Comparability of luminaire lifetime data
For 24/7 opening hours, long luminaire life spans are desired. However, very long luminaire life spans are also known from tunnel lighting where the maintenance of a lighting system requires a great deal of logistical effort. In such applications, a long service life can be quite economical, as the planning, manufacture and installation of the lighting system only has to be paid for once. How can a luminaire manufacturer position itself if luminaires with 75,000 hours instead of the usual 50,000 hours useful life are to be compared?
The service life specification has three parameters:
1. The luminous flux maintenance,
e.g. L90. 90% of the luminous flux is retained.
2. The average rated service life.
The percentage of all LEDs that fall below the luminous flux maintenance value of at most 50% is indicated as B50. On average, the specified luminous flux maintenance is thus achieved. If this specification is not indicated, B50 always applies. To ease comparability, the ZVEI recommends always assuming B50. Since all LED manufacturers determine the data for B50, B10 or B20 specifications should be rejected (only 10% or 20% of all LEDs fall below the luminous flux maintenance limit), since these values would have to be generated by a further extrapolation. A projection of a projection is always vague at best.
3. The useful life indication.
When does the luminous flux not fall below the specified value? 50,000 hours is assumed for professional luminaires.
- ZVEI, Leitfaden Planungssicherheit in der LED-Beleuchtung (Guide to Planning Reliability in LED Lighting), 3rd Edition, Free Download.
- The IEC 62717:2014 standard defines the performance requirements of LED modules for general lighting.
Stages of service life determination
A luminous flux maintenance test over at least 6,000 hours provides the data’s basis. Only 25 LEDs are subjected to this LM 80 report. Whether these first 25 LEDs also correspond to the millions of subsequent LEDs of this design is not rechecked. The LM 80 test is performed for two temperatures. For example, for 85°C, and 105°C. The luminous flux maintenance is determined for each temperature.
The luminaire manufacturer then checks the LED's temperature when it is installed in the luminaire in a thermally adjusted state. If, for example, it drops below the 85°C temperature threshold, the corresponding luminous flux loss for the luminaire applies.
The third step is the extrapolation, defined in TM 21, to six-times the duration of the TM 80 data. LM 80 data for 6,000 hours is required for a service life of 36,000 hours. For 50,000 hours, data must be available for at least 8,334 hours. Etc. To support the luminaire data sheet information, the luminaire manufacturer can provide LM 80 data and temperature measurement records.
Example 1: At 85°C the LED loses 3% of luminous flux after 8,334 hours. 3% x 6 = 18%.
Luminous flux maintenance = 82%.
If the LED's temperature test in the luminaire shows < 85°C, the value is indicated in our data sheets as follows: L80 (luminous flux maintenance after 50,000 hours > 80%).
Example 2: At 85°C the LED loses 1.5% of luminous flux after 8,334 hours.
1.5% x 6 = 9%. Luminous flux maintenance = 91%.
If the LED's temperature test in the luminaire shows < 85°C, the value is indicated in our data sheets as follows: L90 (luminous flux maintenance after 50,000 hours > 90%).
Comparison of L80 and L90 luminaires
Now to the actual question: Is an L90/50,000 hours luminaire better than a L80/75,000 hours luminaire?
Table 2's comparison of L90/50,000 hrs vs. L80/75,000 hrs shows that the L90/50,000 luminaire, after 50,000 hrs has 3.85%, after 75,000 hrs 6.25% and after 100,000 hrs has 9.11% more luminous flux.
Caution: For this comparison to be valid, ensure that the LM 80 test data for both luminaires are available for at least 1/6 of the service life specification!
Requirements of the L90
The highest requirement would be for luminaires with L90/100,000 hours. These 3 points would have to be considered:
1. Is there LM80 data of the LED over 16,667 hours? For this, the LEDs would have to be tested 24 hours a day for 2 years. Most LED manufacturers do not provide this data.
2. Since a L90/100'hr luminaire is equivalent to a L95/50'hr luminaire, the LEDs would probably have to be operated at a lower amperage. Thus, more luminaires would be needed for the same illuminance.
3. In the luminaire design, only converters rated for 100,000 hrs would have to be planned. Sometimes these converters have different dimensions. A new design may therefore be necessary. Such converters are also more expensive due to their higher quality. The higher price is divided between the longer useful life. A cost-benefit analysis is essential.
The market relevance of a longer luminaire life span than 50,000 hours depends on the industry. In retail, where the shop design changes about every 8 years, it is irrelevant. In the lobby of a hotel, you have to consider it. In a petrol station with 24-hour operation, the higher investment costs for a long luminaire service life can be profitable.