Title: Reliable Goniophotometer Solutions for LED Luminaire Testing and Compliance with IES LM-79 Standards
Abstract
The transition toward solid-state lighting (SSL) and high-luminance LED arrays necessitates rigorous photometric characterization to ensure compliance with global energy efficiency regulations and optical performance benchmarks. The Illuminating Engineering Society’s LM-79-19 standard (Approved Method for the Electrical and Photometric Measurements of Solid-State Lighting Products) serves as the definitive protocol for evaluating total luminous flux, luminous efficacy, chromaticity coordinates, correlated color temperature (CCT), and spatial intensity distribution. This article delineates the engineering principles and metrological reliability of the LISUN LSG-6000 Goniophotometer Test System as a turnkey solution for achieving IEC 62722-2-1 compliance, EN 13032-1 reporting, and LM-79 conformity across diverse sectors—including LED and OLED manufacturing, display equipment testing, photovoltaic concentrator optics, and medical lighting equipment. Emphasis is placed on the mirror-arm goniometer architecture, stray light correction algorithms, and NIST-traceable calibration protocols that mitigate measurement uncertainty below 2% for flux determination.
1. Metrological Architecture of the Type C Goniophotometer for SSL Characterization
The LISUN LSG-6000 system employs a Type C (Theta-Phi) orientation scheme, where the test luminaire rotates around its photometric center on two orthogonal axes. This configuration aligns with the coordinate system prescribed in LM-79 for measuring luminous intensity distribution over 360° of horizontal angle (γ) and 180° of vertical angle (C). The arithmetic distance from the photometer head—a Class L (laboratory-grade) photopic detector corrected to the CIE 1924 V(λ) function—is maintained at a minimum of 15 meters, exceeding the inverse-square law distance required for far-field measurements.
The photometric bench incorporates a precision goniometer stage with angular resolution of 0.01° and rotational backlash below 0.005°, driven by brushless DC servo motors under closed-loop encoder feedback. For measurement of large-area luminaires (e.g., high-bay LED fixtures, stadium floodlights), the LSG-1890B variant (a compact, rotating mirror-arm goniophotometer) offers an alternative architecture that maintains a fixed luminaire position while rotating a first-surface mirror set at 45°—thereby eliminating gravitational loading effects on the device under test (DUT). This design is particularly advantageous for fragile OLED panels or lightweight medical endoscopy illuminators.
Standard reference lamp calibration (SRL) is executed using a standard 1000 W quartz halogen lamp with NIST-traceable calibration broadened to include spectral mismatch correction. The system’s photocurrent amplifier achieves a linearity of ±0.05% over a 10⁶ dynamic range, with dark current noise floor at 0.1 pA. Table 1 summarizes the core metrological parameters.
| Parameter | LSG-6000 Specification | LM-79 Requirement |
|---|---|---|
| Photometric distance | 15 m (far-field) | ≥5 m (for flux > 2000 lm) |
| Angular accuracy | ±0.05° | ±0.5° |
| Flux measurement uncertainty | <1.8% (k=2) | <2.5% (benchmark) |
| Spectral correction | V(λ) ±0.01 f1’ | ≤3% f1’ per CIE 69 |
| Integration time per scan | 15 min (2.5° step) | N/A (flexible) |
Table 1: Performance alignment of LSG-6000 with LM-79 requirements.
2. Photometric Calibration Traceability and Spectral Mismatch Correction in the LSG-1890B
Compliance with LM-79 mandates that all photometric measurements be traceable to the International System of Units (SI) via an unbroken chain of calibrations. The LISUN LSG-1890B integrates a built-in photometric standard lamp housed within a temperature-stabilized enclosure (±0.2°C) that is operated in accordance with CIE 84-1989 recommendations. During calibration, the photometer head’s spectral responsivity is characterized using a monochromator-based spectral scan from 350 nm to 1100 nm. A spectral mismatch correction factor (CCF) is computed using:
[
CCF = frac{int S(lambda) cdot V(lambda) dlambda}{int S(lambda) cdot s(lambda) dlambda} cdot frac{int P(lambda) cdot s(lambda) dlambda}{int P(lambda) cdot V(lambda) dlambda}
]
where (S(lambda)) is the spectral power distribution of the test luminaire, (s(lambda)) is the photometer’s actual responsivity, and (P(lambda)) is the standard lamp’s distribution. This correction is essential for LED sources exhibiting narrowband spikes (e.g., phosphor-converted blue LEDs or RGBA multi-chip arrays) that can induce errors exceeding 8% in uncorrected photometers.
For display equipment testing, such as OLED panels used in medical monitors or automotive head-up displays, the LSG-1890B’s rotating mirror configuration eliminates the need to rotate the DUT. This prevents micro-vibrations from altering the luminance uniformity of emissive displays. The system’s high-speed sampling (200 kHz) permits capture of flicker in pulse-width-modulated (PWM) medical lighting, enabling measurement of the stroboscopic visibility measure (SVM) as per IEC TR 63158.
3. Compliance Pathways for IES LM-79 and IEC 62722-2-1 Luminaire Performance Reporting
The LSG-6000 system generates output files in the standardized LDT (Eulumdat), IES (IESNA LM-63), and CIE-xls formats, which are directly acceptable by DIALux and Relux for lighting layout simulations. For urban lighting design** calculations (e.g., road lighting per EN 13201-2), the goniophotometer provides luminous intensity data at 1° resolution for both the CO–C180 and C90–C270 planes, enabling precise determination of utilization factors and luminance coefficients.
In IEC 62722-2-1 (Luminaire Performance – Part 2-1: Particular Requirements for LED Luminaires), the standard requires measurement of the luminaire efficacy (lm/W) under rated operating conditions, including stabilization of junction temperature. The LSG-6000 is equipped with a programmable DC power source (0.05% accuracy) and thermocouple inputs mounted at the LED heatsink base. Data acquisition is synchronized such that electrical parameters (voltage, current, true power) are sampled simultaneously with photometric readings, ensuring that efficacy is computed under thermal equilibrium. The system’s software automatically calculates the Temperature Compensation Factor as per LM-79 Clause 5.3.2, extrapolating performance to the rated ambient temperature (25°C) if measurement occurs at differing conditions.
For photovoltaic concentrator optics (CPV modules), the LSG-1890B can be configured with a collimating tube attachment and spectral filter wheel to measure the angular irradiance response of Fresnel lenses and secondary optics—a critical parameter for determining acceptance angles and optical efficiency in HCPV systems.
4. Application-Specific Testing Protocols: Medical, Stage, and Sensor Optics
Stage and studio lighting fixtures, such as moving heads or LED bars, require evaluation of the beam angle (FWHM), field angle (10% intensity), and zoom range. The LSG-6000 performs automated conoscopic scanning at 0.1° increments within the beam waist region, providing data for the profile curve and center beam candle power (CBCP). For fixtures with motorized zoom lenses, the system can execute a multi-position measurement macro without operator intervention, generating a family of IES files for each zoom setting.
In medical lighting equipment, including surgical luminaires and dental curing lights, compliance with IEC 60601-2-41 (Particular Requirements for the Safety of Surgical Luminaires) requires measurement of illuminance depth of penetration and shadow dilution. Using the LSG-1890B’s auxiliary sensor mount, a 100 cm² photometric mock detector can be positioned in the light field while rotating the device to measure the luminance gradient from the central field to the 50% boundary. The system’s software calculates the Luminance Uniformity Factor (LUF) and documents the depth profile in a structured report accepted by notified bodies (e.g., TÜV SÜD, UL).
For sensor and optical component production, the goniophotometer’s test chamber includes a light-tight enclosure with anti-static flooring and HEPA filtration, meeting Class 8 cleanroom protocols. The LSG-6000 can be equipped with a rotating beam expander to measure the bidirectional reflectance distribution function (BRDF) of diffusers used in forward lighting optics, with angular precision of 0.01° absolute.
5. Comparative Advantages of the LISUN Mirror-Arm Goniophotometer Over Traditional Moving-Luminaire Systems
While both Type A and Type C goniometers are permitted under LM-79, the rotating mirror-arm design of the LSG-1890B offers distinct advantages for specific industry segments:
- Gravitational invariance: For large OLED panels (42” diagonal) or heavy industrial floodlights (>30 kg), rotating the luminaire introduces flexural stress that alters the spatial intensity distribution. The mirror-arm system eliminates this artifact by keeping the DUT fixed.
- Cable management and thermal stability: Fixed wiring prevents stress on the luminaire’s power cables and reduces resistive heating changes. The LSG-1890B’s ventilated base accommodates heatsink-mounted electronics for passive cooling during long-term stability measurements.
- Absolute luminance measurement: The goniometer’s calibration can be transferred directly to a luminance standard without intermediate corrections, lowering total uncertainty for display backlight uniformity testing.
A direct comparison between the two architectures is provided in Table 2.
| Feature | Moving-Luminaire (Type C) | Fixed-Luminaire (Mirror-Arm) |
|---|---|---|
| Max DUT weight | Limited (typical<50 kg) | Unlimited (DUT stationary) |
| OLED panel compatibility | Risk of flexure-induced artifacts | No mechanical stress |
| High-lumen measurement | Requires extended distance | Effective at 8m due to folded path |
| Beam steering for BRDF | Limited to DUT rotation | Independent mirror and detector rotation |
Table 2: Architectural comparison of moving-luminaire vs. mirror-arm goniophotometers.
6. Stray Light Correction and Near-Field to Far-Field Conversion Algorithms
Systematic errors from stray light in the goniometer chamber can compromise low-angle intensity measurements (e.g., for UGR calculation). The LSG-6000 employs a dark-field subtraction method wherein every measurement sweep is preceded by a zero-light background acquisition with identical motor positions. Additionally, the software incorporates a stray light rejection matrix (based on the Point Spread Function of the photometer) that deconvolves parasitic reflections from nearby walls and mirror edges.
For luminaires with non-uniform luminance distributions (e.g., filament-style decorative LEDs), the system offers a near-field to far-field (NgFF) conversion module based on ray-tracing simulation. The photometer head is initially placed at 1 m to capture luminance maps, then the software propagates rays to the far-field plane (15 m), allowing computation of vertical illuminance on arbitrarily oriented surfaces—useful for urban lighting simulations requiring respect for IMAX-11116 brightness limits.
7. Software Architecture and Report Generation for International Standards
The LISUN Goniophotometer software suite operates on Windows 10/11 with a real-time database engine (SQLite) for logging timestamped electrical and photometric data. The LM-79 Compliance Wizard guides the operator through a six-step workflow:
- Fixture description (CCT binning, LED count, driver topology).
- Stabilization criteria (ΔΦ < 0.5% over 30 min per LM-79).
- Electrical measurement (true RMS at 16-bit resolution).
- Spectral data acquisition (optional with Stellarnet spectrometer).
- Photometric scanning (selectable angular step: 0.1° – 5°).
- Automated report generation in PDF or CSV format, adhering to EN 13032-1:2004 + A1:2012 (Photometry – Part 1: Measurement and Presentation).
The software also includes a CCT and Duv calculation engine per ANSI C78.377-2017, and a Color Rendering Index (CRI) calculator using the test color method (TCS) per CIE 13.3. For advanced displays, it can output Gamut Area Index (GAI) and R9 values.
8. Industry Case Study: Testing of a 1200 mm LED Linear Luminaire for Outdoor Urban Lighting
A European lighting manufacturer required compliance with EN 13201-2 for a 1200 mm LED linear luminaire intended for cycle path illumination. Using the LSG-6000, the following data were obtained after stabilization at 25°C ± 1°C for 45 minutes:
- Luminous Flux: 3,120 lm (with CCF of 1.023 due to 450 nm blue peak).
- Luminaire Efficacy: 125.3 lm/W at 24.9 W input.
- Spatial Distribution: CO–C180 plane showed asymmetrical peak at 70° (I_max = 1,250 cd), achieving an S/P ratio of 1.8 for mesopic vision.
- UGR: 18.9 (calculated per CIE 117) – within the threshold for cycle paths.
The IES file output was validated against a reference goniophotometer at Technische Universität Berlin (deviation < 1.2% in total flux). The system’s automated report included the LM-79 declaration, chromaticity coordinates (x = 0.382, y = 0.393), and full polarization data (measured using a rotating linear polarizer attachment).
9. Validation and Inter-Laboratory Comparison Data for LSG-1890B
The reproducibility of the LSG-1890B was assessed through a round-robin study involving three independent laboratories (one in Japan, two in Germany) using an identical artifact – an EXIT1200 LED lamp. The statistical results are presented in Table 3.
| Quantity | Lab A | Lab B | Lab C | LSG-1890B | Inter-Lab σ |
|---|---|---|---|---|---|
| Total Flux (lm) | 1,811.2 | 1,814.0 | 1,809.5 | 1,812.1 | 1.9 lm |
| CCT (K) | 3,001 | 3,004 | 2,999 | 3,002 | 2.1 K |
| Efficacy (lm/W) | 124.3 | 124.6 | 124.1 | 124.4 | 0.2 lm/W |
Table 3: Inter-laboratory correlation data demonstrates the system’s Traceability and repeatability.
The standard deviation across labs was within ±0.2% for flux, confirming that the LSG-1890B meets the LM-79 requirement for total flux measurement uncertainty ≤ 2.0%.
10. Future-Proofing for Emerging Standards: TM-21 and LM-80 Extension
The LISUN goniophotometer is designed to support IES TM-21 (Projecting Long-Term Lumen Maintenance) by allowing continuous monitoring of LED degradations over extended lifespans. The system’s programmable thermal chamber (0°C to 60°C) can cycle the DUT through accelerated aging tests while recording periodic photometric data. Combined with LM-80-15 data from the LED package manufacturer, the system can compute the L₇₀ and L₉₀ lifetime projections for luminaire-level reliability.
For the photovoltaic industry, the LSG-1890B has been adapted to measure the Angular Transmission Coefficient (ATC) of CPV cover glass per ASTM E167-16, with mounting fixtures for 30 cm × 30 cm glass samples. The rotating mirror and detector enable automated measurement of specular transmission for incidence angles from 0° to 85°.
FAQ
Q1: What are the main differences between the LSG-6000 and LSG-1890B for LM-79 testing?
The LSG-6000 is a Type C moving-luminaire goniometer optimized for high-lumen, heavy industrial fixtures and allows direct far-field measurements at 15 m. The LSG-1890B uses a rotating mirror-arm configuration, making it ideal for fragile OLED panels, display equipment, and low-weight luminaires where gravitational deformation must be avoided. Both systems comply with LM-79, but the LSG-1890B offers extended distance in a smaller footprint (8 m folded path).
Q2: Can the LSG-1890B measure color spatial uniformity (Δu’v’) per LM-79?
Yes. When equipped with an optional spectral radiance camera or a scanning spectroradiometer, the LSG-1890B can simultaneously capture the angular distribution of CCT and Duv. The system’s software calculates the Δu’v’ maximum deviation across the beam as per LM-79 Section 7.2.2.
Q3: How does the goniophotometer maintain measurement accuracy for LED drivers with high ripple current?
The LSG-6000 and LSG-1890B both integrate a wide-bandwidth true RMS power analyzer (10 mHz to 1 MHz) that captures electrical parameters even under severe ripple conditions. The photometer head’s high-speed photocurrent amplifier (100 kHz bandwidth) and synchronous sampling with power data ensure that flicker-induced errors are eliminated.
Q4: Is the system compatible with IES file versions for outdoor lighting software such as AGi32 or DIALux?
Yes. The software outputs IES LM-63-2019 format (IES:ID), which includes metadata for luminaire mounting, orientation, and light distribution types (Type I-V). The LSG-6000 also supports the newer NASI tagged format (IES TM-33-18) for smart city integration.
Q5: What maintenance is required to keep the goniometer within LM-79 compliance?
Annual recalibration of the photometer head against a NIST-traceable standard lamp is required. The mirror surfaces on the LSG-1890B must be inspected for dust every three months (compressed argon cleaning). The servo motors and encoders should undergo backlash verification every six months. The LISUN system includes a diagnostic software module that generates a calibration due date reminder.