Light Source Lumen Depreciation and Illuminance Measurement

Light source lumen depreciation and illuminance measurement is an essential photometric evaluation used to quantify how lamps, LED modules, and luminaires maintain their light output over extended periods of operation. This testing enables manufacturers to verify lumen maintenance ratings such as L70, validate illuminance performance against design requirements, and demonstrate compliance with IEC, IES, CIE, and GB standards. The following article explains the technical principles, applicable sample types, measured parameters, test methods, and typical applications of this evaluation.

Technical Overview

Lumen depreciation, commonly referred to as light decay, describes the progressive loss of luminous flux emitted by a light source as it ages. Unlike incandescent lamps, which generally fail abruptly, LED and other solid-state light sources tend to dim gradually as the semiconductor junction, phosphor layers, optical encapsulants, and driver electronics degrade under sustained electrical and thermal stress. Because this decline directly determines the usable service life of a lighting product, it must be measured and projected rather than inferred from catastrophic failure rates alone.

Illuminance measurement complements lumen depreciation data by quantifying the luminous flux incident on a working surface, expressed in lux (lumens per square meter). Together, these two evaluations answer two fundamental engineering questions: how much light a product delivers at any given point in its lifetime, and whether that output continues to satisfy the illumination requirements of the application, whether an office, roadway, sports facility, industrial workshop, or horticultural environment.

Testing is typically performed by accredited third-party laboratories using calibrated photometric equipment operated under controlled ambient conditions, in accordance with internationally recognized standards published by IEC, IES, CIE, ISO, and national frameworks such as GB, EN, UL, and JIS.

Testing Samples

LED Packages, Chips, and Modules

Bare LED packages, chips, and light engines are the fundamental building blocks of solid-state lighting. They are subjected to long-duration lumen maintenance testing at controlled case temperatures, such as 55°C, 85°C, and a manufacturer-selected temperature, following LM-80-style protocols. Data at multiple temperature set points enables reliable projection of depreciation behavior inside finished products.

Self-Ballasted LED Lamps

Finished replacement lamps, including LED tubes, A-type bulbs, candle lamps, PAR lamps, and downlight retrofits, are tested as complete systems with their integrated drivers. Evaluation covers initial photometric performance, lumen maintenance during extended operation, and illuminance output at defined mounting distances.

LED Luminaires

Complete fixtures such as street lights, high-bay luminaires, panel lights, floodlights, tunnel lights, and area lights are tested for total luminous flux, light distribution, and maintained illuminance. Thermal management of the full luminaire assembly has a decisive influence on measured decay rates, making whole-fixture evaluation particularly valuable.

Conventional and Specialty Light Sources

Fluorescent tubes, HID lamps, halogen lamps, and induction lamps can also be evaluated to establish baseline depreciation curves for comparison or retrofit justification. Specialty sources such as plant growth lamps and UV-A modules may be assessed for illuminance or application-specific radiometric parameters.

Testing Items

The scope of measurement typically includes the following photometric and life-performance parameters:

• Initial luminous flux (lumens) and initial illuminance (lux) at specified distances and geometries
• Lumen maintenance ratio at defined aging intervals, such as 1,000, 3,000, 5,000, and 6,000 hours or longer
• Lumen maintenance life projections, including L70, L80, and L90 values with associated confidence levels
• Relative light decay curve and average depreciation rate per 1,000 hours of operation
• Center illuminance, average illuminance, and illuminance uniformity over a defined measurement plane
• Correlated color temperature (CCT) shift and chromaticity coordinate drift over time
• Luminaire efficacy (lm/W) at initial and aged conditions
• Photometric stability under elevated ambient or case temperature conditions

Testing Methods

Integrating Sphere Photometry

Total luminous flux is measured inside an integrating sphere equipped with an auxiliary lamp and a calibrated photometer or spectroradiometer, following the principles of CIE 84 and IES LM-79. This method provides rapid, high-repeatability flux readings suitable for tracking relative depreciation across repeated aging checkpoints.

Goniophotometric Measurement

A goniophotometer characterizes the spatial light distribution, luminous intensity, and zonal flux of a lamp or luminaire in accordance with CIE 121 and EN 13032 series requirements. Repeated goniophotometric scans at different life stages reveal how distribution patterns and directional illuminance change as the source ages.

Extended Aging and Lumen Maintenance Testing

Samples are operated continuously or under duty-cycled conditions at rated electrical input, with case temperature monitored and recorded. Photometric readings are taken at prescribed intervals, and the resulting maintenance data are fitted to an exponential decay model for life projection, consistent with IES TM-21 methodology. LED module and luminaire performance are further evaluated against IEC 62717, IEC 62722-2-1, and corresponding GB/T standards.

Illuminance Measurement

Illuminance is measured with calibrated lux meters or spectroradiometers using standardized grid layouts on the working plane, following CIE 194 guidance and national lighting design standards such as the GB 50034 series. Laboratory measurements are conducted in darkened enclosures with controlled reflection characteristics, while field verification assesses installed performance under real operating conditions.

Key Reference Standards

• IES LM-80: measurement of lumen maintenance of LED packages, arrays, and modules
• IES LM-79: electrical and photometric measurements of solid-state lighting products
• IES TM-21: projection of long-term lumen maintenance from collected data
• IEC 62717 and IEC 62722-2-1: performance requirements for LED modules and luminaires
• CIE 84, CIE 121, and CIE 194: measurement of luminous flux, distribution, and illuminance
• GB/T 24824 and related GB/T methods for LED module photometric measurement
• EN 13032 series: measurement and presentation of photometric data of lamps and luminaires

Applications

Lumen depreciation and illuminance measurement results support decision-making across the entire lighting product lifecycle:

• Research and development, where decay data guide thermal design, driver selection, and phosphor optimization
• Quality control and incoming inspection of LED packages, modules, and finished goods
• Certification, market access, and tender documentation demonstrating compliance with performance requirements
• Substantiation of long-term lumen warranties and service-life claims, such as five-year maintenance guarantees
• Lighting design validation, confirming that maintained illuminance levels meet specified codes for workplaces, roadways, sports venues, and public spaces
• Retrofit justification, comparing projected light output of replacement LED systems against legacy light sources

By combining laboratory photometry with projected maintenance data, manufacturers and project owners can make verifiable, standards-based claims about how a lighting product will perform throughout its installed life.

FAQ

What does the rating L70 mean?

L70 denotes the elapsed operating time at which a light source is projected to emit 70 percent of its initial luminous flux. L80 and L90 correspondingly represent the times to 80 percent and 90 percent maintenance. These thresholds are widely used because a 30 percent loss is generally considered the point at which diminished light output becomes perceptible in many general lighting applications.

How long does a lumen depreciation test take?

Formal lumen maintenance testing of LED packages requires a minimum of 6,000 hours of aging, with 10,000 hours or more providing stronger projection confidence. Life estimates beyond the actual test duration are extrapolated using recognized projection methods, with limits placed on how far extrapolation may extend relative to the collected data.

What is the difference between luminous flux and illuminance?

Luminous flux measures the total quantity of visible light emitted by a source in all directions, expressed in lumens. Illuminance measures the amount of light falling on a surface per unit area, expressed in lux. A source with high flux may still produce inadequate illuminance in an application if its distribution, optics, or installation geometry are unsuitable.

Why is temperature control so important in light decay testing?

LED light output and degradation rate are strongly dependent on junction and case temperature. If temperature is not controlled and reported, maintenance data cannot be compared between laboratories or applied meaningfully to product design. Standards therefore require temperature set points to be stabilized and documented throughout the aging period.

Can light decay be predicted without waiting for years of testing?

Yes, within defined limits. Mathematical projection of measured maintenance data allows life estimates several times longer than the test duration, provided the decay curve is stable and the sample count meets standard requirements. High-stress screening methods are also used during development for rapid comparative evaluation, although certified life claims must rest on standard-compliant long-duration data.