light level to reach a minimum lux value at the end of the lifetime, they over-dimension the luminaire installation, often by using more luminaires. Therefore, the lux value in the beginning of the operating life of the lighting installation is higher than what it is at the end.
The actual lifetime of a LED luminaire is important for cost savings
For many of Luxo's products, the lumen degradation is better than the industry standard and more than 80% of the initial output remains after 50,000 hours. The LLMF is then raised to 0.8 or higher and the lighting designer does not need to over-dimension the lighting installation. 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 still meeting the lighting requirements. In our datasheets this is shown as Lx = 50,000 hours at 77⁰F (25⁰C) ambient temperature (Ta).
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. IEC 62722 states both the test method and minimum required time for testing LED lifetimes. The minimum test time is 6,000 hours where the luminous flux is recorded every 1,000 hours. These values are extrapolated using a method stated in IES TM21.
Life time for LED modules
The lifetime of a LED module 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 in B- and C-values.
Luxo normally states two lifetimes1:
- Lifetime L70 at maximum Ta. This value is normally minimum 50,000 hour
- Lifetime L70 at Ta=25. This lifetime depends on the module’s temperature condition.
B value
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 a minimum of 50% of the LED modules and that the remaining 50% may have a lower lumen value. The value B10 means that a minimum of 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. Luxo uses B10 as the standard value when stating LED lifetimes.
C value
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.
F value
The F value depicts the combined failure fraction. This is the combination of both gradual (B) and total failures (C).
Driver lifetime
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. Luxo only uses drivers from quality suppliers. 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 50,000h with an estimated failure rate of 10% during this time.
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 the environment is 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 reflective factors in the room, how clean the environment is and finally the cleaning interval of the room. Normal values for interior use are between 0.95–0.97 for clean environments.
LLMF is corresponds to the L-value. For example L80 reflects an 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). Luxo states the L-value after 50,000 hours at Ta 25 deg. Since the LLMF is based on average values it is stated as B50 such as Lx B50.
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.
Example 1:
Recessed LED luminaire used in an office landscape. This luminaire has a lifetime of L90 = 50,000 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
Example 2:
Recessed LED downlight used in a corridor. This luminaire has a lifetime of L80 = 50,000 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 not exceed the lifetime at max Ta. For example, a downlight may have a lifetime of L70 100,000 hours at 77⁰F (25⁰C) and 75,000 hours 95⁰F (35⁰C) (max Ta). An enclosed installation would call for a lifetime of 75,000 hours.
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