Title: Type A Goniophotometer: Accurate Luminous Intensity Distribution Measurement for LED Lighting Testing
Abstract
The characterization of luminous intensity distribution (LID) is a fundamental requirement for the photometric evaluation of solid-state lighting (SSL) sources, including LEDs and OLEDs. The Type A goniophotometer, distinguished by its axis configuration and measurement geometry, remains the gold standard for acquiring precise spatial light distribution data. This article delineates the operational principles, metrological advantages, and standardization compliance of the Type A goniophotometer, with specific reference to the LISUN LSG-1890B High-Precision Goniophotometer Test System. Coverage extends from instrumentation architecture to validation against international photometric standards (IES LM-79-19, CIE S 025/E:2015, EN 13032-1, JIS C 8105-5), and industry-specific applications spanning general lighting, medical illumination, sensor optics, and photovoltaic concentrator testing.
1. Fundamental Architecture of the Type A Goniophotometric Coordinate System
The Type A goniophotometer is defined by its rotational axes: the photodetector (or mirror) rotates in a vertical plane (γ-axis), while the test specimen rotates in a horizontal plane (C-plane). This configuration conforms to the C-γ coordinate system recommended by CIE 121 and IES LM-79, where C represents the azimuthal angle (0° to 360°) and γ represents the vertical angle (0° to 180°).
Unlike Type B (θ-φ) or Type C (B-β) systems, the Type A design is optimized for non-rotationally symmetric sources such as linear LEDs, surface-mount device (SMD) arrays, and chip-on-board (COB) modules. The fixed spatial relationship between the photometer head and the source during horizontal rotation minimizes inter-reflections and maintains constant path length, thereby reducing systematic errors. The LISUN LSG-1890B employs this exact kinematic architecture, with a rotational accuracy of ±0.1° and an angular resolution of 0.01°, ensuring repeatable data for rigorous photometric binning.
2. Measurement Principle of the LSG-1890B: Dual-Axis Servo Control and Photometric Data Acquisition
The LSG-1890B integrates a high-speed servo motor system with an absolute encoder feedback loop for both C and γ axes. The photometric sensor is a Class A (CIE) photopic-corrected silicon photodiode with a spectral responsivity closely matched to the V(λ) function. The measurement chain comprises:
- Photometer head: Calibrated luminous intensity range from 0.001 cd to 200,000 cd (with optional neutral density filters).
- Data acquisition: 16-bit analog-to-digital converter sampling at 1 kHz, with a dark-current compensation cycle every 5 seconds.
- Angular control: Closed-loop proportional-integral-derivative (PID) algorithm maintaining positional stability within ±0.05° under dynamic loading.
Measurement commences with a preliminary luminance mapping at coarse resolution (e.g., 5° steps) to estimate peak intensity, followed by a fine scan at user-defined intervals (as low as 0.1°). The system automatically calculates total luminous flux by integrating the intensity distribution over all solid angle segments, using the formula:
[
Phi = int_0^{2pi} int_0^{pi} I(gamma, C) sin gamma , dgamma , dC
]
where ( I(gamma, C) ) is the measured luminous intensity in candelas. The LSG-1890B achieves an uncertainty of less than ±2% for flux measurement, as verified by inter-laboratory comparisons following CIE 84-1989 guidelines.
Table 1. Key Specifications of the LISUN LSG-1890B Type A Goniophotometer
| Parameter | Specification |
|---|---|
| Measurement distance | 2.0 m to 30 m (variable) |
| Luminous intensity range | 0.001 cd – 200,000 cd |
| Angular coverage | C: 0°–360°, γ: –90° to +90° |
| Angular resolution | 0.01° |
| Spectral bandwidth | 380 nm – 780 nm |
| Photometric grade | CIE Class A (integrating detector) |
| Supported standards | IES LM-79, CIE S025, EN 13032-1, JIS C 8105-5, GB/T 29294 |
3. Compliance with International Photometric and Luminance Standards
A Type A goniophotometer must demonstrably fulfill the metrological requirements of standards that govern SSL testing. The LSG-1890B is designed in accordance with:
- IES LM-79-19 (Illuminating Engineering Society): Specifies absolute photometry, requiring Goniophotometer-based measurement at multiple angular planes. The LSG-1890B’s self-centering chuck and laser alignment system guarantee that the LED source’s light center is coincident with the rotational axes, a mandatory condition for LM-79 compliance.
- CIE S 025/E:2015 (Test Method for LED Lamps, LED Luminaires and LED Modules): Defines measurement geometry, distance-to-detector ratio (≥15× source diameter), and ambient temperature control (25°C ± 1°C). The LSG-1890B incorporates a temperature-controlled darkroom and an infrared (IR) temperature probe for junction temperature monitoring.
- EN 13032-1 (Light and Lighting – Measurement and Presentation of Photometric Data): Requires reporting of intensity distribution in LDT and IES TM-27 (IESNA:LM-63) formats. The bundled LSG software automatically exports in .ies., .ldt, and .cie formats.
- JIS C 8105-5 (Japanese Industrial Standard): Pertaining to the photometric measurement of LED lamps for general lighting. The LSG-1890B’s C-plane rotation matches the JIS method for measuring luminous intensity distribution with a mirror-type Goniophotometer.
4. Industrial Applications: From General Lighting to Medical and Optical Systems
4.1 Lighting Industry and Urban Lighting Design
For manufacturers of streetlights, floodlights, and architectural luminaires, the LSG-1890B enables rigorous verification of UGR (Unified Glare Rating) and LOR (Light Output Ratio). Type A measurement data supports the generation of polar curves and isocandela diagrams required for Dialux and Relux simulation. The system’s ability to test luminaires weighing up to 30 kg and spanning 1.5 m in diameter accommodates large-area LED panels used in facade illumination.
4.2 LED & OLED Manufacturing and Display Equipment Testing
OLED panels exhibit Lambertian or near-Lambertian emission profiles; the LSG-1890B’s high angular resolution (0.01°) resolves fine angular luminance variations critical for display uniformity analysis. In LED binning, the system’s rapid scan (<20 minutes for a full 0°–360° scan at 2° intervals) supports high-throughput binning according to ANSI C78.377 chromaticity quadrangles.
4.3 Medical Lighting Equipment
Surgical lights and examination lamps require precise beam spread angles and illuminance uniformity. The LSG-1890B measures the 10% and 50% beam width angles in conformity with IEC 60601-2-41. The cosine-corrected photometer head ensures accurate illuminance values at the target plane (e.g., 1 m from source).
4.4 Sensor and Optical Component Production
In manufacturing photodiodes, CCD arrays, or LiDAR emitters, the spatial emission pattern must match the acceptance angle of the receiver. The LSG-1890B’s absolute photometry over a dynamic range of 1:10^8 enables characterization of high-power IR LEDs (850 nm, 940 nm) and VCSEL arrays.
4.5 Photovoltaic Industry
Concentrated photovoltaic (CPV) modules rely on the angular distribution of sunlight simulators. While not directly measuring solar cells, the LSG-1890B can qualify the intensity profile of artificial sun sources used in solar simulator calibration.
4.6 Stage and Studio Lighting
Moving heads and color-mixing fixtures produce complex beam shapes. The LSG-1890B’s ability to perform automated C-plane scans at user-defined γ intervals generates precise beam angle data for photometric database entries.
5. Competitive Advantages of the LSG-1890B Over Alternative Goniometric Architectures
5.1 Type A vs. Near-Field Goniophotometers
Near-field systems use CCD cameras to capture luminance maps of the source surface, requiring sophisticated ray-tracing algorithms to compute far-field distribution. The LSG-1890B’s direct far-field measurement eliminates algorithmic propagation errors. For sources with strong spatial luminance gradients (e.g., micro-LEDs or segmented arrays), Type A systems provide an inherently lower uncertainty in total flux (±2% vs. ±5%–10% for near-field systems).
5.2 Voltage and Environmental Integration
The LSG-1890B features an integrated programmable AC/DC power supply and a calibrated reference photodiode for luminous flux drift monitoring. This eliminates the need for external measurement instruments, reducing measurement chain uncertainty. A built-in temperature chamber (option) can maintain 15°C–45°C for IES LM-80-21 lumen maintenance testing.
5.3 Aliasing Immunity
Because Type A measurement records intensity values at discrete solid angle positions, it avoids spatial aliasing issues common in mirror-based scanning systems. The LSG-1890B’s four-quadrant encoder provides 1,200 counts per revolution, ensuring no missing data points in the C-γ grid.
5.4 Software Ecosystem
The LSG-1890B software package includes:
- Real-time polar diagram and 3D intensity surface rendering.
- Automatic computation of Zonal Lumen Densities (ZLD) per CIE 140.
- Export to all major photometric formats (IES/LDT/CIE).
- Integration with LISUN’s LMS series spectroradiometer for combined spectral and spatial measurement.
Table 2. Comparison of Goniophotometer Types for LED Testing
| Feature | Type A (LSG-1890B) | Near-Field (Imaging) | Type B (Mirror) |
|---|---|---|---|
| Angular accuracy | ±0.05° | ±0.2° (camera lens) | ±0.15° |
| Flux uncertainty | ±2% | ±5% | ±3% |
| Weight limit | 30 kg | 10 kg (typical) | 50 kg |
| Measurement speed | Medium (20 min full scan) | Fast (5 min) | Slow (40 min) |
| Source size limit | < 30 cm diam. | No limit (scaling) | < 50 cm diam. |
6. Measurement Protocol for LED Lighting Testing using the LSG-1890B
A typical experimental workflow is as follows:
- Specimen mounting and alignment: The LED luminaire is mounted on the test table with its photometric center aligned to the laser crosshair. The γ rotation axis is set to 0° when the source is facing the photometer.
- Dark offset calibration: With the source off, the photometer records dark current for each sensor element over 60 seconds.
- Reference measurement: Calibration using a standard lamp traceable to NIST or National Metrology Institute (NMI) for absolute intensity.
- Angular scan programming: User selects C-plane density (e.g., every 15°) and γ step (e.g., every 1°). For asymmetric sources, automated C-plane interpolation is enabled.
- Data logging and correction: Measurement data is corrected for ambient temperature drift (sensor self-heating) and inverse-square law distance factor.
- Computation and export: The system computes total luminous flux, beam angle (50% and 10% of maximum intensity), and cumulative lumen percentage.
7. Standards Compliance Verification for Global Markets
The LSG-1890B’s Type A configuration directly addresses the angular measurement requirements of EN 13032-1 for European CE marking, IES LM-79-19 for North American ENERGY STAR certification, and JIS C 8105-5 for Japanese PSE marking. Specifically:
- For EU regulation (EU) 2017/1369 regarding energy labeling of light sources, the LSG-1890B provides the luminous flux measurement (Φ) with an expanded uncertainty (k=2) of 2.1%, compliant with the 3% limit.
- For medical device approval per IEC 60601-2-41, the system measures the distance-dependent illuminance decay and beam center position with a repeatability of 0.3%.
- In OLED display testing, the system measures angular luminance falloff (half-luminance angle) consistent with the VESA FPDM 2.0 standard.
8. Frequently Asked Questions (FAQ)
Q1: Why must the test distance be at least 15 times the source diameter for Type A goniophotometer measurements?
A: This condition ensures far-field approximation: the photometer observes the source as a point, and the inverse-square law applies without correction. For smaller distances, the goniophotometer would measure the source as an extended object, introducing systematic errors in luminous intensity distribution.
Q2: Can the LSG-1890B measure the chromaticity distribution (CCT, CRI) of an LED luminaire simultaneously?
A: The base model measures photometric intensity only. However, when paired with the LISUN LMS-9000 spectroradiometer via a fiber-optic coupling, the system can perform synchronized spectral-radiometric scans. The software then correlates spatial intensity data with spectral power distribution to produce pseudo-color maps of CCT and CRI across the beam.
Q3: How does the LSG-1890B handle thermal drift during long-duration measurements?
A: The system incorporates a temperature sensor mounted on the photometer head. A software-based linear correction algorithm applies a negative coefficient of 0.02% per °C based on pre-calibrated thermal drift data. Additionally, the power supply stabilizes drive current to within ±0.1% over 8 hours.
Q4: What is the maximum angular step resolution for gonio-photometric data accepted by IES TM-27 (LM-63) files?
A: IES TM-27-2020 recommends an angular resolution of at least 1° for the γ (vertical) plane and 5° for the C (horizontal) plane, but the format allows any step. The LSG-1890B’s minimum step of 0.01° enables fine scanning for high-precision applications such as LiDAR emitter characterization, albeit with increased measurement time.
Q5: Is the Type A goniophotometer suitable for measuring the intensity distribution of OLED panels that emit light from both sides?
A: Yes. The LSG-1890B supports dual-sided measurement by performing two separate scans (positive and negative γ ranges) and combining the data vectors. The software can produce polar diagrams representing the total hemispherical emission, with each hemisphere (top and bottom) plotted on separate axes.