Lumen Depreciation and LED Lighting

When replacing lighting systems, the goal is to reduce operating costs while maintaining or improving light quality. Three primary strategies can help achieve this:

• Switching from less efficient lighting technology to LED
• Selecting DLC-listed lighting fixtures for energy efficiency
• Implementing dimming controls to reduce energy consumption

All lighting technologies-including Metal Halide, High Pressure Sodium, fluorescent, and LED-experience lumen depreciation over time, though at different rates.

Fluorescent and HID lights typically degrade faster than LEDs, which is why many facility owners are converting from fluorescent to LED systems.

Understanding Lumen Depreciation

Lumen depreciation refers to the gradual decrease in light output over a fixture’s operational life. Several factors influence the rate of depreciation:

• Lighting technology type
• Total operating hours
• Ambient temperature conditions
• Continuous operation (24/7 usage)
• LED drive current levels
• Installation environment factors

Performance Comparison: While LED technology offers the best overall lumen maintenance by design, fluorescent lights perform reasonably well compared to Metal Halide and HPS systems, which experience the fastest lumen depreciation rates.

Example of an LED high bay fixture designed for long-term lumen maintenance

What is L70?

L70 is a rating that indicates how many operating hours it takes for an LED fixture to decrease to 70% of its initial light output. In simpler terms, it tells you when the fixture will lose 30% of its original brightness.

Example: A fixture producing 20,000 initial lumens with an L70 rating of 50,000 hours should still produce approximately 14,000 lumens after 50,000 hours of operation.

How is L70 Calculated?

The lighting industry uses a standardized process called TM-21 to calculate L70 ratings. Since testing lights for tens of thousands of hours would be impractical, this method:

• Tests LED performance for 6,000 hours at three different temperatures
• Uses mathematical modeling to simulate long-term performance
• Extrapolates results to predict L50, L70, and L90 values
• Applies conservative estimates to ensure reliability

What are Good L70 Numbers?

Quality LED fixtures should have an L70 rating of at least 50,000 hours. We recommend avoiding fixtures with L70 ratings below this threshold, as they may require more frequent replacement and higher long-term costs.

How Does L70 Differ from L50 or L90?

These ratings indicate different levels of lumen depreciation:

• L50: Time to reach 50% of initial lumens
• L70: Time to reach 70% of initial lumens (industry standard)
• L90: Time to reach 90% of initial lumens

L70 is the standard metric used for comparing LED fixture longevity.

Does L70 Mean the Fixture Will Stop Working?

No. L70 only indicates when the fixture will produce 70% of its original light output. The fixture will continue operating beyond this point, though with gradually decreasing brightness.

Understanding L70 vs. Actual Fixture Life

An LED fixture with an L70 rating of 200,000 hours doesn’t guarantee 200,000 hours of operation. LED drivers, which power the LEDs, are typically the limiting component. Quality drivers generally last 50,000-70,000 hours, potentially failing before the LEDs reach their L70 point.

When drivers fail, you can either replace the driver or the entire fixture, depending on cost-effectiveness and available efficiency improvements.

Traditional Lighting Lumen Depreciation

Metal Halide and High-Pressure Sodium bulbs experience much faster lumen depreciation:

• Rated life: Approximately 15,000 hours
• L50 point: Around 7,500 hours (50% of rated life)
• Performance: Noticeable lumen loss often occurs within 6 months of operation

Key difference: While Metal Halide bulbs start very bright, they lose lumens rapidly compared to LED systems.

Maintenance Factor Planning

The maintenance factor estimates how much initial light output decreases over time in a lighting installation. This calculation is critical for determining proper fixture quantities and planning maintenance schedules.

Proper maintenance factor planning helps avoid:

• Over-installation of fixtures to compensate for depreciation
• Unnecessary capital costs and energy consumption
• Inadequate lighting levels over time

Professional Consultation: Lighting design and calculations should involve qualified lighting professionals to ensure compliance with applicable codes and standards for your specific application.