A Technical Examination of IESNA LM-79 and LM-80 Standards for Solid-State Lighting Metrology

Introduction to Standardized Photometric and Colorimetric Testing

The proliferation of solid-state lighting (SSL) technologies, primarily light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs), has necessitated the development of rigorous and repeatable testing methodologies. The Illuminating Engineering Society of North America (IESNA) has established a suite of standards that form the cornerstone of SSL performance verification. Among these, IESNA LM-79 and IESNA LM-80 provide the foundational protocols for characterizing the initial performance and luminous depreciation of lighting products, respectively. These standards are indispensable for manufacturers, designers, and testing laboratories across diverse sectors, including automotive lighting, aerospace illumination, and medical equipment, ensuring that performance claims are substantiated by empirical, standardized data. The integrity of data derived from these tests is wholly dependent on the precision and accuracy of the measurement instrumentation employed.

The Architectural Framework of IES LM-79-19

IES LM-79-19, entitled “Approved Method: Optical and Electrical Measurements of Solid-State Lighting Products,” delineates the procedures for testing integrated LED lamps and luminaires under controlled conditions. It is critical to note that LM-79 is an “absolute” measurement standard; it requires the acquisition of data directly from the product in its operational state, rather than relying on derived calculations from individual components. The standard mandates specific environmental conditions, electrical settings, and thermal stabilization periods to ensure reproducibility. The key parameters measured under LM-79 include total luminous flux (in lumens), luminous efficacy (in lumens per watt), chromaticity coordinates (x, y, and u’, v’), Correlated Color Temperature (CCT), and Color Rendering Index (CRI). The methodology prescribes the use of specific apparatus, primarily integrating spheres or goniophotometers, coupled with spectroradiometers for comprehensive colorimetric analysis.

Dissecting the LM-80-21 Lumen Maintenance Methodology

While LM-79 provides a snapshot of initial performance, IES LM-80-21, “Approved Method: Measuring Lumen Maintenance of LED Light Sources,” addresses the long-term temporal light output stability of LED packages, arrays, and modules. It is a “relative” measurement method focused on the light source itself, not the complete luminaire. The LM-80 procedure involves operating LED specimens at a minimum of three case temperatures—typically 55°C, 85°C, and a third temperature selected by the manufacturer—for a minimum test duration of 6,000 hours, with data collection points at least every 1,000 hours. The resulting data set, which charts normalized luminous flux against time, is used to project long-term lumen maintenance, often extrapolated using the IESNA TM-21-11 projection method to estimate the L70 lifetime (the point at which light output depreciates to 70% of its initial value). This data is paramount for product lifecycle forecasting and warranty substantiation.

Integrating Sphere Systems as the Foundation for LM-79 Compliance

The integrating sphere is a critical apparatus for conforming to the total luminous flux measurement requirements of LM-79. Its function is to spatially integrate light from a source, creating a uniform radiance distribution on the sphere’s inner surface, which is then measured by a spectrometer. For accurate measurements, the sphere’s interior is coated with a highly reflective, spectrally neutral diffuse material, such as BaSO4 or PTFE. The system must be properly calibrated using a standard lamp of known luminous flux. The selection of an appropriate sphere diameter is crucial; it must be sufficiently large to minimize self-absorption errors caused by the test device, a factor particularly important for luminaires with significant physical dimensions or disparate light distributions.

The Indispensable Role of Spectroradiometry in Comprehensive Characterization

A spectroradiometer integrated within the sphere system is what enables the full suite of LM-79 photometric and colorimetric measurements. This instrument measures the spectral power distribution (SPD) of the light source—the radiant power as a function of wavelength. From the SPD, all key colorimetric quantities are derived computationally. These include chromaticity coordinates (defining the color point on the CIE diagram), CCT (describing the warmth or coolness of white light), CRI (assessing the fidelity of object color appearance under the source), and newer metrics such as TM-30 Rf (fidelity index) and Rg (gamut index). The precision of the spectroradiometer, including its wavelength accuracy, bandwidth, and stray light rejection, directly dictates the reliability of the reported color data.

The LPCE-3 Integrating Sphere Spectroradiometer System for High-Accuracy SSL Testing

For laboratories and manufacturers requiring rigorous adherence to LM-79 and LM-80 test conditions, the LISUN LPCE-3 Integrating Sphere Spectroradiometer System represents a comprehensive solution. The system is engineered to facilitate precise optical, electrical, and colorimetric measurements of LED lamps and luminaires. Its architecture is designed to meet the stringent requirements of the standards while offering the flexibility needed for research and development applications.

System Specifications and Configuration:
The LPCE-3 system typically comprises a high-reflectance integrating sphere, a high-precision CCD array spectroradiometer, a programmable AC/DC power supply, and a master control and analysis software suite. The spheres are available in various diameters (e.g., 0.5m, 1m, 1.5m, 2m) to accommodate different sample sizes and minimize spatial non-uniformity errors. The spectroradiometer features a wide wavelength range, typically from 380nm to 780nm, covering the entire visible spectrum with a fine wavelength resolution. The integrated power supply provides stable, metered input to the device under test, allowing for simultaneous measurement of electrical parameters (voltage, current, power, power factor) as mandated by LM-79.

Testing Principles and Workflow:
The operational principle of the LPCE-3 aligns directly with LM-79 protocols. The device under test (DUT) is mounted inside the integrating sphere. After a prescribed thermal stabilization period, the spectroradiometer captures the SPD of the integrated light. The software then calculates all photometric and colorimetric values from this SPD. For luminous flux, the system is first calibrated with a standard lamp traceable to NIST or other national metrology institutes. The software automates data collection, correction for self-absorption (using an auxiliary lamp method), and report generation, ensuring a repeatable and auditable testing process.

Industry-Specific Applications of Standardized LED Metrology

The application of LM-79 and LM-80 testing, facilitated by systems like the LPCE-3, extends far beyond general illumination.

• Automotive Lighting Testing: Ensuring the luminous intensity, color consistency, and long-term reliability of LED headlamps, daytime running lights, and interior lighting is critical for safety and compliance with regulations such as FMVSS 108 and ECE standards.
• Aerospace and Aviation Lighting: Navigation lights, cabin lighting, and cockpit displays require absolute color and intensity stability under extreme environmental conditions. LM-80 data is essential for certifying components that must operate reliably for tens of thousands of hours.
• Display Equipment Testing: The performance of LED backlight units for LCDs and direct-view LED screens is characterized for uniformity, color gamut, and brightness, directly impacting image quality.
• Medical Lighting Equipment: Surgical lights and diagnostic illumination demand high CRI and precise CCT to ensure accurate tissue differentiation and color rendition. LM-79 verification is a critical step in the medical device approval process.
• Marine and Navigation Lighting: The performance of marine signal lights is governed by international maritime conventions (COLREGs), which specify luminous intensity and color chromaticity boundaries, verifiable through LM-79 testing.
• Photovoltaic Industry: While not for illumination, spectroradiometer systems are used to measure the spectral responsivity of solar cells and the SPD of solar simulators, ensuring accurate efficiency ratings.

Comparative Advantages of an Integrated Testing Platform

The primary advantage of an integrated system like the LPCE-3 lies in its synergy and automation. By combining the sphere, spectroradiometer, and power supply under a unified software platform, it eliminates inter-instrument variability and streamlines the testing workflow. This integration enhances measurement repeatability and reduces potential operator error. The system’s software is designed to incorporate the mathematical corrections and data processing algorithms required by the standards, delivering directly compliant reports. Furthermore, the system’s capability to test both optical and electrical parameters simultaneously provides a holistic view of product efficacy, a key metric for energy-conscious industries.

Navigating the Complexities of LM-80 Test Management

Conducting an LM-80 test is a resource-intensive endeavor requiring precise environmental control. Systems like the LPCE-3 are not the environmental chamber but are the measurement core used at each data collection interval. A complete LM-80 solution involves placing LED packages on temperature-controlled heat sinks within an environmental chamber. At defined intervals (e.g., every 1,000 hours), samples are removed, allowed to stabilize at room temperature, and then their photometric and colorimetric properties are measured using the LPCE-3 system. The high repeatability of the LPCE-3 is critical here, as any measurement drift over the thousands of hours of testing would invalidate the long-term depreciation trend.

The Criticality of Traceable Calibration and Measurement Uncertainty

Adherence to LM-79 and LM-80 is not merely about following a procedure; it is about establishing metrological traceability. All measurements must be traceable to national or international standards. The calibration of the integrating sphere system with a standard lamp is the foundational step for this traceability. A comprehensive measurement uncertainty analysis, as recommended by guides like the ISO/IEC Guide 98-3 (GUM), must accompany any test report. Factors such as sphere non-uniformity, spectrometer drift, temperature fluctuations, and electrical measurement inaccuracies all contribute to the final uncertainty budget, which quantifies the confidence in the reported results.

Conclusion: The Unifying Role of Standardized Metrology in SSL Advancement

The IESNA LM-79 and LM-80 standards have provided the essential framework that has enabled the SSL industry to mature from a disruptive technology into a dominant, trusted lighting solution. They provide a common language of performance and reliability that facilitates fair competition, informed specification, and continued innovation. The practical implementation of these standards relies on sophisticated, accurate, and reliable measurement systems. Integrated solutions, such as the LISUN LPCE-3, serve as the critical bridge between theoretical standard and practical, auditable data, empowering industries from automotive to medical to confidently design, validate, and deploy the next generation of solid-state lighting products.

FAQ Section

Q1: What is the minimum required integrating sphere size for testing a typical LED streetlight luminaire according to LM-79?
LM-79 does not prescribe an exact sphere size but emphasizes that the sphere must be large enough to minimize self-absorption errors. For a typical streetlight luminaire, which is large and may have a non-uniform distribution, a sphere with a diameter of 1.5 meters or 2 meters is generally recommended to ensure spatial integration is achieved and measurement accuracy is maintained.

Q2: Can the LPCE-3 system measure the flicker percentage of an LED lamp?
Yes, the high-speed sampling capability of the spectroradiometer within the LPCE-3 system, when combined with appropriate software algorithms, can capture the temporal light output waveform. This allows for the calculation of flicker metrics such as percent flicker and flicker index, which are increasingly important for lighting quality and human health considerations.

Q3: Why is thermal stabilization of the device under test so critical in LM-79 testing?
The photometric and colorimetric output of an LED is highly dependent on its junction temperature. Without a sufficient stabilization period, the measurements will not represent the steady-state performance of the product. LM-79 requires operation until the luminous flux output stabilizes to within 0.5% over a 30-minute period, ensuring that the reported data is repeatable and representative of real-world operating conditions.

Q4: How does the auxiliary lamp method in an integrating sphere system correct for self-absorption?
Self-absorption occurs because the device under test (DUT) absorbs a portion of the light reflected inside the sphere, which a calibration standard lamp would not. The auxiliary lamp method involves measuring the sphere’s response with and without the DUT present (but powered off) while the auxiliary lamp is lit. This ratio provides a correction factor that is applied to the DUT’s measurement, compensating for the absorption effect and yielding an accurate luminous flux reading.

Q5: Is it permissible to use LM-80 data obtained for an LED package to claim the lifetime of a final luminaire?
Not directly. LM-80 data characterizes the LED source itself under controlled temperatures. In a luminaire, the LED is subjected to a different thermal, electrical, and optical environment. While the LM-80 data is a crucial input, the luminaire’s L70 lifetime must be projected using the TM-21 method on the LM-80 data, but the claim must be qualified based on the specific operating conditions within the luminaire, as per guidelines like IES TM-28.