Facts of life
The reference value that we use to indicate the duration of an incandescent bulb is known as its average life.
Does that mean that a bulb with an average life of 5,000 hours must last 5,000 hours?
Not exactly. Average life is determined by means of a statistical estimate, and to calculate it we use a large enough sample of lamps so it can be considered representative. We experimentally measure the time elapsed until 50 per cent of the bulbs stop working, and the obtained value is what we define as average life.
Thus, if one of these bulbs lasted, for instance, 4,000 hours, we could not say it was a faulty device. In fact, it is most probable that half of the bulbs we use during our lifetime last less than the stated time. And the other half will probably last longer.
This is a useful fact whenever the features of the light casted by the source keep within acceptable parameters —both in quantity and in quality—, until it entirely stops working. But, with technologies that experiment a noticeable decrease of performance as their elements get old, average life is not a recommended concept. The case would happen that, after a determined time period, we would keep having the light source on, but the light casted would be insufficient or inadequate. That’s the reason why we define another time frame, known as useful life —a period equal or inferior to the average life—, that determines the maximum working hours accepted, past which replacement is advised.
The mentioned depreciation affects discharge lamps and, in a smaller extent —in fact, it is not even considered—, incandescent lamps. But the most paradigmatic case are LEDs, because in their working pattern there is not a moment in which they stop working completely, but they experiment a steady and sustained process of flux reduction.
Source: Adapted from Bullough, J.D., 2003. Lighting Answers: LED Lighting Systems. Troy, NY. National Lighting Product Information Program, Lighting Research Center, Rensselaer Polytechnic Institute
Then, when does the LED stop fulfilling its function?
We consider that the LED has exceeded its useful life when it has suffered a 30 per cent depreciation —in other words, when its light flux is reduced to a 70 per cent of the flux initially emitted. The time elapsed until it reaches that point is known as useful life period (L70). However, it is important to note that the setting of the 70 per cent as a limit is due to a convention adopted by the industry: it does not respond to any technical basis. Thus, it is better to consider it only as a reference value, especially useful to compare the performance of the different LED. In practice, we should be responsible enough as to know to what extent the designed equipment can admit a specific flow reduction and, based on that, we will be able to determine the real life of our project, which could be higher or lower than L70.