Lumen depreciation: what nobody accounts for

The light output of every LED luminaire declines over time. This isn't a defect but normal aging; yet it rarely appears in quotes, even though it decides whether the plan holds up.

What do L80 and L90 mean?

The L80B10 rating says: 90 percent of the luminaires still retain 80 percent of their original light output after the specified number of hours. So an L80B10 50,000-hour luminaire in two-shift operation drops to 80 percent after roughly ten to twelve years.

The B value is the failure rate: B10 means that ten percent of the units perform worse than this. On a project with a large number of fixtures, that ten percent is already a visible patch on the hall ceiling.

Why does it matter in design?

If an office is designed for 500 lux and the light level drops to 80 percent over ten years, 400 lux remain in the end: less than the standard requires. That's why a photometric calculation works with a maintenance factor, typically between 0.8 and 0.9.

The factor includes not only LED aging but also dust buildup. The diffuser of a luminaire in an industrial environment can absorb 10-15 percent of the light in two years if it isn't cleaned. The plan therefore also has to assume a cleaning cycle.

Constant light output

In practice, all this means that at the start the light level is higher than the target value. In a dimmable system, this isn't a problem: the luminaires initially dim themselves down, and ramp up as they age.

This is constant light output, and it can save as much as 10-15 percent of energy over the lifetime. On top of that, the light level stays consistent throughout, so users don't notice the gradual decline. For larger projects, it's therefore worth thinking in terms of a dimmable system from the start.