Comprehensive Characterization of Photometric and Colorimetric Performance: The Role of IESNA LM-79-19 Approved Test Methods

Introduction to Standardized Photometric Evaluation

The accurate and repeatable measurement of solid-state lighting (SSL) products is a cornerstone of product development, quality assurance, and regulatory compliance across multiple industries. The Illuminating Engineering Society of North America (IESNA) standard LM-79-19, “Electrical and Photometric Measurements of Solid-State Lighting Products,” provides the definitive methodological framework for this critical task. This document prescribes the approved methods for measuring total luminous flux (lumens), electrical power characteristics (watts, volts, amps), luminous efficacy (lumens per watt), and chromaticity coordinates under controlled conditions. Unlike legacy methods for incandescent or fluorescent sources, LM-79-19 explicitly addresses the directional, spectral, and thermal sensitivities inherent to LED and OLED-based luminaires and integrated LED lamps. Compliance with this standard is not merely a benchmark; it is an essential requirement for ENERGY STAR® certification, DesignLights Consortium (DLC) qualification, and credible product data sheets. This article delineates the technical requirements of LM-79-79 testing, with a specific examination of integrated sphere-spectroradiometer systems, exemplified by the LISUN LPCE-3 High Precision Spectroradiometer Integrating Sphere System, as a primary apparatus for absolute photometric and colorimetric measurement.

Architectural Principles of Integrating Sphere Photometry

The integrating sphere, or Ulbricht sphere, operates on the principle of spatial integration through diffuse reflectance. A light source placed within the sphere emits radiation that undergoes multiple diffuse reflections on the sphere’s interior coating, typically composed of barium sulfate (BaSO₄) or polytetrafluoroethylene (PTFE). This process creates a uniform radiance distribution across the sphere’s inner surface, independent of the original spatial or angular characteristics of the source. A detector, shielded from direct illumination by a baffle, samples this uniform radiance. For absolute measurement of total luminous flux, the sphere system must be calibrated using a standard lamp of known luminous flux. The sphere’s efficacy is quantified by its spatial non-uniformity and absorption error, which are minimized through precise coating application and optimal geometric design of baffles and the source mounting assembly. The sphere diameter must be sufficiently large to accommodate the test luminaire without violating the inverse square law approximation and to minimize self-absorption errors—a critical consideration for products with large physical dimensions or distinct housing colors.

Integration of Spectroradiometry for Comprehensive Spectral Analysis

While traditional sphere photometers use filtered photopic detectors to measure luminous flux, modern LM-79-19-compliant systems increasingly integrate array spectroradiometers. This configuration enables simultaneous measurement of all required photometric and colorimetric quantities from a single spectral power distribution (SPD) capture. The spectroradiometer disperses the light sampled from the sphere’s port via a diffraction grating onto a CCD or CMOS array, generating a precise waveform of spectral irradiance. From this SPD, software calculates:

• Photometric Quantities: Luminous flux (Φ_v) by weighting the SPD with the CIE standard photopic luminosity function V(λ).
• Colorimetric Quantities: Chromaticity coordinates (CIE 1931 x,y and CIE 1976 u’,v’), correlated color temperature (CCT), and color rendering index (CRI Ra).
• Spectral Metrics: Peak wavelength, dominant wavelength, and spectral purity.
  This integrated approach eliminates errors associated with filter mismatch and provides a complete spectral fingerprint of the device under test (DUT), which is indispensable for applications where color quality and spectral content are critical.

The LISUN LPCE-3 System: Configuration and Metrological Specifications

The LISUN LPCE-3 High Precision Spectroradiometer Integrating Sphere System represents a turnkey solution engineered for full LM-79-19 compliance. The system is configured with a precision-machined integrating sphere, a high-sensitivity CCD spectroradiometer, a constant current/voltage programmable power supply, and a computer running dedicated analysis software.

Key System Specifications:

• Integrating Sphere: Available in multiple diameters (e.g., 1.0m, 1.5m, 2.0m) to accommodate various DUT sizes. The interior is coated with a proprietary, highly stable diffuse reflective material with a reflectance >95% from 380nm to 850nm.
• Spectroradiometer: Wavelength range typically 380-780nm (extendable to 1000nm for near-IR applications), with a wavelength accuracy of ±0.3nm and a high signal-to-noise ratio essential for low-light-level measurement.
• Software Capabilities: Automates the calculation of all LM-79-19 metrics, including luminous flux, electrical parameters, CCT, CRI, CIE 1931/1976 chromaticity, and peak/dominant wavelength. It features data logging, spectral overlay, and report generation in standardized formats.

The system’s competitive advantage lies in its holistic calibration chain, traceable to National Metrology Institutes (NMIs), and its software’s ability to correct for sphere imperfections and self-absorption effects through advanced algorithms, ensuring laboratory-grade accuracy in both R&D and high-throughput production environments.

Industry-Specific Applications and Testing Protocols

The universality of LM-79-19 principles finds application in a diverse array of fields, each with unique testing nuances.

LED & OLED Manufacturing and the Lighting Industry: Here, the LPCE-3 system is deployed for binning LEDs by flux and chromaticity, verifying final luminaire performance against datasheets, and conducting accelerated life testing (LM-80) by taking periodic measurements. Consistency in CCT and CRI across production batches is paramount.

Automotive Lighting Testing: Beyond total flux, the spectral analysis capability is crucial for measuring the chromaticity of signal lamps (brake lights, turn signals) to ensure compliance with stringent ECE/SAE regulations. The sphere can measure complete headlamp units or individual LED modules.

Aerospace and Aviation Lighting: Testing focuses on extreme reliability and specific chromaticity for navigation lights and cabin lighting. The system must often operate in conjunction with thermal chambers to simulate performance at altitude-induced temperature extremes.

Display Equipment Testing: For backlight units (BLUs) in LCDs or direct-view OLED panels, measurement of luminous flux and color uniformity of the light source assembly is critical. The integrating sphere provides the total light output, a key metric for display efficiency.

Photovoltaic Industry: While not for light output, similar sphere systems with extended-range spectroradiometers are used for measuring the absolute spectral responsivity of photovoltaic cells, a critical parameter for efficiency calculations.

Urban Lighting Design: Designers utilize LM-79 data to accurately model the performance of streetlights and area lights in simulation software, ensuring required illuminance levels are met while predicting energy consumption and light pollution spectra.

Marine and Navigation Lighting: Compliance with International Maritime Organization (IMO) and COLREGs standards for luminous intensity and chromaticity of maritime signal lights is verified using spectroradiometric sphere systems.

Medical Lighting Equipment: For surgical lights and phototherapy devices, precise measurement of spectral irradiance (μW/cm²/nm) is as important as luminous flux. The system ensures the device delivers the therapeutic wavelength band effectively and safely.

Critical Pre-Test Stabilization and Environmental Controls

LM-79-19 mandates that all measurements be performed on thermally and electrically stabilized products. The DUT must be operated at its rated voltage/current in a controlled ambient temperature (25°C ± 1°C is typical) until its light output varies by less than 0.5% over a 30-minute interval. This stabilization period, which can range from 30 minutes to several hours, is necessary because LED performance is intrinsically temperature-dependent. The integrating sphere itself must be situated in a dark, draft-free environment to prevent stray light and thermal perturbations. The LPCE-3 system’s software often includes real-time monitoring functions to graphically confirm stabilization has been achieved before formal data acquisition begins.

Analysis of Electrical Parameters and Luminous Efficacy

Concurrent with optical measurements, LM-79-19 requires precise measurement of input electrical characteristics. A calibrated digital power meter or the integrated programmable power supply of a system like the LPCE-3 measures the DUT’s RMS voltage (V), current (A), power (W), power factor, and frequency. Luminous efficacy (lm/W), a primary metric of energy efficiency, is then calculated as the ratio of total luminous flux to input electrical power. Accurate electrical measurement is non-trivial for SSL products, as many incorporate drivers that can introduce harmonic distortion, making true RMS measurement capabilities essential.

Advanced Colorimetric Calculations from Spectral Data

The spectroradiometric data enables sophisticated color analysis. The software calculates the CIE 1931 (x,y) and the more perceptually uniform CIE 1976 (u’,v’) chromaticity coordinates. Correlated Color Temperature (CCT) is determined by finding the temperature of the Planckian radiator whose chromaticity is nearest to the DUT’s on the CIE 1960 UCS diagram. The Color Rendering Index (CRI, Ra) is computed by comparing the appearance of 8 standard color samples (R1-R8) under the DUT and under a reference illuminant of the same CCT. Additional indices like TM-30-18 (Rf, Rg) can also be derived from the high-resolution SPD.

Uncertainty Analysis and Measurement Traceability

A complete LM-79-19 report includes a statement of measurement uncertainty, in accordance with the Guide to the Expression of Uncertainty in Measurement (GUM). Key uncertainty contributors for an integrating sphere-spectroradiometer system include:

• Calibration uncertainty of the standard lamp.
• Sphere spatial non-uniformity.
• Spectroradiometric linearity, stray light, and wavelength accuracy.
• Temperature instability of the DUT and environment.
• Electrical measurement accuracy.
  A system like the LPCE-3, with NMI-traceable calibration and characterized sphere properties, allows laboratories to establish a rigorous uncertainty budget, often achieving uncertainties below 3% (k=2) for total luminous flux and ±0.0015 for chromaticity coordinates, which is necessary for inter-laboratory comparisons and certified testing.

Validation and Inter-Laboratory Comparison Protocols

To ensure ongoing accuracy, laboratories must implement regular validation procedures. This includes periodic recalibration of the system using standard lamps, verification using check standards (stable LED references), and participation in inter-laboratory comparison rounds. The stability of the sphere coating is monitored over time. The software in advanced systems provides tools for these routine checks, ensuring the measurement integrity required for accredited laboratory status (e.g., ISO/IEC 17025).

Conclusion

IESNA LM-79-19 provides the indispensable foundation for the credible evaluation of SSL products. The integration of spectroradiometry within the traditional integrating sphere apparatus, as implemented in systems like the LISUN LPCE-3, represents the state of the art, delivering comprehensive photometric, electrical, and colorimetric data from a single, stable measurement. This capability is critical across the spectrum from high-volume manufacturing to specialized applications in automotive, aerospace, medical, and scientific fields. As lighting technology continues to evolve, the principles enshrined in LM-79-19 and the precision of the instruments designed to fulfill its requirements will remain central to innovation, quality, and energy efficiency.

FAQ Section

Q1: What is the primary advantage of using a spectroradiometer inside an integrating sphere instead of a traditional photopic head?
A spectroradiometer captures the complete spectral power distribution (SPD) in a single measurement. From the SPD, all photometric (lumens, efficacy) and colorimetric (CCT, CRI, chromaticity) parameters can be derived mathematically with high accuracy. This eliminates filter mismatch errors associated with photopic detectors and provides a full spectral dataset essential for analyzing color quality and spectral-specific applications.

Q2: For testing large or oddly shaped luminaires, what sphere size is recommended, and how does the LPCE-3 system address self-absorption errors?
LM-79-19 recommends the sphere diameter be at least 1.5 times the largest dimension of the DUT. For very large streetlights or industrial luminaires, a 2m sphere is often necessary. The LPCE-3 software incorporates correction algorithms for self-absorption—the error introduced because the DUT’s housing absorbs more light than the calibration standard lamp. This is achieved by measuring the sphere’s response with and without an auxiliary lamp, allowing for a computational correction that significantly improves accuracy for non-neutral-colored luminaires.

Q3: How does the system ensure accurate testing of products with pulsed or dimmable drivers?
The LPCE-3’s spectroradiometer must be operated in an appropriate integration mode (e.g., a long enough integration time) to capture the average output of a pulsed waveform. For dimmable products, LM-79-19 requires testing at 100% rated power. The system’s programmable power supply provides stable, precise input to the DUT, and the software’s real-time monitoring confirms that the optical output has stabilized at the chosen setting before measurement commences.

Q4: Can the LPCE-3 system be used for testing beyond the visible spectrum, relevant to photovoltaic or horticultural lighting applications?
Yes, while the standard configuration targets the visible (380-780nm) range for photopic measurements, the system can be specified with an extended-range spectroradiometer (e.g., 200-1100nm). This allows for measurement of ultraviolet (UV) components for disinfection lighting, far-red spectra for horticulture, and near-infrared (NIR) output for PV cell testing or special sensors. The sphere coating must maintain high reflectivity across the extended range.