Lumen Depreciation and LED Lighting

When we talk about replacing lights, we try to reduce the current costs that old, inefficient lights incur. We also try to keep (if not improve) the 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 including Metal Halide, High Pressure Sodium, fluorescent and even LED suffer from lumen depreciation. Fluorescent and HID lights degrade the fastest, and that is why many warehouse owners are converting fluorescent to LED.

Lumen depreciation is a process in which the total lumens produced decrease over time. The rate at which lumens depreciate depends on 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 fluorescent and LED lamps are the lowest to depreciate their lumens? Metal Halide Fixtures and HPS are the fastest to depreciate their 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 use 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 an LED Fixture include an L70 rating in its specifications. L70 is a calculation of how many hours it would take for a fixture’s LED chips to lose 30% of their initial lumens.

A fixture produces 20,000 lumens. Its L70 is 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 tens of thousands of hours to figure out lumen degradation.

So, scientists developed a testing process that runs for 6000 hours at three different ambient temperatures. The TM-21 calculates via simulation the life of the LED chips. It then takes the results and extrapolates them to L50, L70, and L90. To add 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. It’s only a sign of when the fixture will be at 70% of its 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 an 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 the L50 for Metal Halide and High-pressure Sodium Bulbs?

The period for L50 in these types of bulbs is about half 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 a maintenance factor? It is an estimate of how much the initial lumens are reduced within a lighting scheme. This is very important to calculate as it will tell us how much light there will be in a space a few years down the line. This allows us to plan and install the proper and correct number of fixtures into a building. And then you must understand at what point you might have to address diminished light levels.

When 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.