The Definitive Guide to IES File Testing: Standards, Procedures, and Contract Laboratory Services

Introduction: The Foundation of Modern Lighting Design

In the complex ecosystem of modern illumination, a lighting fixture is only as valuable as the data that defines its performance. This data is universally standardized and transferred via the IES file format. An IES file, derived from the Illuminating Engineering Society (IES) of North America, is not merely a static document; it is the comprehensive, electronic fingerprint of a luminaire’s light distribution characteristics. For lighting manufacturers, specifiers, engineers, and, critically, the contract laboratories that validate their claims, a deep, technical understanding of IES file generation and testing is paramount.

The stakes are high. Accurate IES data is essential for energy compliance (Energy Star, DLC), design modeling (AGI32, DIALux), and project feasibility. Errors in testing or file generation can lead to costly redesigns, project failure, and, worst of all, non-compliance with regional and national lighting regulations. This comprehensive guide serves as a technical breakdown of IES file testing, the governing standards, the intricate testing procedures, and the indispensable role of accredited contract laboratories.

Part I: Decoding the IES Standard (LM-63)

At its core, the IES file format is defined by the ANSI/IES LM-63 standard, titled “Approved Method: IES Standard File Format for the Electronic Transfer of Photometric Data and Related Information.” This standard ensures that photometric data collected anywhere in the world can be accurately and consistently interpreted by simulation and design software. The file itself is a plain text (ASCII) document structured into three distinct sections:

1. The Header and Keyword Section

The file always begins with a version identifier (e.g., IESNA:LM-63-2002 or IESNA:LM-63-2019). This is followed by a series of mandatory and optional keyword fields enclosed in brackets, which serve as metadata. These fields provide crucial context about the product and the testing environment, including:

• $$TEST$$: The unique test report number, linking the IES file directly back to the full photometric report.
• $$MANUFAC$$ and $$LUMCAT$$: Manufacturer and Luminaire Catalog Number, critical for product identification.
• $$ISSUEDATE$$: The date the test was officially completed.
• $$TESTLAB$$: The name and location of the accredited testing facility.
• $$INPUTWATTAGE$$: The measured total input power (in Watts) of the device under test (DUT), a figure vital for efficacy calculation.

Any omission or mismatch in these keywords, especially concerning input wattage or test dates required by certification bodies like the DesignLights Consortium (DLC), can result in immediate data rejection.

2. The Scaling and Configuration Data

Immediately following the keyword section, a fixed block of numeric data defines the structure of the photometric distribution data to follow. This includes:

The configuration data sets the stage for the raw intensity distribution data. An incorrect photometric type or a non-unity multiplier is a critical flaw that a reputable contract laboratory must flag and correct.

3. The Photometric Data Array

The final, and most data-intensive, section comprises the measured luminous intensity (candela) values.

• Vertical Angles List: A sequential list of vertical angles (e.g., $0.0^\circ, 2.5^\circ, 5.0^\circ, \dots, 90.0^\circ$) at which measurements were taken. For a typical Type C test, this range runs from nadir ($0^\circ$) to zenith ($180^\circ$), or $0^\circ$ to $90^\circ$ for semi-cutoff fixtures.
• Horizontal Angles (C-Planes) List: A list of horizontal angles (e.g., $0^\circ, 45^\circ, 90^\circ, \dots, 360^\circ$) that represent the rotational slices of the fixture. The final angle determines the rotational symmetry (e.g., $0^\circ$ for axially symmetric, $90^\circ$ for quadrant symmetry).
• Candela Values: The raw luminous intensity values, listed sequentially. The data is organized in groups, with all candela values for the first horizontal angle listed first, followed by the next horizontal angle, and so on.

This dense array of numerical data, often hundreds or thousands of values long, is the actual light distribution profile used by lighting software to render the light’s shape and intensity in a 3D space.

Part II: The Mandate of Absolute Photometry (LM-79)

The rise of Solid-State Lighting (SSL), particularly LEDs, introduced a fundamental shift in how luminaires must be tested. Unlike traditional lamps, where the light source (the bulb) could be measured separately from the fixture (the luminaire), the LED chip is inextricably linked to the heat sink, housing, and driver electronics—all of which affect its performance. This necessitated the creation of ANSI/IES LM-79, the “Approved Method for the Electrical and Photometric Measurements of Solid-State Lighting Products.”

The Absolute Testing Requirement

LM-79 mandates absolute photometry. In pre-LED relative photometry, a test was performed on the bare lamp, and a separate test was performed on the fixture with a reference lamp. The final result was derived by calculation. Absolute photometry, however, requires the testing of the entire, complete luminaire—driver, heat sink, optics, and light source—as a single unit. This ensures that thermal effects and integrated optics are inherently included in the final IES file data.

Key Photometric and Electrical Metrics Measured under LM-79

A contract laboratory executing an LM-79 test is responsible for capturing several interdependent metrics, all of which are reflected in the final IES report:

1. Total Luminous Flux ($\Phi_v$): The total light output of the luminaire, measured in lumens (lm), typically using a large integrating sphere.
2. Luminous Efficacy ($\eta$): Calculated as Luminous Flux divided by Input Power ($\text{lm/W}$). This is a critical metric for energy efficiency and compliance programs.
3. Luminous Intensity Distribution ($\mathbf{I_v}$): The central data set (the IES file), measured in candela (cd) across the photometric web using a goniophotometer.
4. Chromaticity: Includes Correlated Color Temperature (CCT), Color Rendering Index (CRI), and Chromaticity Coordinates ($x, y, u’, v’$). These are essential for evaluating light quality.
5. Electrical Characteristics: Measurements of input voltage, input current, power consumption (Watts), and Power Factor (PF).

Crucial Testing Procedures: Stabilization and Thermal Control

The accuracy of an LM-79 test hinges on strict environmental control. The IES standard dictates the following:

• Ambient Conditions: The test environment (often a large goniophotometer chamber) must be maintained at an ambient temperature of $25^\circ \text{C} \pm 1^\circ \text{C}$.
• Stabilization: The Device Under Test (DUT) must be continuously powered until both the electrical input and the light output have fully stabilized. For SSL products, this thermal stabilization can take hours. A quality contract laboratory monitors the lumen output using an integrating sphere or light sensor until the drift is less than $0.2\%$ over 30 minutes.

This rigorous stabilization process is vital because LED light output is temperature-dependent; failure to stabilize results in artificially inflated or inaccurate photometric data.

Part III: The Goniophotometer and Data Integrity

The heart of IES file generation is the goniophotometer. This precision instrument measures luminous intensity from the DUT at every single defined vertical and horizontal angle, mapping the fixture’s 3D light distribution pattern.

Type C Goniophotometry

The Type C configuration is the prevailing standard for general and architectural lighting. In this system:

• Vertical Angles ($\gamma$): Measured from the center of the luminaire, starting at the nadir ($0^\circ$) and moving upwards.
• Horizontal Angles (C-Planes): These planes rotate around the vertical axis of the luminaire, typically starting at $0^\circ$ (the parallel axis of the fixture) and proceeding through $90^\circ$ (the perpendicular axis).

The goniophotometer takes intensity measurements at the cross-section of these C-planes and vertical angles (the photometric web), producing the thousands of candela values that populate the final IES data array.

Data Integrity and Contract Laboratory QA

For lighting manufacturers relying on contract laboratories, the integrity of the IES data is critical to their product launch and certification. A specialized lab provides indispensable Quality Assurance (QA) checks that go beyond simply running the test:

1. Symmetry Validation: The lab verifies the expected symmetry based on the fixture design. A round high-bay fixture is expected to be axially symmetric (the candela values should be nearly identical across all C-planes). If the measured data deviates significantly, it indicates either a measurement error or an optical/manufacturing defect in the DUT.
2. BUG Rating Analysis: For outdoor and roadway lighting (per IES TM-15), the IES file is used to calculate the Backlight, Uplight, and Glare (BUG) rating. The lab must ensure the intensity values in the upper hemisphere and back zones of the photometric web are accurately captured, as exceeding limits (e.g., U4 or G2 ratings) can render the product non-compliant for many jurisdictions.
3. IES File Structure Compliance: Before delivery, the lab uses specialized photometric software (like Photometric Toolbox) to validate the LM-63 structure—checking for character limits, correct data formatting, proper use of the TILT keyword, and ensuring the vertical and horizontal angle counts match the candela array size.

Advanced Analysis: TM-30 Color Reporting

While IES LM-63 primarily deals with intensity, modern lighting quality assessment, driven by IES TM-30, focuses on color rendition fidelity ($R_f$) and gamut ($R_g$). An advanced contract laboratory will integrate TM-30 analysis into the LM-79 report package, using the spectroradiometric data collected during the sphere test to provide a comprehensive color quality assessment that supplements the raw IES file.

Part IV: The Contract Laboratory’s Compliance Role

In the highly regulated lighting industry, the use of an accredited contract laboratory is not just a preference; it is a necessity for market access and risk mitigation.

Accreditation and Traceability

The cornerstone of a reputable contract laboratory is ISO/IEC 17025 accreditation. This international standard specifically outlines the requirements for the competence of testing and calibration laboratories. Accreditation ensures that the lab’s equipment is properly calibrated, measurement uncertainty is calculated and minimized, and the staff are technically proficient.

Compliance programs, such as the DesignLights Consortium (DLC) and the U.S. Environmental Protection Agency’s ENERGY STAR, mandate that photometric testing (LM-79) and LED component lumen maintenance testing (LM-80) be conducted by NVLAP (National Voluntary Laboratory Accreditation Program) or A2LA (American Association for Laboratory Accreditation) accredited facilities.

Mitigating Risk for Manufacturers

By outsourcing IES file testing to an accredited contract laboratory, manufacturers gain three primary advantages:

1. Objectivity and Credibility: Third-party validation removes any perception of bias, lending instant credibility to the IES data file. This is essential when presenting data to specifiers or regulatory bodies.
2. Reduced Time-to-Market: Maintaining an in-house goniophotometer and integrating sphere, along with the staff expertise required for continuous calibration and standard adherence, is a tremendous operational overhead. Specialized labs provide rapid turnaround times for testing and reporting.
3. Guaranteed Compliance: A contract laboratory acts as a regulatory gatekeeper, ensuring that the photometric data output is not only technically accurate but also structurally compliant with the precise requirements of certification programs like DLC V5.1, which demands specific keyword fields and multiplier rules.

The laboratory’s final report is the legally binding document that validates the IES file. This report includes a full data summary (efficacy, CCT, CRI, power factor), a detailed description of the testing equipment used, and, crucially, a reference to the specific LM-79 and LM-63 standards followed, establishing the audit trail necessary for global commerce.

Conclusion: Data as Currency

The IES file is the primary currency of the lighting industry. It translates physical performance into actionable digital data, enabling engineers to design highly efficient, compliant, and comfortable lighting environments. For lighting product providers, the choice of a contract laboratory for IES file testing dictates the success of their product.

The process of generating this file is a precise, multi-step technical exercise, governed by the LM-63 file structure and the rigorous absolute photometry principles of LM-79. Manufacturers must look beyond simple price points and partner with ISO 17025-accredited contract laboratories that offer not just the measurement, but the comprehensive data validation, compliance checking, and technical consultation required to turn a physical luminaire into a certifiable, digitally-marketable product. In an industry where light performance is measured to the thousandth of a candela, data integrity is the ultimate non-negotiable asset.

If your organization requires certified IES LM-63 and LM-79 testing for luminaire compliance, submit your testing request today and connect with our network of accredited photometric laboratories.