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Lumen Depreciation in LED Lamps and Fixtures

One of the benefits of a lighting retrofit is increased life of the lamp or fixture. Both LED and fluorescent lighting fixtures have much longer lifespans than lighting fixtures that were available just a few years ago. For some businesses, the savings in maintenance costs can be even more valuable than the savings due to energy.

So to make a solid, educated choice at your facility, you need to understand what manufacturers mean when they talk about the lifespan of a fixture.

Lumen Depreciation is Essentially Dimming:

The first thing you need to understand is that every light source gets dimmer over time. This is referred to as lumen depreciation.

The first time a lamp is turned on will be its brightest. This is called “initial lumens”. At the end of the lamp or fixtures ‘rated lifespan’, the light will be at its dimmest, which is referred to as the lamp or fixture’s “end of life” or EOL lumens. The difference between a fixture’s initial lumens and end of life lumens is referred to as “lumen maintenance”. If a lamp had a lumen maintenance of 50%, that would mean it is expected to lose half of its brightness during its rated life. If it had a lumen maintenance of 90%, that would mean it is expected to lose just 1/10 of its light output during its rated lifespan.

Different lighting technologies have different levels of lumen depreciation. Metal-halide lamps typically have lumen maintenance ratings of 50%-70%. LED Lamps and fluorescent lamps have much higher lumen maintenance ratings, thus much longer expected lifespans.

LED Lifespans and How They Are Determined:

With traditional lighting, the lamp is the first source of failure. Just like incandescent bulbs in your home burn out after a year or two of use, in commercial and industrial facilities with HID and fluorescent fixtures, the lamp will burn out after a certain time period. So with HID, fluorescent and most other lamps, the lifespan has been defined as the length of time that it’s expected to take for 50% of the lamps in a large group to burn out.

That means that if you were installing 50 HID high bays, their predicted lifespan would be how long you expect it to take for 25 of them to go out.

The Illuminating Engineering Society of North America (IESNA) has named this rating method LM-40-01, the Approved Method for Life Testing of Lamps.

Unfortunately, or fortunately depending on what you manufacture, that doesn’t work for LED lighting. LED lighting doesn’t “fail” in the same way as other lighting technologies. Instead of going out like a traditional light source, LEDs just get dimmer and dimmer over time. Most likely, the initial point of failure in an LED fixture will be the driver and not the light source or lamp.

As a result, IESNA developed lifespan rating standards for LED lamps based on the amount of time it takes for the light source to dim below usable light levels. Usable light levels are usually defined as L90, L80 or L70 — the point at which the fixture has dimmed to 90%, 80% or 70% of its original output.

IESNA has developed two standards used in rating the lifespan of LED lamps. The first, LM-80 is a standard for measuring lumen maintenance and depreciation in LED Diodes. A test protocol called TM-21 takes the LM-80 data of an LED diode, and uses algorithms to predict the lifespan of a diode. There are two different types of TM-21 ratings, both “reported” and “calculated”.

Reported TM-21 values have an upper limit of 6-times the number of LM-80 test hours. So if an LED chip is tested for 6,000 hours, its maximum reported TM-21 lifetime would be 36,000 hours. If the chip was tested for 10,000 hours, its maximum reported TM-21 would be 60,000 hours.

Calculated TM-21 lifetimes are the results of algorithms calculated based on the LM-80 data, not actual burn-hours of chip testing.

What Does This Mean For You?:

Now that we understand how different lifespans are rated, what do those differences mean for your facility?

When a traditional lamps reach the end of their lifespan, they fail. The light won’t come on anymore. When an LED light source reaches the end of its lifespan, it still comes on, but it’s dimmer than it was the first time you turned it on.

If retrofitting to fluorescent lighting, ask yourself if your facility will be okay operating with 50% of the lamps in the fixtures out. If the answer is no, you should plan on re-lamping prior to the rated lifespan.

If retrofitting to LED lighting, ask yourself if your facility will be okay with its lighting at 70% of its original output. If the answer is no, you should plan on performing lighting maintenance prior to your LED fixture or lamps rated lifespan.

If it all sounds confusing, that’s because it is. Call a member of The Lumen Squad at Orion Lighting Solutions out to your facility so you can enjoy the benefit of having a lighting professional assist you today.