What You Need to Know About LED Rated Life and Lumen Depreciation

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What You Need to Know About LED Rated Life and Lumen Depreciation

When LED fixtures (or lamps) are installed in a building, thinking about replacement is likely the last thing on the minds of staff. With LED rated life ranging from 25,000 hours (for lamps) to over 50,000 hours (for fixtures) the typical time to replacement is measured in years.

However, facility managers should be aware at the outset of LED installations of the different requirements for maintaining and monitoring LED lighting compared to fluorescent, halogen or HID.

With traditional incandescent, halogen, fluorescent and HID light sources, end of life often meant when the lamp goes dark. This is not the case with LED.

The graph below shows the typical life of different light sources. Two things to note. First, the slope of the line indicates declining lumen output, called lumen depreciation*, with increasing hours of operation. Second, the sudden drop to zero for incandescent, fluorescent and HID compared to the gradual slope down toward zero for LED.

The graph represents a different maintenance challenge for facility staff. Instead of replacing lamps when they “burn out”, with LED, replacement needs to be scheduled before light output has declined to the point that it is noticeable to the occupants of the space.

When manufacturers of LED light bulbs and integrated LED fixtures list the rated life in the spec sheet, you will normally see the designation 'L70' next to the rated life. L70 is an industry standard, meaning the lumen output has declined to 70% of the initial lumens.

For example, if you install LED troffers with a rated life of 50,000 hours @ L70, you should assume that the lumen output of most of the troffers will drop below optimal (70% of initial lumens) at around 50,000 hours of operation. The LED light source may continue to emit light until it has been operating for 80,000 hours or more, but it will cease producing effective lighting at around 50,000 hours.

As a facility manager, you may ask how do I know when the fixtures in a space drop to 70% of initial lumen output. There is only one accurate way - with alight meter that shows lux (or footcandles) that can be converted to lumens.

Before LED lighting, light meters were likely rarely found in facilities. Today they are an essential tool.

Here is an example of how a light meter is necessary for LED lighting maintenance.

If the specification sheet say an LED fixture has initial lumens of 29,000, then the likely fixture or LED light source replacement needs to happen when the light meter shows an average of 20,000 lumens in the space (29,000 initial lumens x .70 = 20,300 lumens). If a light meter is not used, the only alternative is to wait until the occupants start complaining of low light. Remember, in most cases LEDs do not ‘burn out’, they simply get dimmer and dimmer over time.

You may ask, why 70%? Studies have been done that show most people who work in a space do not notice there is insufficient light until the light level drops to around 70% of initial output. However, different tasks or different space lighting requirements may be more sensitive to a decline in illumination. In these cases a 20% drop instead of 30% (L70) in lumen output may be the trigger for lamp or fixture replacement. If so the rated life shown of 50,000 hours @L70 may not apply. The replacement will need to happen sooner.

* Lumen Depreciation Defined

Lumen depreciation is the lumen output lost at various points in the operating life of the lighting component. It is usually expressed as a percent of the initial lumen output.

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