You might have heard the terms “lumen maintenance” or “lumen depreciation” or (if you’re more advanced) “L70” in reference to lighting – particularly LED lighting. These terms have to do with lifespan, which is particularly important for lighting for commercial, municipal, educational, healthcare, or industrial applications, where short operating lives come at a higher premium.
Like the metal halide (MH), high pressure sodium (HPS), and fluorescent/compact fluorescent (CFL) technology typically used in these settings, LEDs decrease in brightness over time. L70 is a calculation of when the output of a light source dips down to 70% of the initial lumens – after which point, the bulb or luminaire is considered no longer useful, and should be replaced.
The above chart maps the typical trajectory of lumen depreciation across many professional-grade lighting technologies compared to LEDs. L70 at 50,000 hours is the baseline for everything we carry; however, most of our newer industrial products such as LED high bays are rated to last two to ten times longer.
(Note that halogen/incandescent bulbs fail suddenly, or catastrophically, rather than slowly fade in brightness. This is because their light comes from filaments, which simply burn out, rather than from the gas-based chemical reactions of the ballasted lighting technologies, or from the electricity conducted through solid state Light Emitting Diodes.)
The slow lumen depreciation of LEDs is significant for several reasons. The most obvious one is that it supplements energy savings towards a return on investment. By saving on bulb replacements and on maintenance workers’ time – which is an especially important consideration for the workplace – LED lights pay for themselves. And the advertised lifespan of, say, a fluorescent T8 tube, is not as relevant if one third of it is spent transmitting dim, flickering light.
Secondly, lumen maintenance is worth noting when comparing the brightness of different lighting technologies. People are slowly disabusing themselves of the idea that Wattage measures brightness, and instead they are looking at lumen counts.
However, the advertised lumen output of a MH, HPS or fluorescent bulb is not an accurate indication of how bright it will be, since that value declines so drastically and so quickly. Occasionally, fluorescent lighting suppliers advertise a mean (average) value in addition to initial lumen output. When this is not available, one should determine their own value based on their needs and capabilities. For example, if you replace bulbs at the L70 mark, the lumen output at 85% initial lumens (or the halfway point between 100% and 70%) may provide a more accurate picture of how bright a fluorescent, MH or HPS light source will be.