Lighting Measurements – An In-depth Guide Part 1
When retrofitting your LEDs, lightbulbs have various illuminance. Theres different ways to measure the lighting of…
Part 2 of our four-part commercial LED lifespan series. Click here to review Part 1.
In commercial lighting specifications, “L” identifies lumen maintenance: the percentage of an LED’s original light output remaining after a stated number of operating hours. Also called brightness retention, this describes how much light the LED still produces compared with when it was new. L70 indicates 70% retention, L80 indicates 80%, and L90 indicates 90%.
These ratings help evaluate useful life, meaning the operational period before lighting falls below the levels required for its application. Useful life differs from catastrophic failure, which occurs when an LED or another component fails completely and stops producing light. A fixture can remain operational while becoming too dim for the space it serves.
Key Takeaways
- L70, L80, and L90 identify retained light output and must be paired with operating hours.
- Higher retention indicates stronger projected brightness maintenance when hours and operating conditions match.
- LM-80 measures LED component performance; TM-21 uses those measurements to project future output.
- Component lumen maintenance does not establish complete fixture reliability or guarantee a failure date.
- Commercial lighting plans should account for declining output before illumination falls below required levels.
What Is LED Lumen Maintenance?
LED lumen maintenance measures retained light output against the source’s initial output. Lumens measure the total visible light a source emits.
LED lumen depreciation describes the corresponding loss. A source with 80% lumen maintenance has experienced 20% lumen depreciation.
Calculate retention using this formula:
Lumen Maintenance (%) = (Current Lumen Output ÷ Initial Lumen Output) × 100
For example, a source that initially produces 10,000 lumens and later produces 8,000 lumens has 80% lumen maintenance.
What Is the Difference Between L70, L80, and L90?
The number after “L” identifies the percentage of original light output remaining at the stated operating time.
| Rating | Maintained Output | Lumen Depreciation | Example Specification |
| L70 | 70% | 30% | L70 at 100,000 hours |
| L80 | 80% | 20% | L80 at 75,000 hours |
| L90 | 90% | 10% | L90 at 50,000 hours |
These specifications are illustrative, not universal product benchmarks.
An L70 rating of 50,000 hours means the source is projected to reach 70% of its initial output at approximately that operating time under the stated conditions. It does not remain at full brightness until that hour and suddenly become dimmer.
For the same source and conditions, L90 is generally reached before L80, followed by L70.
Is L80 Better Than L70?
An L80 LED rating indicates better projected brightness retention than L70 when both apply to the same operating hours and comparable conditions.
However, an L80 claim at 30,000 hours cannot be directly ranked against an L70 claim at 100,000 hours. Compare the percentage, hours, temperature, and supporting data together.
What Does L70B50 Mean on a Specification Sheet?
Specifications sometimes pair a lumen maintenance threshold, written generally as Lp, with a population-based Bx rating.
For example, L70B50 at 50,000 hours describes a projected median lumen maintenance life: approximately half the relevant population remains at or above 70% of initial output at that time, while half falls below it.
The B value addresses variation in gradual depreciation. It generally excludes abrupt failures and does not mean every finished fixture was physically tested for the full stated lifetime. Confirm whether the declaration applies to LED modules or complete fixtures.
Why Do LEDs Gradually Lose Light Output?
LED light output degradation develops as the semiconductor and materials within the LED package age. Operating temperature and electrical current influence the rate of deterioration.
Thermal management helps move heat away from the LEDs. Poor heat dissipation or operation beyond the fixture’s temperature rating can accelerate aging. Temperature fluctuations can also change immediate output and contribute to long-term stress.
Dust accumulation and deteriorating lenses can further reduce delivered illumination. These losses occur alongside LED component depreciation and require separate consideration.
How Does Lumen Maintenance Change a Fixture’s Output Over Time?
Consider a fixture that initially delivers 20,000 lumens. This hypothetical timeline illustrates its output at successive retention thresholds.
| Illustrative Operating Point | Brightness Retained | Calculated Output |
| New installation | 100% | 20,000 lumens |
| 30,000 hours | 90% | 18,000 lumens |
| 60,000 hours | 80% | 16,000 lumens |
| 90,000 hours | 70% | 14,000 lumens |
These hours are examples, not product predictions. Actual depreciation is generally non-linear and depends on operating conditions, including thermal fluctuations. The “bathtub curve” describes changing failure rates across a product population, rather than the shape of an individual LED’s brightness decline.
This simplified example assumes fixture output follows LED source retention proportionally, without additional dirt, optical losses, or component failures.
How Do LM-80 Testing and TM-21 Projections Work?
The Illuminating Engineering Society (IES) publishes methods that support consistent measurement and projection of LED lumen maintenance.
| Comparison | LM-80 | TM-21 |
| Method | Physical laboratory testing | Mathematical projection protocol |
| Main purpose | Measure output and color maintenance over time | Estimate future lumen maintenance |
| Evidence used | Measurements from LED samples under controlled conditions | LM-80 measurement data |
| Main limitation | Evaluates LED components rather than complete fixture reliability | Estimates performance beyond measured hours within reporting limits |
What Is LM-80 Testing?
IES LM-80 measures the light output and color maintenance of LED packages, arrays, and modules at controlled temperatures and electrical currents.
Reports commonly include 6,000–10,000 hours or more of observations. These measurements document performance during the test period; they do not independently establish a complete fixture’s operating life.
Make Smarter LED Retrofit Decisions with a Better Understanding of Lighting Lifespan
Understanding L70, L80, and L90 lumen maintenance ratings is an important part of evaluating how LED lighting systems will perform over time. While LED fixtures may continue operating for years, their light output gradually decreases, making it essential to consider long-term illumination, maintenance requirements, and replacement planning when selecting lighting solutions. A well-planned LED retrofit can help commercial facilities maintain consistent lighting performance while maximizing energy efficiency and long-term savings.
If you haven’t already, read Part 1 of our Commercial LED Lifespan Series to learn how LED lifespan and rated life are determined. Then, explore our LED Retrofit Solutions to see how Action Services Group helps organizations select and implement energy-efficient lighting systems designed for long-term performance. Connect with our lighting experts to discuss your facility’s lighting needs and opportunities to improve efficiency, reliability, and return on investment.
👉 Read Part 1: How Long Do Commercial LED Lights Last? Understanding LED Lifespan and Rated Life
What Is TM-21 Testing?
Although often called “TM-21 testing,” IES TM-21 is a calculation method that projects long-term lumen maintenance from LM-80 data.
Its reported projection limit is no more than six times the underlying test duration, with applicable sample-size and method requirements. For example, 10,000 hours of qualifying measurements can support a reported projection up to 60,000 hours.
Claims beyond the applicable limit are extrapolations outside TM-21 reporting limits and should not be presented as verified TM-21 results. Buyers should also confirm that the projection matches the LEDs’ operating temperature and current within the proposed fixture.
Does an L70 Rating Predict Complete Fixture Reliability?
An L70 rating does not cover every way a fixture can fail.
Finished fixtures also contain drivers, connections, lenses, seals, and other components. A failed driver can stop illumination while the LEDs still retain substantial output. Moisture intrusion, electrical damage, and optical deterioration can also shorten service life.
Review component projections alongside fixture construction, environmental ratings, driver information, and warranty terms.
Frequently Asked Questions
Does L80 mean an aging LED uses 20% less electricity?
No. A 20% reduction in output does not establish an equivalent reduction in electricity consumption. An aging fixture may produce fewer lumens while drawing approximately the same power.
Can cleaning restore lost LED brightness?
Cleaning can recover light blocked by dirt on accessible optical surfaces. It cannot reverse aging within the LED package. Follow the manufacturer’s cleaning instructions.
Are lumen maintenance percentages the same as foot-candles?
No. Lumen maintenance compares source output over time. Foot-candles measure light reaching a surface, which also depends on mounting height, spacing, beam distribution, and surrounding surfaces.
Plan for the Light Your Facility Will Need Over Time
Warehouses need sustained visibility for picking and movement. Retail spaces rely on adequate illumination for merchandise and navigation. Parking areas need sufficient coverage for pedestrians, vehicles, and potential hazards.
Lighting designers account for declining illumination through a Light Loss Factor (LLF), which combines relevant allowances such as lumen depreciation and dirt accumulation. For example, an overall LLF of 0.80 means the design anticipates 80% of initial calculated illumination under the assumed conditions. It is not automatically equivalent to an L80 rating.
Contact Action Services Group to evaluate your lighting and plan an LED upgrade that supports adequate illumination, practical maintenance, and long-term facility performance. Reach out today by calling 610-558-9773, emailing [email protected], or scheduling a call that fits your needs by clicking the button below.
