Light Attenuation Testing and Analysis: A Comprehensive Experimental Study of Lumen Depreciation in

Light attenuation testing is a critical reliability experiment that quantifies the gradual loss of luminous flux in LEDs, lamps, and luminaires over extended operating periods. This technical article explains the principles, sample requirements, measurement parameters, test methods, and data analysis procedures used in professional light attenuation (lumen depreciation) testing conducted in accredited third-party laboratories. Understanding these procedures helps manufacturers project useful lifetime metrics such as L70 and demonstrate compliance with international standards.

Technical Overview

Light attenuation, also known as lumen depreciation or luminous flux decay, refers to the progressive reduction in light output that occurs in solid-state lighting devices as they accumulate operating hours. Unlike traditional light sources that typically fail abruptly, LED-based products tend to dim gradually over time, which makes light attenuation the dominant end-of-life criterion for LED systems. The light attenuation test is therefore one of the most important reliability experiments in the lighting industry.

Several physical and chemical mechanisms drive lumen depreciation in LED devices. These include degradation of the semiconductor chip active region, thermal quenching of radiative recombination, photo-thermal yellowing or cracking of the silicone encapsulant, phosphor sedimentation and conversion efficiency loss, delamination at the chip-phosphor or phosphor-encapsulant interfaces, moisture-induced corrosion of metallization layers, and degradation of optical elements such as lenses and diffusers. Because the rates of these mechanisms are strongly temperature-dependent, light attenuation experiments are conducted under strictly controlled thermal conditions.

The internationally recognized framework for light attenuation testing is built around standards published by the Illuminating Engineering Society (IES), including IES LM-80 for LED packages, arrays, and modules; IES LM-84 for LED lamps and luminaires; and IES TM-21, TM-28, and TM-26 for projecting long-term lumen maintenance from measured data. In parallel, IEC 62717 (LED modules for general lighting) and IEC 62612 (self-ballasted LED lamps) define performance requirements, while Chinese national standards such as GB/T 24824, GB/T 24908, and GB/T 31831 specify measurement methods and depreciation limits for domestic products. Programs such as ENERGY STAR reference these methods to verify claimed lifetimes of 25,000 hours or more.

Testing Samples

LED Packages, Chips, and Arrays

Bare LED chips, packaged LEDs, and chip-on-board (COB) arrays represent the fundamental building blocks evaluated in light attenuation studies. These samples are typically powered on a temperature-controlled test board at specified case temperatures such as 55 °C, 85 °C, or 105 °C, following the requirements of IES LM-80. At least 20 units per test condition are required to achieve statistically meaningful projections.

LED Modules and Light Engines

LED modules with integrated or separate drivers are tested as complete electro-optical subsystems according to IEC 62717 and IES LM-80 guidance for modules. The module case temperature (Tc) is monitored and maintained at the rated maximum value defined by the manufacturer to simulate worst-case operating conditions.

Integrated LED Lamps and Luminaires

Self-ballasted LED lamps, downlights, street lights, high-bay luminaires, and panel lights are evaluated using IES LM-84 and IEC 62612 procedures. Because complete products include drivers, thermal management structures, and secondary optics, their depreciation profiles reflect system-level interactions rather than component behavior alone.

Specialty and Application-Specific Samples

Automotive lighting modules, horticultural grow lights, UV-LED assemblies, and display backlight units may be tested under application-specific temperature, humidity, and drive-current profiles agreed between the manufacturer and the accredited third-party laboratory performing the experiment.

Testing Items

A comprehensive light attenuation experiment measures the following parameters at each scheduled readout interval:

• Luminous flux (lumens) and luminous flux maintenance factor, expressed as the percentage of initial (hour 0) light output remaining at each readout
• Luminous efficacy (lumens per watt) calculated from simultaneous photometric and electrical measurements
• Chromaticity coordinates (x, y), correlated color temperature (CCT), and Duv, together with the color shift Δu'v' relative to the initial measurement
• Color rendering index (CRI, Ra and R9) to detect spectral degradation of the phosphor system
• Forward voltage, drive current, and input power to identify electrical drift and driver instability
• Case temperature (Tc) or solder point temperature (Ts) recorded continuously or at each readout to confirm thermal condition compliance
• Spectral power distribution (SPD) captured by a spectroradiometer for detailed analysis of wavelength shifts and phosphor band changes
• Catastrophic failure count and functional defect observations, including flicker, dead pixels, and intermittent operation

Testing Methods

Sample Preparation and Initial Characterization

Test units are first inspected for mechanical damage, aged for a short stabilization period (typically 0 to 100 hours) where required by the applicable standard, and then measured under reference conditions of 25 °C ambient temperature. The initial photometric and electrical data recorded at hour 0 serve as the baseline for all subsequent depreciation calculations. Photometric measurements are performed with calibrated integrating sphere systems (4π or 2π geometry) coupled to array spectroradiometers, or with goniophotometers for total luminous flux verification, in accordance with IES LM-79 and GB/T 24824 measurement practice.

Stress Conditions and Chamber Operation

Samples are then placed in temperature-controlled life-test chambers and operated continuously at the specified stress condition. IES LM-80 defines three standard case temperatures: 55 °C, 85 °C, and a third temperature selected by the manufacturer, typically at or above the maximum rated Tc. Relative humidity is controlled to remain below levels that would cause condensation (commonly below 65 % RH) unless a combined temperature-humidity protocol such as IEC 60068-2-78 (damp heat) is specifically required. Drive current is set to the rated value, and all electrical supplies are monitored for stability throughout the test.

Readout Schedule

Photometric readouts are typically performed at intervals of approximately 1,000 hours, consistent with the IES LM-80 requirement of at least 300 data points across the test population and readouts no less frequent than every 1,000 hours. A standard LM-80 campaign runs for a minimum of 6,000 hours, with extended campaigns of 10,000 hours or longer increasingly common for high-reliability products. During each readout, the samples are removed from stress, allowed to stabilize at reference ambient conditions, and measured with the same instruments and fixtures used at hour 0 to minimize measurement uncertainty.

Data Analysis and Lifetime Projection

The collected lumen maintenance data are analyzed using the exponential decay model described in IES TM-21 for LED packages:

Φ(t) = exp(−αt), where α is the decay rate constant derived from a least-squares fit to the interval between 1,000 hours and the final test hour (typically the 5,000-to-6,000-hour window for a 6,000-hour study). From the fitted decay constant, the time to reach 70 %, 80 %, or 90 % of initial flux (L70, L80, L90) is calculated, and results are reported within the TM-21 projection limit of six times the actual test duration. For example, a 6,000-hour LM-80 study supports a projected L70 claim up to 36,000 hours, expressed as L70(6k) > 36,000 hours. Analogous methods in TM-28 (lamps) and TM-26 (non-LED sources) apply similar exponential or logarithmic projection frameworks at the product level. Reported projections always state the sample size, test temperature, drive current, and projection limit to preserve traceability.

Failure Criterion and Reporting

A sample is classified as failed for depreciation purposes when its luminous flux falls below the selected maintenance threshold (commonly 70 %) or when it exhibits catastrophic failure. The final test report presents time-series depreciation curves, color shift trajectories (Δu'v' versus time), fitted decay constants, projected L-values with confidence context, and full descriptions of test conditions, instrument calibration status, and measurement uncertainty budgets.

Applications

• Lifetime claim verification for LED packages and lamps seeking ENERGY STAR, DLC, or regional market qualification, where documented LM-80 and TM-21 data are mandatory
• Quality control and lot acceptance in LED manufacturing, comparing depreciation behavior between production batches to detect process drift in die bonding, phosphor dispensing, or encapsulation
• Thermal design validation, using depreciation data measured at multiple case temperatures to extract the temperature acceleration of the decay constant and guide heat sink and PCB optimization
• Accelerated reliability engineering, extrapolating high-temperature attenuation behavior through Arrhenius-type acceleration models to estimate lifetimes at real-world operating temperatures
• Durability verification for horticultural, automotive, industrial, and outdoor lighting, where combined thermal, humidity, and vibration stresses demand demonstrated long-term flux stability
• Research and development of new materials, including low-yellowing silicones, high-stability phosphors, and gallium nitride chip structures, where quantitative attenuation rates serve as the key figure of merit
• Warranty substantiation and tender documentation, providing independent laboratory evidence supporting claimed lifetimes of 25,000 to 50,000 hours in commercial and municipal lighting projects

FAQ

What is the minimum test duration for a valid light attenuation experiment?

IES LM-80 requires a minimum of 6,000 hours of continuous operation for LED packages and modules. Lifetime projections under IES TM-21 may only extend to six times the actual test duration, so a 6,000-hour study supports claims up to 36,000 hours, while a 10,000-hour study extends the supported claim to 60,000 hours.

How many samples are required for light attenuation testing?

For LED package testing under LM-80, a minimum of 20 units per temperature condition is required, drawn from three production lots where practical. Larger sample sizes reduce statistical uncertainty and enable more confident projections at the product level.

Why are multiple case temperatures tested?

Because lumen depreciation is strongly accelerated by temperature, testing at 55 °C, 85 °C, and a manufacturer-selected higher temperature allows the laboratory to characterize the thermal acceleration behavior. This enables interpolation of expected lifetime at any operating temperature within the tested range.

What do L70, L80, and L90 mean?

These terms denote the projected operating time at which luminous flux declines to 70 %, 80 %, or 90 % of its initial value. L70 is the conventional end-of-life criterion for general lighting, while L90 is often specified for applications such as museums and horticulture where tighter light output control is required.

Can light attenuation tests be accelerated to shorten project timelines?

Yes, within defined limits. Testing at elevated case temperatures and drive currents provides acceleration, but projections must remain inside the framework defined by TM-21 and related standards. Highly accelerated protocols beyond these limits yield comparative engineering data rather than certifiable lifetime claims.

What measurement uncertainty should be expected?

Well-maintained integrating sphere systems operated by accredited third-party laboratories typically achieve photometric measurement uncertainty of approximately 2 % or better for luminous flux. Uncertainty is minimized by using identical instruments, fixtures, and stabilization procedures at every readout throughout the experiment.

How is color shift evaluated alongside lumen depreciation?

Chromaticity coordinates are measured at every readout, and the shift Δu'v' relative to the initial point is tracked versus time. Common acceptance limits range from 0.007 for demanding applications to 0.013 for general lighting, and sustained color drift frequently reveals phosphor or encapsulant degradation before significant flux loss appears.