How to Read LED Lifetime: L70, L80, B10, B50 and TM-21
Short answer
LED lifetime is written as 'Lx By': L shows the percentage of initial luminous flux that remains (L70 = 70%, L80 = 80%), and B the percentage of luminaires that fall below that limit (B10 = 10%, B50 = 50%). '50,000 h L80B10' means that after 50,000 hours at most 10% of luminaires have fallen below 80% of their initial flux, a stricter criterion than 'L70B50'.
LEDs do not burn out suddenly like an incandescent lamp; they slowly lose light over time. So LED lifetime is not measured by 'after how many hours does it fail' but by 'after how many hours how much light has it lost'. Expressions on datasheets such as '50,000 h (L80B10)' are the short answer to that question.
L and B values
| Abbreviation | Meaning | Example |
|---|---|---|
| L70 | Luminous flux falls to 70% of the initial value | 100 lm → 70 lm |
| L80 | Luminous flux falls to 80% of the initial value | 100 lm → 80 lm |
| L90 | Luminous flux falls to 90% of the initial value | 100 lm → 90 lm |
| B10 | 10% of luminaires fall below this limit | 10 out of 100 luminaires |
| B50 | 50% of luminaires fall below this limit (median) | 50 out of 100 luminaires |
The two values are read together: '50,000 h L80B10' means that after 50,000 hours at least 90% of luminaires still give more than 80% of their initial light. If no B value is given, B50 is usually meant, so the claim describes an average luminaire.
Comparing two claims
For the same period, L80B10 is a stricter claim than L70B50. Two products stating '50,000 h L70' and '50,000 h L80B10' are not of the same quality. When comparing, look at the L and B values together, not at the hours alone.
Where do these numbers come from?
- LM-80: a test in which the LED package is run for at least 6,000 hours at defined temperatures and its light loss measured.
- TM-21: a method for projecting forward from LM-80 data. The projection is reported up to at most 6 times the test duration.
- Operating temperature: the claim assumes the LED runs at a certain temperature inside the luminaire. If the housing does not dissipate heat well, the real lifetime falls short of the claim.
Drivers and other failures
The Lx By value describes only the LED's light loss. A drying driver capacitor, connection faults or moisture are outside this calculation, and these often decide a luminaire's real life. So the driver brand and temperature class should also be asked; for details, see our LED luminaire buying guide.
In Harmony ELS datasheets
The datasheets of our catalogue products state an LED lifetime of 50,000 hours (L80B10). This value assumes the LED runs at the target temperature in the luminaire housing; for thermal design, see our electronics and components page.
Frequently Asked Questions
What does L80B10 mean?
At the end of the stated period (for example 50,000 hours) at most 10% of luminaires fall below 80% of their initial luminous flux; the other 90% stay above 80%.
Which is better, L70B50 or L80B10?
For the same period L80B10 is the stricter claim: it allows less light loss (20%) and fewer luminaires (10%). L70B50 accepts 30% loss and half of the luminaires.
What is TM-21?
A method that projects lifetime from the LED lumen loss data measured in the LM-80 test. The projection may be reported up to 6 times the test duration; a 10,000-hour test supports a claim of at most about 60,000 hours.
Does LED lifetime include driver lifetime?
No. The Lx By value describes the LED's light loss; driver, capacitor and connection failures are assessed separately. The real life of a luminaire often depends on driver quality and operating temperature.