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When we talk about replacing lights, we try to reduce the current costs old, inefficient lights incur. And keeping (if not improving) our current quality of light in place. Here are some obvious options to achieve this:

Switching away from less efficient lighting technology to LED
Buying DLC listed lighting fixtures
Dimming the lights

All types of lighting – Metal Halide, HPS, Florescent or LED, all suffer from lumen depreciation.

Lumen depreciation is a process in which total lumens produced decreases over time. The rate at which lumens depreciate depends on so many things.

Type of lighting technology used
Total hours used
Ambient heat
Are the lights on 24/7
How hard the LEDs are driven
Various other factors

Did you know that florescent and LED lamps are the lowest to depreciate it’s lumens. Metal Halide and HPS are the fastest to depreciate it’s lumen output?

While LED is the winner overall in lumen depreciation by design, fluorescent lights aren’t too far behind and not nearly as bad as Metal Halide or HPS lights. Here at LED Lighting Supply, we would recommend you get LED for a lot more reasons than just slower lumen depreciation. Planning for lumen depreciation and light loss between lighting maintenance schedules is crucial.

What is L70?

You will sometimes see LED Fixtures include in their specifications L70. L70 is a calculation of how many hours it would take for the fixtures LED chips to lose 30% of its initial lumens.

A fixture produces 20,000 lumens. Its L70’s rated at 50,000 hours. This means that after running this fixture for 50,000 hours, it should still be producing 14,000 lumens.

How is L70 Calculated?

A process, called TM-21, was developed to help calculate L70 numbers. It would be impractical to run lights for 10’s of thousands of hours to figure put lumen degradation. 

So scientists developed a testing process that runs for 6000 hours at three different ambient temperatures. The TM-21 calculates through simulation the life of the LED chips. It then takes the results and extrapolates them to L50, L70 and L90. To add in a level of safety, the typical TM-21 numbers are generally conservative. 

What are good – or – reasonable L70 numbers?

A high quality LED fixture should have an L70 of at least 50,000 hours or higher. Avoid fixtures with an L70 that is less than 50,000 hours.

How does L70 differ from L50 or L90?

The only difference is the calculation to 50%, 70% or 90% of initial lumens. The standard you should pay attention to is L70.

Does L70 mean the fixture will stop working?

No. This is not a measurement or sign when the fixture will stop working. Its only an sign of when the fixture will be at 70% of initial lumens.

I saw a fixture with an L70 of 200,000 hours. Does that mean the fixture will run for 200,000 hours?

No. The weakest link of a LED fixture is the driver. The best drivers will last around 70,000 hours. So a driver might fail well before the L70 rating of the LED chips is ever reached. At this time, you could replace the driver, or it may make sense to replace the LED fixture. LEDs are always getting more efficient, with more features.

What is L50 for Metal Halide and High Pressure Sodium bulbs?

The time period for L50 in these types of bulbs is about 1/2 of the rated bulb life. These bulbs are typically rated for about 15,000 hours of total use. So at about 7500 hours, these bulbs are at L50.

Brand new Metal Halide bulbs are very bright. But they lose lumens very quickly. Even after only 6 months of use, there is a noticeable loss in lumen output from these fixtures.

Maintenance Factor of LEDs

What is maintenance factor? It is an estimate on how much the initial lumens reduces within a lighting scheme. This is a very important to calculate as it will tell us how much light their will be in a space a few years down the line. This allows us to plan and install the proper and correct amount of fixtures into our building. And then understand at what point you might have to address diminished light levels.

Trying to offset lumen depreciation, installations will add more lights to offset this. This leads to obvious increases in capital costs and not-so-needed extra energy consumption.

Want to know more about Maintenance Factors? Read our blog post on Maintenance Factor to get a full understanding of what it is.


Dwayne Kula


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About the Author

Dwayne Kula is President of LED Lighting Supply. On any given day, Dwayne is writing content for the site and helps manage the marketing initiatives that are on-going. He has a Software Engineering degree and still dabbles in writing software for the company as needed. When not working, he enjoys spending time with his family, working out, playing the occasional game of golf and exploring New England.

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