How to read a photometric report
A photometric report is a standardized document that records the measured light output, distribution pattern, color characteristics, and electrical performance of a luminaire. Produced through laboratory testing per IES LM-79-19, it contains the data that lighting designers use to calculate whether a fixture will deliver the right amount of light, in the right pattern, to the right surfaces in a given space.
Every commercial lighting specification relies on photometric data. When a lighting designer places fixtures in a layout and generates a foot-candle grid, the calculations behind that grid come from the photometric report. When an inspector checks whether a parking lot meets the required light levels, the verification traces back to photometric testing. When a facility manager evaluates competing proposals, the photometric data is the only objective basis for comparison.
The challenge: photometric reports contain polar plots, candela tables, zonal lumen summaries, and efficiency calculations that can look impenetrable if you haven't been trained to read them. They don't have to be. Each section of the report answers a specific question about what the fixture does, and once you know which questions map to which sections, the report becomes a practical tool rather than an obstacle.
This guide breaks down each component of a photometric report, from the photometric layout you receive from a manufacturer to the underlying LM-79 test data it's based on.
What a photometric report contains
A complete photometric report, whether from an independent testing lab or a manufacturer's in-house facility, follows the measurement framework defined in IES LM-79-19 (Approved Method: Optical and Electrical Measurements of Solid-State Lighting Products). Here is what each section tells you and why it matters.
| Report section | What it tells you | Who uses it |
|---|---|---|
| Test summary / cover sheet | Fixture ID, test date, lab name, ambient temperature (25°C ±1°C per LM-79), orientation during test. Establishes the test conditions. | Specifiers verifying test validity |
| Luminous flux (lumens) | Total light output of the complete luminaire, measured in an integrating sphere or by goniophotometer. This is luminaire lumens, not LED chip lumens. | Everyone: the starting point for any calculation |
| Input power (watts) | Total electrical power consumed by the luminaire including driver. Used to calculate efficacy. | Energy code compliance, operating cost estimates |
| Luminous efficacy (lm/W) | Lumens divided by watts. The fixture's efficiency. A 150 lm/W fixture produces 50% more light per watt than a 100 lm/W fixture. | DLC qualification, utility rebate eligibility, code compliance |
| Candela distribution table | Luminous intensity (candela) at every measured angle, organized by C-plane (horizontal rotation) and gamma angle (vertical tilt). The raw spatial data. | Lighting designers running calculations |
| Candela distribution plot (polar graph) | Visual representation of the candela table. Shows the light distribution shape: narrow, wide, symmetric, asymmetric, forward-throw, etc. | Quick visual assessment by specifiers and reviewers |
| Zonal lumen summary | Lumens emitted in each 10° vertical zone (0–10°, 10–20°, etc.). Shows where the light goes: how much down, how much to the sides, how much up. | Classifying distribution type, identifying backlight/uplight |
| Coefficient of utilization (CU) table | Percentage of luminaire lumens that reach the work plane, indexed by room cavity ratio (RCR) and surface reflectances. Used in the lumen method for layout calculations. | Lighting designers calculating fixture quantity |
| Color data (CCT, CRI, chromaticity) | Correlated Color Temperature (in Kelvin), Color Rendering Index (Ra), and CIE chromaticity coordinates (x, y). Defines the color appearance and quality. | Specifiers matching color requirements; DLC and ENERGY STAR compliance |
| IES electronic file (.ies) | The candela matrix in IES LM-63 format. Imported directly into AGi32, Visual, DIALux, or Relux for point-by-point illuminance calculations. | Lighting designers generating photometric layouts |
LM-79 vs. LM-80: LM-79 measures a fixture's performance at a single point in time. LM-80 measures LED lumen depreciation over thousands of hours. They answer different questions: LM-79 tells you how bright the fixture is today. LM-80 (combined with IES TM-21 projections) estimates how bright it will be in 50,000 hours.
How to read a candela distribution plot
The candela distribution plot is the fingerprint of a luminaire. It shows how light spreads from the fixture in every direction, plotted on a polar graph. The shape of this curve determines whether a fixture throws light wide (good for area lighting), narrow (good for high-bay or spot applications), or asymmetrically (good for wall wash or roadway patterns).
The plot uses a coordinate system defined by IESNA RP-16. The vertical axis represents the nadir (0°, directly below the fixture) at the bottom and the zenith (180°, directly above) at the top. Concentric rings represent candela values. Each curve on the plot represents a vertical slice through the distribution, called a C-plane. For symmetric fixtures, one or two C-planes tell the whole story. For asymmetric distributions (like a roadway fixture), multiple C-planes show how the throw pattern differs in each direction.
The left plot shows a Type V symmetric distribution, typical of round high-bay fixtures or square troffers. Light spreads evenly in all horizontal directions, so the C0 and C90 planes are nearly identical. This pattern is suited to open spaces where the fixture sits centered above the area it serves: warehouse bays, gymnasiums, retail floors.
The right plot shows a Type III asymmetric distribution, common in roadway and parking lot fixtures. The C0 plane (along the road) shows a strong forward throw with peak intensity at roughly 65° from nadir, pushing light outward along the street. The C90 plane (across the road) is much narrower, containing light to the roadway width. This is the pattern used on area and parking lot fixtures.
Watch the scale. Two polar plots can look identical but represent very different fixtures if the candela scales differ. Always read the ring labels. A plot that looks "wide" at a 1,000 cd scale is a very different fixture than one that looks "wide" at a 10,000 cd scale.
Reading foot-candle grids and illuminance maps
The foot-candle grid is the most practical section of any photometric layout. It shows the calculated illuminance at each point on the work plane (typically 30 inches above the floor for offices, floor level for warehouses) after the design software applies the fixture's IES data to the actual space geometry. This is what the inspector checks, what the code references, and what tells you whether the lighting will actually work.
Three numbers in the calculation summary matter most.
Average maintained foot-candles (fc avg) is the mean illuminance across all calculation points after applying light loss factors (lamp lumen depreciation and luminaire dirt depreciation). For a warehouse aisle, the IES Lighting Handbook (10th Edition) recommends 10–30 fc. For detailed manufacturing assembly, 50–100 fc. The average must meet the target for the specific task.
Minimum foot-candles (fc min) identifies the darkest point in the layout. In the diagram above, the lower-right corner reads 9 fc. In a warehouse with a 20 fc target, that corner is a problem. This is where dark spots hide, and where the inspector's light meter will tell you the layout doesn't comply.
Uniformity ratio (max:min) measures how evenly light is distributed. A ratio of 3.6:1 (32 fc max ÷ 9 fc min in the example) may be acceptable in a warehouse but would fail in an office where 3:1 or better is expected. Lower ratios mean more even light.
Five metrics to check in every photometric report
Whether you're a specifier reviewing an LM-79 test report, a contractor evaluating a manufacturer's layout, or a facility manager comparing proposals, these five data points determine whether the photometric data is trustworthy and whether the design will perform.
Luminous efficacy (lm/W): Divide total lumens by input watts. Current commercial LED fixtures range from 100–200 lm/W. The DLC Premium threshold requires efficacy above category-specific minimums (e.g., 135 lm/W for linear ambient fixtures). If the report shows efficacy below 100 lm/W for a new LED product, question the data.
Average maintained illuminance: The average foot-candle value after light loss factors (LLF) are applied. LLF accounts for lumen depreciation over time (LLD, typically 0.7–0.85 for LEDs at L70 rated life) and dirt accumulation on the fixture (LDD, typically 0.85–0.95 for enclosed fixtures in clean environments). If the layout shows initial values without LLF, the installed performance will be 15–30% lower than what's on the page.
Uniformity ratio: Max-to-min is the most commonly cited ratio. For indoor commercial spaces, target 4:1 or better. For parking lots and exterior areas, 10:1 to 15:1 may be acceptable depending on jurisdiction. Average-to-minimum ratios are also used; they're less sensitive to outlier points and typically range from 1.5:1 to 3:1 for commercial interiors.
CRI (Color Rendering Index): Minimum 80 for most commercial applications per DLC requirements and general industry practice. Retail, healthcare, and education spaces often specify CRI ≥ 90. The LM-79 report will list the measured CRI (Ra) and individual R-values. If R9 (deep red rendering) is below 25, skin tones and warm-colored merchandise will appear washed out.
Test lab accreditation: LM-79 reports should come from a laboratory accredited under the National Voluntary Laboratory Accreditation Program (NVLAP) or an equivalent ISO 17025 accredited program. DLC and ENERGY STAR both require NVLAP-accredited test data. A report from a non-accredited lab cannot be used for rebate applications or code-compliance documentation.
IES recommended illuminance levels by space type
When reviewing a photometric layout, the first question is whether the calculated foot-candle values meet the target for the space type. The following table summarizes maintained illuminance recommendations from the IES Lighting Handbook (10th Edition). These are horizontal illuminance values at the work plane unless otherwise noted. Code requirements (ASHRAE 90.1, IECC, local amendments) set maximum lighting power density, but the IES sets the illuminance targets the layout should achieve.
| Space type | Recommended fc (maintained) | Measurement plane | Notes |
|---|---|---|---|
| General office | 30–50 fc | Desk height (30") | Higher end for detailed reading tasks; lower for computer-primary work |
| Open-plan office | 30–50 fc | Desk height (30") | Uniformity ratio ≤ 3:1 to avoid hotspots near windows |
| Conference room | 30–50 fc | Table height (30") | Dimming to 10–15 fc for presentations |
| Warehouse, general storage | 10–30 fc | Floor level | Higher for pick-and-pack operations, lower for bulk storage |
| Warehouse, detailed tasks | 30–50 fc | Task surface | Label reading, quality inspection areas |
| Manufacturing, medium assembly | 50–100 fc | Work surface | Per IES; specific tasks may require higher |
| Retail, general sales floor | 30–50 fc | Floor level + vertical | Accent lighting on displays may be 3–5x ambient |
| Parking lot | 1–5 fc | Pavement | IES RP-20 recommendations; varies by security level |
| Parking garage | 5–10 fc | Pavement | Higher at entries/exits, ramps, and pedestrian areas |
| Classroom | 30–50 fc | Desk height | Board/display wall may need 50–70 fc vertical |
| Healthcare, general corridor | 5–10 fc | Floor level | Night mode as low as 1–2 fc; avoid abrupt transitions |
| Healthcare, exam room | 50–100 fc | Exam table height | Task lighting at exam area often supplemental |
Maintained vs. initial: IES recommendations are for maintained values, meaning after light loss factors (lumen depreciation + dirt depreciation) are applied. If a photometric layout shows only initial foot-candles without LLF, the actual in-service performance will be 15–30% lower. Always ask whether the layout uses initial or maintained values.
Common mistakes when reading photometric reports
Confusing luminaire lumens with LED lumens. Marketing materials sometimes list "LED lumens" (light output from the chip alone), which can be 10–25% higher than the luminaire lumens measured in LM-79 testing. The luminaire lumen value accounts for losses through optics, lenses, and thermal effects. Always use the LM-79 luminaire lumen figure for calculations.
Ignoring mounting height in the layout assumptions. A fixture that produces 30 fc at a 20-foot mounting height will produce roughly 13 fc at 30 feet (illuminance decreases with the square of distance). If the photometric layout uses the wrong mounting height, every foot-candle value on the grid is wrong. Verify the mounting height in the calculation header matches the actual ceiling height.
Comparing reports with different light loss factors. One manufacturer's layout using a 0.95 LLF (clean environment, short maintenance cycle) will show 20% higher foot-candle values than a competitor using 0.78 LLF for the same space. The difference isn't the fixture; it's the assumption. Compare LLF values before comparing foot-candle results.
Evaluating only the average, not the minimum. A layout with 40 fc average and 8 fc minimum has a dark corner. The 40 fc average looks fine on paper, but the 8 fc zone is a safety concern, a code violation, or both. Check the minimum value and its location on the grid.
Using photometric data from a different fixture variant. A fixture available in 4000K and 5000K, or in 100W and 150W configurations, will have different LM-79 reports for each variant. The IES file for the 150W 5000K version cannot be used to represent the 100W 4000K version. Confirm the test report model/wattage/CCT matches the specified product exactly.
Trusting a layout with no calculation grid visible. Some proposals include a floor plan with fixtures placed on it but no point-by-point foot-candle grid, no summary statistics, and no uniformity ratio. That is a fixture layout, not a photometric analysis. Without the calculation data, there is no evidence the light levels will meet the target.
Frequently asked questions
What is the difference between a photometric report and a photometric plan?
A photometric report (LM-79 test report) documents the measured optical and electrical performance of a single luminaire in a laboratory: total lumens, candela distribution, efficacy, CCT, and CRI. A photometric plan (or photometric layout) applies that fixture data to a specific space, showing foot-candle grids, fixture placement, mounting heights, and uniformity ratios. The report describes the fixture. The plan describes the space.
What file format are photometric data files in?
The standard format is IES LM-63 (.ies file), which contains the candela distribution matrix for a luminaire. This file is imported into lighting design software (AGi32, Visual, DIALux, Relux) to calculate illuminance in a space. Some manufacturers also provide .ldt files (EULUMDAT format, common in European projects) or TM-33 XML-based files. The .ies file is the most widely used in North America.
How many foot-candles do I need for a commercial space?
It depends on the task. The IES Lighting Handbook (10th Edition) recommends 30–50 fc for general office work, 50–100 fc for detailed tasks like drafting or manufacturing assembly, 10–30 fc for warehouse aisles, and 30–50 fc for retail sales floors. Exterior applications use lower ranges: 1–5 fc for parking lots, 5–10 fc for parking garages. These are maintained average values at the work plane, not initial.
What is a good uniformity ratio for commercial lighting?
For most indoor commercial spaces, a max-to-min uniformity ratio of 4:1 or lower is acceptable. Office and retail environments often target 3:1 or better. Parking lots and exterior areas may allow 10:1 to 15:1 depending on the jurisdiction and IES recommended practice document (RP-20 for parking facilities, RP-8 for roadway lighting). Lower ratios mean fewer dark spots and more even visual conditions.
What does LM-79 testing measure?
IES LM-79-19 (Approved Method: Optical and Electrical Measurements of Solid-State Lighting Products) measures the total luminous flux (lumens), luminous intensity distribution (candela at every angle), input power (watts), luminous efficacy (lm/W), correlated color temperature (CCT), color rendering index (CRI), and chromaticity coordinates of a complete LED luminaire. Testing is absolute, meaning the luminaire is tested as a complete system including driver and thermal management, at a controlled ambient of 25°C ±1°C.
Can I request a photometric layout for free?
Many commercial lighting manufacturers provide complimentary photometric layouts for projects specifying their products. Jarvis Lighting's photometric and project support service produces foot-candle grids, fixture schedules, and compliance documentation at no cost. Submit floor plans or site drawings with dimensions and target light levels; turnaround is typically 2–3 business days.
Get a photometric layout for your project
Reading a photometric report is step one. Getting one produced for your specific space is step two. Jarvis Lighting's engineering team generates complimentary photometric layouts for commercial and industrial projects: foot-candle grids overlaid on your floor plan, uniformity calculations, fixture schedules, and compliance documentation for permit submittals and rebate applications.
Send your plans, dimensions, and target light levels. Typical turnaround: 2–3 business days.
