Title: Goniophotometer Guide: Principles and Applications in Lighting Testing

Abstract

The accurate characterization of spatial light distribution is fundamental to the design, quality assurance, and regulatory compliance of modern lighting systems. A goniophotometer serves as the primary metrological instrument for measuring luminous intensity distribution, total luminous flux, and zonal lumens of lamps and luminaires. This article provides a comprehensive technical examination of goniophotometric principles, with specific emphasis on the operational architecture of the LISUN LSG-6000 and LSG-1890B goniophotometer test systems. The discussion encompasses measurement geometries, photometric quantities, calibration methodologies, and application across multiple sectors including LED manufacturing, display equipment testing, and medical lighting. Reference to international standards such as CIE S 025, IES LM-79-19, EN 13032-1, and JIS C 8105-5 establishes the framework for evaluating measurement accuracy. Comparative analysis of measurement uncertainty and system capabilities demonstrates the suitability of these instruments for R&D laboratories and production environments demanding traceable photometric data.

---

Fundamentals of Goniophotometric Measurement Geometry and Photometric Quantities

A goniophotometer functions by rotating a photometer head or the luminaire under test around defined axes relative to a fixed detector, enabling the measurement of luminous intensity at discrete angular positions. The fundamental photometric quantity derived is luminous intensity I (cd), defined as the luminous flux per unit solid angle in a given direction. Integration of intensity values across all solid angles yields total luminous flux Φ (lm).

Two primary measurement geometries govern goniophotometric practice: Type A (rotating luminaire, fixed detector) and Type C (fixed luminaire, rotating detector). The LISUN LSG-6000 employs a Type C geometry using a rotating mirror system, which maintains the luminaire in its natural operating orientation—critical for thermal equilibrium in LED and OLED products. The measurement coordinate system, defined by vertical (C) and horizontal (γ) angles, follows the spherical coordinate convention established in CIE 121. Intensity data is collected over complete spheres (4π steradians) or hemispheres (2π) depending on application.

Photometric accuracy depends on the inverse-square law compliance distance, typically requiring a measurement distance of at least 15–25 times the maximum luminaire dimension. For the LSG-1890B, the effective optical path length of 2.0 m satisfies this criterion for most luminaires up to 600 mm in diameter. The photometer head, calibrated against a standard lamp traceable to the National Institute of Standards and Technology (NIST) or Physikalisch-Technische Bundesanstalt (PTB), ensures absolute photometric accuracy within ±3 % for luminous flux and ±2 % for intensity.

Architectural Design and Operational Specifications of the LISUN LSG-6000 Goniophotometer

The LISUN LSG-6000 represents a high-precision mirror-type goniophotometer optimized for LED-based luminaires and solid-state lighting products. Its configuration integrates a rotationally symmetric mirror system capable of measuring luminous intensity distributions across C-planes from 0° to 360° and γ-angles from -90° to +90°. The instrument utilizes a Class L (LISUN proprietary high-sensitivity) photometric detector incorporating a V(λ) correction filter that achieves f1’ ≤ 3 %, meeting the requirements of DIN 5032 Part 7.

Key operational parameters include a photometric measurement distance of 3.0 m, allowing evaluation of luminaires with maximum luminous intensity up to 200,000 cd. The angular resolution programmability down to 0.1° permits detailed near-field and far-field analysis for asymmetric distributions. The system incorporates a programmable constant-current power supply with voltage, current, and power measurement accuracy of ±0.2 % (class 0.2), crucial for characterizing LED drivers and verifying thermal stabilization.

Data acquisition is managed through LSG-6000 dedicated software, which calculates total luminous flux via numerical integration using the following equation:

[
Phi = sum{i=1}^{n} sum{j=1}^{m} I(gamma_i, C_j) cdot sin(gamma_i) cdot Deltagamma cdot Delta C
]

Where I(γi, Cj) is the intensity at the i-th γ angle and j-th C-plane. The software outputs photometric files in IES LM-63, EULUMDAT (LDT), and CIBSE TM-14 formats, enabling direct import into lighting design platforms such as DIALux, Relux, and AGi32.

Photometric Characterization and Calibration Standards for the LSG-1890B System

The LSG-1890B is a compact goniophotometer designed for laboratories with space constraints, featuring a 1.8 m optical arm. Its photometric sensor achieves f1’ ≤ 4 % with a linearity deviation below 0.2 % across a dynamic range of 0.01 lx to 100,000 lx. The system is calibrated using a secondary standard lamp calibrated to CIE L-011 (secondary reference lamp classification). Calibration coefficients are stored per angular position via a multifactor polynomial correction algorithm that compensates for stray light and cosine error.

The LSG-1890B supports both absolute and relative photometry. In absolute mode, the luminous flux measurement uncertainty is ±3.5 % (k=2) as verified by interlaboratory comparison programs. The instrument includes an environmental monitoring module that records ambient temperature and humidity during testing, ensuring data traceability to ISO/IEC 17025 conditions. Integration with a goniophoto-colorimeter option enables simultaneous acquisition of chromaticity coordinates (CIE 1931 x, y) at each angular position, essential for characterizing color spatial uniformity in RGB and tunable-white LED products.

Parameter	LSG-6000	LSG-1890B
Measurement Distance	3.0 m	1.8 m
Intensity Range	up to 200,000 cd	up to 50,000 cd
Angular Resolution	0.1°	0.2°
f1’ (Vλ)	≤ 3 %	≤ 4 %
Flux Uncertainty (k=2)	±3.0 %	±3.5 %
Output Formats	IES, LDT, TM-14	IES, LDT, TM-14
Test Stand Capacity	50 kg	25 kg

Compliance with International Lighting Metrology Standards: IEC, IES, EN, and JIS Frameworks

Goniophotometric testing must adhere to standardized measurement protocols to ensure comparability across jurisdictions and manufacturers. The LISUN LSG series instruments are designed to satisfy the following international standards:

• IES LM-79-19 (Illuminating Engineering Society): Specifies electrical and photometric measurements of solid-state lighting products. The LSG-6000 enables Type C photometry with constant-current mode conforming to Section 6.2, requiring measurement after thermal stabilization (ΔT < 0.5°C over 15 minutes).
• CIE S 025/E:2015: Defines test methods for LED lamps and luminaires. Both LSG models support the specified measurement distance ratios (d > 15 × luminaire dimension) and angular step increments of 1° or smaller for general lighting.
• EN 13032-1:2004+A1:2012 (European standard): Requires luminous flux measurement with goniophotometers of at least Class I accuracy. The LSG-6000’s photometric detector classification meets Class A (f1’ ≤ 3 %) as defined in DIN 5032-6.
• JIS C 8105-5:2017 (Japanese Industrial Standard): Applicable to LED luminaires for general lighting, the LSG-1890B’s vertical axis stability of ±0.05° and angular encoder resolution of 0.01° align with the standard’s permissible error limits.
• LM-80-15 / TM-21-19 (for LED lumen maintenance): While goniophotometry is not directly used, the initial luminous flux data from the LSG-6000 provides the baseline for accelerated life testing (ISTMT) in thermal chambers.

These standards mandate measurement of zonal lumens for energy labeling (e.g., EU Regulation 2019/2020 for EcoDesign), where the LSG systems output tabulated data per 10° or 5° zones. The software automatically calculates Luminaire Efficacy Ratings (LER) and Unified Glare Rating (UGR) in compliance with CIE 117 and CIE 190.

Applications in LED and OLED Manufacturing: Lumen Output Validation and Bin Stability

In LED production environments, high-speed batch characterization requires goniophotometers with minimal measurement cycle times. The LSG-6000’s dual-axis servo motors achieve angular velocities up to 12°/s, completing a full 4π scan (1° step) in under 25 minutes. For OLED panels, which demand extremely low levels of measurement error due to Lambertian emission profiles, the system’s low noise floor (0.001 cd/m²) ensures accurate assessment of brightness uniformity.

Critical quality control parameters obtained include:

• Total Luminous Flux: Verified against reference LED modules (lamp type L65, calibrated by NMI).
• Beam Angle: Defined as the full width at half maximum (FWHM) intensity, measured with 0.2° repeatability.
• Color Over Angle: Chromaticity deviation Δu’v’ across central and peripheral angles. For display backlight units, Δu’v’ < 0.004 is required per VESA Flat Panel Display Measurements Standard (FPDM) v2.0. The LSG-6000 with optional colorimeter achieves Δu’v’ uncertainty of ±0.001.
• Spatial Color Uniformity: Measured at 5° increments; deviations beyond 0.006 u’v’ (SDCM) trigger bin reassignment.

Use in Display Equipment Testing: Luminance Distribution and Uniformity Assessment

Flat panel displays—including LCD, OLED, and microLED—require photometric characterization across large viewing angles for compliance with ISO 13406-2 and VESA FPDM. Goniophotometers evaluate luminance and contrast ratio as functions of polar angle and azimuth. The LSG-1890B, when configured with a reduced measurement distance of 0.5 m (using a telescoping fixture), can evaluate panels up to 55 inches.

Measurement data supports calculation of:

• Viewing Angle (χ): Defined as the polar angle where luminance drops to 50 % of normal.
• Contrast Ratio (CR): Luminance of white state divided by black state at each angle.
• Gamma Curve Deviation: Determined by measuring gray-to-gray transition luminance across 256 levels.

The LSG software identifies angular zones with luminance non-uniformity exceeding 20 %, enabling corrective action during display binning. Automotive display compliance with SAE J1757-1 (luminance uniformity for driver assistance systems) relies on angular data acquired at 1° intervals.

Application in Photovoltaic Industry: Angular Response and Solar Simulator Validation

Although PV characterization primarily uses spectroradiometry, goniophotometers serve to measure the angular response of concentrator photovoltaic (CPV) modules and diffuse reflector materials. The LSG-6000’s high dynamic range sensor measures reflected intensity from test surfaces (e.g., white reference tiles) under controlled illumination from a collimated solar simulator (Class AAA per IEC 60904-9). The angular response function f(θ) is defined as:

[
f(theta) = frac{I(theta)}{I(0^circ) costheta}
]

Deviations from ideal Lambertian behavior are quantified as the Angular Uniformity Index (AUI), critical for connecting IEC 61853-1 irradiance matrix testing. The LSG series supports continuous scanning from 0° to 85° for characterizing bifacial modules per IEC TS 60904-1-2.

Role in Stage, Studio, and Urban Lighting Design: Asymmetric Distribution and Glare Analysis

Illumination for theatrical and architectural applications demands luminaires with customized beam profiles—e.g., ellipsoidal reflector spots, PAR cans, or asymmetric wall-washers. Goniophotometric data from the LSG-6000 enables precise calculation of illuminance vectors and adaptivity metrics defined in CIE 194 (outdoor lighting). For stage lighting, the software generates intensity distribution curves (IDC) in polar plots and calculates field angle ratios.

Urban lighting designers utilize the LM-63 file outputs from the LSG to simulate pole spacing, glare ratings (GR per CIE 112), and uniformity parameters (U0, Ul) for road lighting compliance with EN 13201-2. The system’s 3D coordinate data supports direct import into lighting calculation engines such as ReluxPro for tunnel lighting (L20 luminance) and sports field illuminance (Eh > 500 lx).

Medical Lighting and Sensor Applications: Photobiological Safety and Spectral Irradiance

Medical lighting equipment—LED surgical lights, UV curing systems, and dental curing lamps—must meet IEC 60601-2-41 (surgical luminaires) and IEC 62471 (photobiological safety). The LSG goniophotometers, when equipped with a spectroradiometer attachment, measure spectral irradiance Eλ(θ, φ) at each angular position. The system calculates weighted exposure limits (peak emission at 420 nm for blue light hazard) according to ICNIRP guidelines and IEC 62471:2006.

Optical sensors and photodiodes require certified cosine response curves. The LSG-1890B’s detector characterization facility measures relative angular responsivity and computes the cosine error correction factor (CECF) as per CIE 053. For UV intensity mapping in safety cabinets, angular resolution of 0.5° ensures spatial homogeneity reporting better than ±5 %.

Competitive Advantages of the LISUN LSG-Series Compared to Rival Metrology Systems

Relative to competing goniophotometers, the LSG-6000 and LSG-1890B offer distinct technical advantages:

1. Mirror Design versus Direct Detection: Many goniophotometers rotate the luminaire, causing gravitational redistribution of solder joints and thermal imbalance. The LSG mirror-based design eliminates this artifact, improving reproducibility for LED arrays by 0.8 % (standard deviation over 20 measurements).
2. Integrated Power Measurement: Inclusion of a class 0.2 power analyzer eliminates external cabling and synchronization errors, critical for IEC 62301 standby power measurement.
3. Self-Calibration Check: Built-in reference lamp allows daily verification of photometric scale without need for external calibration service, reducing downtime.
4. Multi-Format Export: Direct XML support for CIE 102 (universal interchange format) and EMC data formats enables seamless integration into laboratory information management systems (LIMS).
5. Cost-Effectiveness: Initial capital cost is approximately 40 % lower than comparable systems from competitors (e.g., Instrument Systems LZT or TechnoTeam LMK), while maintaining measurement uncertainty within the EN 13032 requirement envelope.

Conclusion

The LISUN LSG-6000 and LSG-1890B goniophotometer systems represent precision metrological instruments that fully satisfy the measurement requirements of international photometric standards across industries from semiconductor lighting to medical devices. The scientific basis of goniophotometric measurement, combined with the robust design characteristics of these instruments—including mirror-type geometry, high-resolution angular encoding, calibrated photometric detection, and multi-format software output—provides manufacturers, test laboratories, and R&D facilities with reliable tools for photometric characterization. The capability to produce IES-compliant data for modeling, validation, and certification ensures that these systems contribute directly to regulatory compliance and product quality optimization.

---

Frequently Asked Questions (FAQ)

Q1: What is the typical calibration interval recommended for the LISUN LSG-6000 goniophotometer?
A1: Annual recalibration is recommended to maintain measurement traceability. The built-in self-check feature using an internal reference lamp allows monthly stability verification in accordance with ISO 17025 practices.

Q2: Can the LSG-1890B measure luminaires with asymmetric light output, such as streetlights or floodlights?
A2: Yes. The Type C rotation mechanism enables scanning of arbitrary C-planes between 0° and 360°, while the photometer head rotates through γ from -90° to +90°, accommodating highly asymmetric distributions for outdoor and architectural luminaires.

Q3: How does the LSG-6000 ensure compliance with the IES LM-79-19 requirement for electrical stabilization prior to photometric measurement?
A3: The integrated programmable power supply monitors voltage and current in real-time. The software delays data acquisition until the output power variation is less than 0.2 % over a 15-minute period, meeting LM-79 stabilization criteria.

Q4: What is the maximum luminaire weight and physical dimension that the LSG-6000 test stand can accommodate?
A4: The test stand can support luminaires up to 50 kg in weight with a maximum diameter of 0.8 m. For larger profiles, external fixtures can be attached to the rotating arm via the M6 threaded base plate.

Q5: Is it possible to obtain spectroradiometric data (e.g., color temperature, CRI) simultaneously with photometric scans using the LSG-1890B?
A5: Yes. An optional fiber-optic input and spectrometer module can be integrated, enabling concurrent capture of spectral data at each angular coordinate. The software calculates CCT, CRI (Ra), TM-30 Rf and Rg, and chromaticity coordinates at each measured position.