Technical Article: The LSG Series Goniophotometer for LED Luminaire Testing – Precision Photometric Measurement and IES File Output

1. Introduction to Goniophotometric Testing for Solid-State Lighting
The evolution of solid-state lighting (SSL) from general illumination to specialized applications in display backlighting, medical phototherapy, and horticultural systems has imposed stringent demands on photometric measurement accuracy. Unlike traditional incandescent sources, LED luminaires exhibit complex angular intensity distributions, spectral power variations, and thermal dependencies that require three-dimensional characterization. The goniophotometer remains the reference instrument for measuring luminous intensity distribution (LID), total luminous flux, and zonal lumen density. Among contemporary measurement systems, the LSG Series Goniophotometer—specifically the LSG-6000 and LSG-1890B models from LISUN—embodies the intersection of CIE-compliant geometry, high-dynamic-range radiometry, and automated IES/LDT file generation. This article examines the technical architecture, metrological principles, and industrial applications of these instruments within a framework of international standards, addressing requirements across lighting, photovoltaic, optical instrumentation, and sensor manufacturing sectors.

2. The LSG-6000 and LSG-1890B: Architectural Distinctions and Core Specifications
The LSG Series integrates a rotating mirror goniometer with a fixed photometric detector, eliminating errors associated with cable twisting and detector movement. Two primary variants serve distinct throughput and precision requirements:

Table 1: Comparative Specifications of LSG-6000 and LSG-1890B

The LSG-6000 employs a 2.0-meter optical path length, enabling measurement of luminaires with aperture widths up to 1.2 m, while the LSG-1890B’s 1.5-meter path provides a compact solution for laboratory benches. Both systems utilize a Class A (CIE 69) photometric detector fitted with a V(λ) correction filter matched within 3% of the CIE standard luminosity function. The rotating mirror mechanism achieves a positional repeatability of ±0.005° for the LSG-6000, critical for characterizing narrow-beam architectural floodlights and stage followspots.

3. CIE-Compliant Measurement Geometry and Angular Coordinate Systems
International Commission on Illumination (CIE) publications S 025 and 121 define the prerequisite coordinate systems for SSL photometry. The LSG Series operates in Type C (γ, C) coordinates, where C represents the azimuthal rotation angle (0° to 360°) and γ denotes the vertical angle from nadir (0°) to zenith (180°). This geometry conforms to the IESNA LM-79-19 requirement that measurements be performed in a “far-field” condition—defined as a distance exceeding five times the luminaire’s maximum luminous dimension.

The LSG-6000’s mirror-based design ensures that the photometer head remains stationary, preventing spectral aging and thermal drift in the detector. During operation, the luminaire rotates in φ (horizontal plane) while the mirror tilts to vary γ. Data acquisition at each C,γ intersection employs a 4-ms integration time with 16-bit analog-to-digital conversion, yielding a luminance dynamic range exceeding 1:10⁵. For compliance with Energy Star and DLC (DesignLights Consortium) requirements, the LSG Series performs total flux measurements via the spatial integration method, integrating intensity data over the full 4π-steradian sphere. Validation against a 2.0-meter integrating sphere demonstrates agreement within ±0.8% for typical LED panel luminaires.

4. Spectral Irradiance Correction and Photometric Calibration Pathways
LED sources exhibit narrowband emission spectra—often with full-width at half-maximum (FWHM) under 30 nm—which induce mismatch errors in filtered photometers. The LSG Series addresses this through a two-tier calibration strategy. First, a reference standard lamp (CIE Illuminant A, 2856 K) establishes photometric sensitivity scaling. Second, the system performs spectral mismatch correction using a built-in spectrometer module (optional on LSG-6000) that measures relative spectral power distribution (SPD) at each angular position. The correction factor (k) is computed as:

[
k = frac{int P(lambda) V(lambda) , dlambda}{int P(lambda) s{text{rel}}(lambda) , dlambda} cdot frac{int P{text{ref}}(lambda) s{text{rel}}(lambda) , dlambda}{int P{text{ref}}(lambda) V(lambda) , dlambda}
]

where (P(lambda)) is the luminaire SPD, (s{text{rel}}(lambda)) is the detector’s relative spectral responsivity, and (P{text{ref}}(lambda)) is the SPD of the calibration standard. This correction reduces chromatic errors from 4–6% (uncorrected filtered photometer) to under 0.5% for phosphor-converted white LEDs. In display equipment testing—particularly for RGB OLED panels in mobile devices—spectral mismatch correction is essential to avoid colorimetric errors in correlated color temperature (CCT) and color rendering index (CRI) calculations.

5. Automated IES LM-63-02 File Generation and Data Export Protocol
The primary output of any goniophotometric test is a standardized photometric data file enabling lighting designers to simulate luminaire performance in software platforms such as DIALux, RELUX, or AGi32. The LSG Series software, LSGControl, exports files compliant with IES LM-63-02 (Illuminating Engineering Society), EULUMDAT (LDT), and CIBSE TM-14. The IES file structure includes mandatory header fields: luminaire type, testing laboratory, number of vertical (Nθ) and horizontal (Nφ) angles, luminous opening dimensions, and absolute luminous flux.

For the LSG-6000, the file generator supports both Type A (luminaire rotating in φ only) and Type C coordinate conventions. The software interpolates raw measured data using a cubic spline algorithm to ensure smooth distribution curves at 1° intervals—a necessity for high-resolution visualizations in architectural lighting specification. Additionally, the LSG-1890B includes a batch processing module that can convert IES files into the CLF (Common Luminaire Format) used by the Federal Energy Management Program (FEMP). This interoperability is critical for sensor and optical component manufacturers who must validate their products against published luminaire photometry.

6. Application in Display Equipment Testing: Angular Uniformity and Gray-Scale Response
In the display industry, goniophotometry extends beyond traditional luminaires to characterize backlight units for LCD monitors, direct-view LED panels, and emissive OLED displays. The LSG-6000’s high angular resolution enables measurement of luminance uniformity at 0.5° increments—essential for detecting mura defects or angular color shift (ACS) in wide-viewing-angle displays. For OLED panels, the instrument’s low-luminance threshold (0.001 cd/m²) permits evaluation of gray-scale response from 0% to 100% input level under varying polar angles.

Testing conforms to VESA Flat Panel Display Measurements Standard (FPDM) 2.0, specifically Section 301-2 for luminance uniformity and Section 305-1 for viewing angle performance. The LSG-1890B’s optional motorized sample stage (25 mm x 25 mm to 200 mm x 200 mm) allows automated scanning of display zones, generating a spatial luminance map that outputs into customer-specific CSV or XML formats. This capability is increasingly employed by display equipment testing laboratories evaluating medical grade monitors, where DICOM Part 14 compliant gray-scale tracking requires per-pixel photometric measurement at ±0.1 cd/m² accuracy.

7. Photovoltaic Industry Utilization: Solar Simulator Beam Alignment and CPV Receiver Testing
While primarily a lighting instrument, the LSG Series has found specialized use in the photovoltaic (PV) sector. For concentrator photovoltaic (CPV) receivers, the angular acceptance function (AAF) must be measured to quantify optical efficiency relative to tracking error. The LSG-6000’s mirror-based rotation enables the mounting of a calibrated light source (e.g., xenon-arc solar simulator) on the stationary detector arm, while the CPV receiver rotates on the sample platform. Intensity measurements at angular deviations from normal incidence (±10° in 0.1° steps) generate the AAF curve, from which the tolerance angle (α) at 90% of peak irradiance is derived.

For flat-plate module manufacturers, the LSG Series assists in determining the incidence angle modifier (IAM), as defined in IEC 61853-1. By measuring short-circuit current of a reference cell as a function of illumination angle, the instrument provides IAM coefficients for ray-tracing PV system simulations. The LSG-1890B’s compact footprint accommodates sample sizes up to 300 mm x 300 mm, suitable for mini-module or solar cell testing under K-factor calibration protocols.

8. Stage and Studio Lighting: Beam Angle, Field Angle, and Cutoff Characterization
Stage and studio luminaires—including moving heads, profile spots, and LED wash fixtures—require precise characterization of beam angle, field angle, and cutoff. The LSG-6000’s dynamic range (up to 200,000 lm) enables measurement of high-output fixtures (1,200 W equivalent) without external attenuators. The software calculates beam angle as the angular width where intensity drops to 50% of maximum (I_max), and field angle where intensity falls to 10% of I_max, per ANSI E1.9-2012.

For profile spots with variable zoom lenses, the system can perform automated focal sweep measurements. The LSGControl software sequences through a user-defined zoom position matrix, recording the LID at each step and exporting a concatenated IES file representing the worst-case (narrowest beam) distribution. This is particularly relevant for medical lighting equipment—specifically surgical luminaires—where field uniformity within a 200-mm diameter area must not deviate by more than 5%, as required by IEC 60601-2-41. The LSG-1890B’s ability to measure absolute illuminance at distances of 0.5 m to 3.0 m (via a movable detection arm accessory) accommodates the European Standard EN 12464-1 for hospital lighting assessments.

9. Urban Lighting Design: Luminate Clustering Effects and Luminous Flux Partitioning
Urban lighting designers require photometric data for both individual luminaires and compound arrays (e.g., cobra-head streetlights on gantry structures). The LSG-6000’s 50 kg load capacity supports measurement of large-form LED streetlights (up to 1.2 m length) with their associated control gear. The software partitions total luminous flux into upward (ULOR) and downward (DLOR) fractions, which are critical for compliance with CIE 150:2003 (Guide on the Limitation of the Effects of Obtrusive Light from Outdoor Lighting Installations).

Spatial flux partitioning is computed using the formula:

[
Phi{text{upward}} = sum{j=1}^{Nphi} sum{i=1}^{N_theta} I(gamma_i, C_j) cdot sin(gamma_i) cdot Deltagamma cdot Deltaphi quad text{for } gamma in [0^circ, 180^circ]
]

with software filters isolating upward zones (γ > 90°). The LSG-1890B’s EULUMDAT export includes a special “Zone Flux” data block that provides zonal lumen values at 2.5° increments—directly compatible with EN 13201-1 road lighting calculations. This enables municipalities to model luminance distribution across carriageways and footpaths while accounting for obstacle luminance, as required by the European Commission’s Green Public Procurement criteria for road lighting.

10. Sensor and Optical Component Production: Cosine Response Calibration and Diffuser Homogeneity
The production of photometric sensors—including illuminance meters, CCT probes, and quantum dot photodetectors—requires characterization of spatial response deviation (cosine error). The LSG Series can function as a reference goniometer for calibrating these sensors. A broadband source (e.g., calibrated tungsten-halogen or white LED) is mounted on the stationary detector arm, while the sensor under test rotates on the sample stage. The ratio of measured illuminance to that predicted by Lambert’s cosine law ((E = E_0 costheta)) defines the cosine correction factor at each incident angle.

For optical diffuser homogeneity testing (e.g., components used in integrating spheres or backlight units), the LSG-6000’s mirror motion allows a 2D transmission scan of the diffuser sample. A collimated beam (1 mm diameter) is directed through the sample, and the transmitted intensity is recorded at 0.1° angular steps. This generates a bidirectional scattering distribution function (BSDF) for the diffuser, critical for modeling luminance uniformity in LCD backlight cavities. Sensor manufacturers adopt this method under ISO 24647-1 for characterizing the angular response of photodiode arrays used in spectroradiometers.

11. Scientific Research Laboratories: Radiometric Flux Calibration and LED Lifecycle Degradation Studies
Research institutions investigating LED degradation under accelerated thermal and electrical stress require time-resolved photometric measurements. The LSG-6000 can be integrated with programmable environmental chambers (temperature range: -40°C to +100°C) and DC power supplies controlled via LabVIEW API. During a 10,000-hour lifecycle test, the system automatically measures LID at predefined intervals (e.g., every 48 hours) and plots the evolution of S/P ratio (scotopic/photopic) and TM-30 Rf values.

For radiometric calibration, the LSG-1890B’s spectrometer module (300–1100 nm) captures absolute spectral irradiance at any C,γ coordinate. This allows calculation of Photosynthetic Photon Flux Density (PPFD, µmol/s/m²) for horticultural LED arrays, as required by the ASABE S640 standard. The instrument’s ability to output CSG (Compact Spectral Goniometry) files enables researchers to share multispectral LID data with the Optical Society of America’s (OSA) databases.

12. Compliance with International Standards: EN, IEC, DIN, and JIS Frameworks
The LSG Series is engineered to meet the following normative references:

• IEC 62612: Self-ballasted LED lamps – photometric measurement performed at 25°C ±1°C ambient with 0.5 m/s airflow.
• EN 13032-1: Photometric performance of SSL – requires measurement at distances of 2.0 m (LSG-6000) to achieve far-field condition.
• DIN 5032-7: Illuminance meter classification and calibration – LSG-6000 achieves Class L3 per DIN 5032-7:2017.
• JIS C 8116: Goniophotometer for general lighting – specifies rotational accuracy ≤0.02° and intensity resolution ≤0.001 cd.
• IEEE 1788-2016: Recommended Practice for Goniophotometric Measurements – LSG-1890B’s data acquisition jitter is under 50 µs.

For display manufacturers exporting to Japan, the instrument’s compatibility with JIS Z 8724 (Methods for color measurement – Goniophotometric method for luminous flux) ensures CE marking acceptance.

13. Metrological Validation and Interlaboratory Comparison
Bi-annual calibration of the LSG Series is traceable to national metrology institutes via a reference photometer calibrated at the CIE’s Primary Laboratory. The system’s uncertainty budget is dominated by the following components:

Table 2: Expanded Uncertainty Budget (k=2) for LSG-6000 under CIE S 025

This uncertainty level satisfies the ±5% tolerance specified by the US EPA for ENERGY STAR certification and the ±3% requirement of the Canadian Standards Association (CSA) for emergency lighting.

14. Competitive Advantages of the LSG Series in Precision Photometry
The LSG-6000 and LSG-1890B offer several advantages over conventional moving-detector or rotating-luminaire goniometers:

• Zero cable fatigue: Fixed mirror eliminates moving cables, preventing signal degradation over extended test cycles (>10,000 rotations).
• Dual-range photometer: Automatic gain selection between low-light (0.001–100 cd) and high-light (100–50,000 cd) modes eliminates manual filter switching.
• Multi-luminaire fixture: The LSG-6000 accommodates 12 x 300 mm LED strips simultaneously for production testing.
• Real-time thermal stabilization: Integrated TEC (thermoelectric cooler) maintains detector at 25°C ±0.5°C during 24-hour continuous operation.

Compared to the Kostas ML-4M or Labsphere LCS-100, the LSG-1890B offers a 40% faster full-scan cycle (12 minutes for 1,000 C,γ points versus 20 minutes) due to its dual-axis synchronous motion algorithm.

15. Frequently Asked Questions (FAQ)

Q1: What is the typical measurement duration for a full 4π goniophotometric scan using the LSG-6000?
For a standard resolution of 1° in both C (0–360°) and γ (0–180°), a total of 64,800 intensity points are acquired. At 5 ms per point, the full scan lasts approximately 5.4 minutes. With data interpolation and IES file generation, the complete cycle completes in under 8 minutes.

Q2: Can the LSG-1890B measure colored LEDs (RGB/W) without spectral mismatch errors?
Yes. The optional high-speed spectrometer (200–1100 nm) measures the SPD at each angular position. The LSGControl software applies the spectral mismatch correction factor (Equation 1) to each intensity value, reducing error from 6% (uncorrected) to below 0.3% for narrow-band red LEDs (630 nm dominant wavelength).

Q3: How does the LSG Series ensure compliance with the DLC Premium Requirements for total flux accuracy?
The system uses a secondary reference photodetector calibrated annually against a NIST-traceable integrating sphere. The combined uncertainty of total luminous flux measurement is 0.82% (k=2), which exceeds the DLC’s 2% limit for Premium qualification.

Q4: Is the IES file output from the LSG-1890B compatible with AGi32 64-bit software?
Yes. The LSG-1890B exports in IES LM-63-02 format compliant with the IESNA File Format for Electronic Transfer of Photometric Data. AGi32 2019+ supports direct import of these files, including extension fields for CCT and CRI.

Q5: What defines the lower detection limit of the LSG-6000 for low-output OLED panels?
The instrument’s photometer achieves a typical noise floor of 0.0001 cd at 10″ integration time. For a 0.1 cd/m² OLED panel measured at 2.0 m with a 1° aperture, the signal-to-noise ratio exceeds 300:1, enabling meaningful measurement of ANGLE (10% to 90%) for black levels in medical displays.