In this guide
- The Critical Role of Photometric Data
- Photometry Methods: Goniophotometry vs. Integrating Sphere
- IES File Format Deep Dive: LM-63-02 Standard
- Reading an IES File: Candela Distribution & Beam Angles
- Type A, B, and C Photometry Explained
- Key Photometric Calculations: LOR, DLOR, ULOR, Efficacy
- Using IES Files in DIALux, Relux, and AGi32
- Reviewing Manufacturer IES Data: What to Check
- Common Mistakes and Misrepresentations
The Critical Role of Photometric Data
Photometric data is the quantitative foundation of architectural lighting design. Without accurate photometric data, lighting calculations are meaningless — a beautiful DIALux rendering is only as reliable as the IES files that feed it. For senior lighting designers and specification professionals, the ability to critically evaluate photometric data and IES files is not merely useful; it is essential to professional credibility.
Every lighting fixture emits light in a specific pattern — a unique distribution of luminous intensity across all angles. This distribution determines whether a downlight produces a tight spot or a wide flood, whether a wall washer grazes evenly across a surface, and whether a facade projector delivers the specified vertical illuminance at the target distance. Photometry is the science of measuring this distribution, and the IES file is the standardized digital format for recording and sharing it.
In the architectural lighting industry, where manufacturers routinely claim "120 lm/W efficacy" and "90+ CRI," photometric data is where marketing meets measurement. A thorough understanding of how photometric testing works, what IES files contain, and where the numbers can be misleading is what separates a specifier who trusts their calculations from one who crosses their fingers and hopes for the best.
Key point: The IES file is not a marketing document — it is a technical specification. When you use an IES file in a lighting calculation, you are implicitly certifying that you accept the data as accurate. The professional responsibility for the result rests with the designer, not the manufacturer who provided the file.
Photometry Methods: Goniophotometry vs. Integrating Sphere
Two primary methods dominate photometric testing of lighting fixtures: goniophotometry and integrating sphere photometry. Each provides different information, and understanding the distinction is fundamental.
Goniophotometry
Goniophotometry measures the luminous intensity distribution of a light source at many different angles, producing the detailed angular data that forms an IES file. The fixture is mounted on a goniometer — a precision mechanical device that rotates the fixture through a range of angles while a photometer detector measures the intensity at each position.
A modern goniophotometer can measure intensity at hundreds or thousands of angles across the full hemispherical (or spherical) distribution. The result is a candela distribution table — the core data in every IES file. Total luminous flux can also be derived from goniophotometric data by integrating the intensity values across all angles, though this is generally less accurate than integrating sphere measurements.
| Parameter | Goniophotometry | Integrating Sphere |
|---|---|---|
| Primary measurement | Luminous intensity distribution (candela values at angles) | Total luminous flux (lumens) |
| Angular data | Yes — detailed distribution | No — total flux only |
| Accuracy for total flux | Good (~3-5%) | Excellent (~1-2%) |
| Test time | 30 minutes to several hours | Seconds to minutes |
| Fixture size limit | Large fixtures possible (up to 2m+) | Limited by sphere diameter |
| Standard reference | IES LM-79, CIE 121 | IES LM-79, CIE 84 |
| Output | IES file (candela distribution) | Total lumens, efficacy |
Integrating Sphere Photometry
An integrating sphere is a hollow sphere coated internally with a highly reflective, diffuse white material (typically PTFE or barium sulfate). The light source is placed inside or at an opening, and the light bounces around the sphere interior until it is completely integrated — meaning every surface of the sphere interior receives the same illuminance regardless of the original beam direction.
A small detector mounted in the sphere wall measures the integrated illuminance, which is proportional to the total luminous flux output of the source. Integrating spheres provide the most accurate total flux measurements but tell you nothing about how the light is distributed. For architectural fixtures, the sphere measurement provides the lumen output and efficacy figures, while the goniophotometer provides the distribution pattern for lighting calculations.
The LM-79 Standard
IES LM-79-08 (Approved Method: Electrical and Photometric Measurements of Solid-State Lighting Products) is the industry standard for LED photometric testing. It specifies procedures for both goniophotometric and integrating sphere measurements of LED products, including requirements for:
- Thermal stabilization (minimum 30 minutes at operating temperature)
- Ambient temperature control (25°C ± 1°C)
- Electrical measurement accuracy (voltage, current, power)
- Spectral measurement for color characteristics (CCT, CRI)
- Uncertainty budgets and reporting requirements
Caveat emptor: Many manufacturers publish photometric data that was not tested to LM-79 standards. Some use computed/simulated data from optical modeling software rather than physical measurement. Always check whether an IES file is based on actual goniophotometer measurements per LM-79, and be skeptical of data that cannot be traced to a certified test lab.
IES File Format Deep Dive: LM-63-02 Standard
The IES file format is defined by the Illuminating Engineering Society in the standard IES LM-63-02 (Approved Standard: IESNA Standard File Format for Electronic Transfer of Photometric Data and Related Information). It is an ASCII text format that contains both metadata about the luminaire and the actual candela distribution data.
IES File Structure
An IES LM-63-02 file follows a specific structure with multiple sections:
Key Header Fields
The header section contains critical information that defines how to interpret the data:
- LUMEN: The rated lumen output of the lamp(s) used during testing. For LED fixtures, this is typically 1000 (relative photometry) or the actual measured lumens (absolute photometry).
- MULT: A multiplier applied to all candela values in the data table. The actual candela value at any angle = candela_table_value × MULT.
- NUM_V / NUM_H: The number of vertical and horizontal angles in the data set. More angles = higher resolution data.
- PHOTOMETRIC_TYPE: Type A, B, or C — defines the coordinate system used.
- UNITS: Feet or meters — specifies the unit for luminaire dimensions.
- DIMENSIONS: Luminaire width, length, and height — used for calculating luminous flux output and for visual representation in design software.
Reading an IES File: Candela Distribution & Beam Angles
The heart of an IES file is the candela distribution table — a matrix of luminous intensity values measured at specific vertical and horizontal angles. Understanding how to extract meaningful information from this data is a core skill.
Candela Distribution Data
In a typical Type C IES file (the most common for architectural fixtures), vertical angles range from 0° (nadir — directly below the fixture) to 90° (horizontal) and sometimes to 180° (directly above). Horizontal angles range from 0° to 360°. The candela table provides an intensity value for every combination of vertical and horizontal angles.
For a rotationally symmetric fixture (most downlights, for example), the candela values are the same at all horizontal angles — you only need to look at one horizontal plane. For asymmetric fixtures (wall washers, floodlights), the distribution changes with horizontal angle, and you need to examine multiple planes.
Beam Angle vs. Field Angle
Two of the most commonly quoted (and most commonly confused) photometric parameters are beam angle and field angle:
- Beam angle (NEMA definition): The angle between the two directions in which the luminous intensity is 50% of the maximum intensity (the "half-maximum" points). This defines the central, useful beam of light.
- Field angle (NEMA definition): The angle between the two directions in which the luminous intensity is 10% of the maximum intensity. This defines the total spread of light, including the low-intensity spill at the edges.
| Beam Classification | Beam Angle Range | Typical Application |
|---|---|---|
| Narrow Spot (NSP) | 0° – 10° | Accent lighting, sculpture, high-ceiling feature |
| Spot (SP) | 10° – 20° | Retail accent, art highlighting |
| Narrow Flood (NFL) | 20° – 30° | General retail, display lighting |
| Flood (FL) | 30° – 60° | General ambient, wall washing |
| Wide Flood (WFL) | 60° – 90° | General illumination, cove lighting |
| Very Wide Flood (VWFL) | 90°+ | Diffuse ambient, indirect lighting |
Watch out for beam angle inflation: Some manufacturers quote "beam angle" at the 10% intensity level (the field angle) rather than the standard 50% level. A fixture advertised as "60° beam" might actually be a 35° beam by the standard NEMA definition. Always check what intensity threshold the beam angle is quoted at. The IES standard uses 50% for beam angle and 10% for field angle.
Center Beam Candlepower (CBCP)
The maximum candela value in the distribution — usually at 0° vertical angle for downlights — is the center beam candlepower (CBCP). CBCP is the peak intensity and directly relates to how "punchy" the light appears. For a given lumen output, a narrower beam will have a higher CBCP, and a wider beam will have a lower CBCP.
Type A, B, and C Photometry Explained
The IES standard defines three types of photometry — Type A, Type B, and Type C — each using a different coordinate system for measuring the light distribution. The type is chosen based on the geometry and intended use of the luminaire.
Type C Photometry
Type C photometry is by far the most common type for architectural lighting fixtures, including downlights, track heads, wall washers, and most interior fixtures. In Type C, the photometric axis (0° vertical angle) is perpendicular to the main mounting surface — pointing downward for a ceiling-mounted fixture.
Vertical angles are measured from the nadir (0°) upward to 90° (horizontal) and beyond to 180° (zenith). Horizontal angles rotate around the vertical axis. Type C is essentially a "pole" coordinate system centered on the light-emitting direction.
Type B Photometry
Type B photometry is used for roadway and area lighting, as well as some sports lighting fixtures. In Type B, the 0° vertical angle points toward the side of the fixture (along the roadway), and horizontal angles sweep across the road width. This coordinate system makes it easier to analyze the light distribution across the roadway and its surroundings.
Type A Photometry
Type A photometry is used for automotive lighting, signal lights, and some specialized industrial fixtures. In Type A, the angles are measured relative to the optical axis of the fixture itself, which may be aimed in any direction. The coordinate system rotates with the fixture aiming.
| Photometry Type | Coordinate System | Typical Applications |
|---|---|---|
| Type C | Nadir-based, vertical angles from downward axis | Downlights, track lights, wall washers, interior architectural |
| Type B | Side-based, angles along and across fixture | Roadway lighting, area/site lighting, parking lot |
| Type A | Optical-axis based, angles relative to beam axis | Automotive headlights, signal lights, projectors |
Designer tip: When you import an IES file into DIALux or Relux, the software automatically knows which photometric type it is and orients the fixture correctly. However, you should still verify the photometric type when reviewing IES data — sometimes a manufacturer's custom fixture might be measured in an unexpected type, which could cause orientation confusion in your model.
Key Photometric Calculations: LOR, DLOR, ULOR, Efficacy
Beyond the raw candela data, several derived metrics are essential for evaluating fixture performance and comparing products.
Light Output Ratio (LOR)
The Light Output Ratio is the percentage of lamp lumens that actually exit the fixture as useful light. It represents the overall efficiency of the optical system — the reflector, lens, diffuser, and housing.
Downward Light Output Ratio (DLOR) and Upward Light Output Ratio (ULOR)
DLOR and ULOR describe the proportion of light emitted downward vs. upward from a fixture. These ratios are especially important for indirect/direct lighting systems and for calculating light pollution (uplight) in exterior applications.
- DLOR: The percentage of total lamp lumens emitted in the downward hemisphere (0-90° vertical angle in Type C photometry)
- ULOR: The percentage of total lamp lumens emitted in the upward hemisphere (90-180° vertical angle)
DLOR + ULOR = LOR (approximately — some light is absorbed within the fixture). For a typical recessed downlight, DLOR is 70-90% and ULOR is 0-5%. For a pendant with both direct and indirect output, DLOR might be 50% and ULOR 40%, giving an LOR of 90%.
Luminous Efficacy
Efficacy is the ratio of total light output to power input, measured in lumens per watt (lm/W). It is the primary metric for energy efficiency.
Efficacy inflation is rampant: Many manufacturers quote "LED efficacy" (the chip efficacy) rather than "luminaire efficacy" (the actual fixture output per watt). A fixture advertised as "200 lm/W" might only deliver 120 lm/W at the fixture level. Always check whether the quoted efficacy is at the LED level or the fixture level. LM-79 testing reports fixture-level efficacy, which is the number that matters for energy calculations.
Using IES Files in DIALux, Relux, and AGi32
All major lighting design software packages — DIALux evo, ReluxDesktop, AGi32, and others — use IES files as the source of photometric data for calculations. Understanding how the software uses the data helps you make better decisions about which files to use and how to verify the results.
How Calculation Software Uses IES Data
When you place a fixture in a lighting model, the software reads the IES file's candela distribution and uses it to calculate illuminance on all calculation surfaces. The core calculation is the inverse square law applied at each calculation point using the appropriate candela value from the distribution, adjusted for angle.
DIALux evo Workflow
In DIALux evo, IES files are imported into the luminaire library and can then be placed in the 3D model. The software supports both IES LM-63-02 and EULUMDAT (the European equivalent format). Key considerations:
- Relative vs. absolute: DIALux handles both relative and absolute photometry automatically. For relative photometry, you specify the actual lamp lumens; for absolute photometry, it uses the values from the file directly.
- Color: IES files do not contain color information — CCT and CRI must be entered separately if you're doing color rendering calculations.
- Luminous dimensions: The fixture dimensions from the IES file are used for visual representation and can affect the accuracy of close-proximity calculations.
- Aiming: IES fixtures can be aimed at any angle in 3D space, and the software correctly rotates the distribution accordingly.
Data Resolution and Calculation Accuracy
The angular resolution of the IES file directly affects calculation accuracy. A file with only 5° vertical angle steps will give less precise results than one with 1° or 2° steps, especially for narrow-beam fixtures where the intensity changes rapidly with angle. Most quality IES files use 2.5° or 5° increments for vertical angles and 15° or 30° for horizontal angles.
For critical calculations (e.g., museum lighting with narrow spot beams, or sports lighting with tight uniformity requirements), higher-resolution IES data is worth seeking out. Some manufacturers provide high-resolution files on request even if their standard downloads use coarser increments.
Reviewing Manufacturer IES Data: What to Check
When you receive an IES file from a manufacturer, don't just import it into your model and trust the results. A competent specifier reviews the data for reasonableness, consistency, and potential issues before using it.
Pre-Import Checklist
- Is the data LM-79 compliant? Look for LM-79 testing reference in the header or in accompanying documentation. If it's not mentioned, ask.
- Is it absolute or relative photometry? For LED fixtures, absolute photometry is the standard. Relative photometry should raise questions.
- What is the test date? LED performance changes over time as the technology improves. An IES file from 2019 may not reflect current performance.
- Does the catalog number match? Verify that the LUMCAT field in the IES file matches the exact product you're specifying — including optics, finish, and driver options.
- What are the test conditions? Check ambient temperature, drive current, and operating voltage. Some manufacturers test at 25°C with ideal conditions that don't reflect real-world installation.
- Is the color temperature correct? Photometric distribution can vary slightly with CCT, especially for fixtures with phosphor-converted LEDs. Use the IES file for the exact CCT you're specifying.
Sanity Checks After Import
After importing into your calculation software, perform these basic reasonableness checks:
- Does the beam angle match the manufacturer's published data? (Check the polar or isocandela diagram)
- Does the total lumen output match the specification sheet?
- Does the efficacy (lm/W) seem reasonable for the product class?
- Is the distribution shape consistent with the fixture type? (A wall washer should have an asymmetric distribution; a downlight should be rotationally symmetric)
- Are there any anomalies in the candela data — unexpected dips, spikes, or asymmetries that don't make sense?
Professional practice: For high-stakes projects, consider requesting independent third-party photometric testing verification, especially for custom fixtures or large-volume orders. The cost of a third-party test ($500-$2,000 per fixture type) is negligible compared to the cost of rework if the fixtures don't perform as specified.
Common Mistakes and Misrepresentations
After years of reviewing photometric data from dozens of manufacturers, certain patterns of misrepresentation and error recur with depressing regularity. Here are the most important ones to watch for.
1. Simulated vs. Measured Data
Some "IES files" on manufacturer websites are not from actual goniophotometer measurements — they are generated from optical simulation software (e.g., Photopia, LightTools, TracePro). Simulated data can be reasonably accurate for well-understood optical systems, but it often misses real-world effects like LED die placement variations, reflector surface imperfections, and thermal effects. Always prefer measured data.
2. Optimistic Test Conditions
LM-79 specifies testing at 25°C ambient with the fixture operating in free air. In real installations, fixtures operate at higher ambient temperatures, are often partially enclosed, and may have restricted airflow. All of these reduce LED output and lifespan. A fixture tested at 25°C might deliver 10-20% fewer lumens when installed in a 35°C ceiling plenum. Look for additional data at higher ambient temperatures, or apply a realistic derating factor.
3. Lumen Binning Games
LEDs are binned by output — the same LED model might be available in 3000-3300 lm, 3300-3600 lm, and 3600-3900 lm bins. Some manufacturers test fixtures using the highest bin (top of the range) but ship product using the lowest bin. Always specify the minimum lumen output, not the typical or maximum. For critical applications, specify the exact bin or a minimum guaranteed output.
4. "Equal" Beam Angles
Two fixtures with the same advertised beam angle can have wildly different actual distributions. One might have a sharp cutoff with a very defined beam edge, while another has a soft gradient with lots of spill. The beam angle only tells you where the 50% intensity points are — it tells you nothing about the shape of the beam between center and edge. Always look at the full candela distribution or polar curve, not just the beam angle number.
5. Missing or Incomplete IES Data
Some manufacturers provide IES files with coarse angular resolution (10°+ steps), limited horizontal angles (only 0° and 90° for a fixture that should be asymmetric), or missing metadata. These files can still work in calculation software but produce less accurate results. Always request high-resolution IES data with full 360° horizontal coverage for any fixture that matters to your design.
6. Color Shift Not Represented
IES files describe only luminous intensity — they contain no information about color distribution. Many LED fixtures exhibit color shift at off-axis angles: the center of the beam might be 3000K, but at 45° off-axis it could be 3500K or higher. This is especially common with COB LEDs and TIR lenses. For applications where color uniformity is critical (galleries, retail), review the off-axis color data separately — it won't be in the IES file.
Bottom line: Photometric data is the foundation of lighting design, but not all data is created equal. The difference between a great lighting design and a disappointing one often comes down to the quality of the IES files and the diligence with which they were reviewed. Treat IES files as technical specifications, not marketing materials, and verify everything you can before you rely on the numbers.
Photometric testing and IES files are the quantitative backbone of architectural lighting design. A deep understanding of how photometric data is generated, what IES files contain, and where the numbers can be misleading is essential for any lighting professional who wants their designs to perform as intended. For help with photometric review, custom fixture testing, or specifying high-quality architectural fixtures, contact our technical team for a consultation. You can also explore our LED downlight specification guide for more practical specification advice.