What Is a Photometric Study? A Complete Guide for Lighting Professionals

A photometric study is a computer-simulated analysis of how light will distribute across a defined space before a single fixture is ever installed. It is the foundation of every credible lighting design — the document that separates specification-grade work from a ballpark fixture count.

If you’ve submitted a lighting permit for a parking lot, received a bid package for a sports facility, or been asked to prove code compliance before breaking ground, you’ve either produced one or needed one. This guide explains exactly what a photometric study is, what data it contains, how to read it, and why it’s required across most commercial and institutional lighting applications.

What a Photometric Study Actually Is

At its core, a photometric study is a simulation. Software models a three-dimensional space — a parking lot, a soccer field, a warehouse floor — populates it with virtual luminaires placed at specified mounting heights and aiming angles, and then calculates the illuminance at every point in the calculation grid.

The output is a set of numeric and graphical results: illuminance values at each grid point, statistical summaries (average, minimum, maximum), uniformity ratios, and often color-coded false-color renderings that make the distribution pattern visually obvious.

What a photometric study is not is a guarantee. It is a prediction, and its accuracy depends entirely on the quality of inputs: the IES photometric data file for the fixture, the accuracy of the mounting height, the reflectance values assigned to surrounding surfaces, and the maintenance factor applied. A study produced with placeholder IES data or incorrect pole heights is worse than no study at all — it creates false confidence.

What Data a Photometric Study Produces

A complete photometric report includes several distinct data outputs, each serving a different purpose.

Illuminance Grid

The illuminance grid is the core deliverable. Software calculates illuminance (in footcandles or lux) at each intersection of a regularly-spaced calculation grid laid across the calculation zone. Grid spacing is typically 5 to 10 feet for outdoor applications and 2 to 5 feet for indoor work. Each grid point represents the horizontal illuminance at working plane height — usually 0 feet for outdoor surfaces (grade) and 2.5 feet for desk-height office applications (per ANSI/IES RP-1-24 for office and ANSI/IES RP-8-22 for exterior applications).

Statistical Summary

From the grid, the software produces statistical summary values:

• Average illuminance (Eavg): The arithmetic mean of all grid point values. This is the number most permitting authorities reference first.
• Minimum illuminance (Emin): The lowest individual grid point value anywhere in the zone. Critical for safety analysis.
• Maximum illuminance (Emax): The peak value, often directly under a fixture. Used to assess glare potential and energy efficiency.
• Uniformity ratio (Eavg/Emin or Emax/Emin): The ratio between values; lower is better. More on this below.

Glare and Light Trespass Analysis

Many studies include a separate light trespass calculation zone at property boundaries, showing how much illuminance spills beyond the project boundary. This is required by most municipal lighting ordinances. Some software also calculates luminous intensity contours at the boundary line to address glare-sensitive neighbors.

Why Photometric Studies Are Required

Municipal Permitting

Most cities and counties with outdoor lighting ordinances — and the list grows every year — require a photometric study as part of the permit application for any new or substantially altered outdoor lighting installation. The ordinance typically specifies maximum average footcandle levels, minimum uniformity ratios, and maximum illuminance at property lines.

In dark-sky-sensitive jurisdictions, the ordinance may also specify maximum correlated color temperature (CCT), full cutoff fixture requirements, and curfew dimming requirements. All of these can be verified in a photometric study.

Sports Facility Design and Bidding

Sports lighting is where photometric studies earn their highest importance. A study for a baseball field isn’t just a permit document — it’s the competitive bid document. Contractors submit photometric studies showing that their proposed fixture count and layout will meet the specified illuminance class under ANSI/IES RP-6-24. A study that meets Class II requirements with 6 poles versus a competitor’s 8-pole layout represents a significant cost difference, and the photometric study is the proof.

Code Compliance Verification

The IES Lighting Handbook and application-specific IES Recommended Practices (RP documents) define illuminance requirements for virtually every application type. A photometric study is the mechanism for verifying compliance before installation.

ROI and Feasibility Analysis

For retrofit and energy upgrade projects, a photometric study demonstrates that the proposed LED replacement maintains or improves the existing illuminance levels while documenting the wattage reduction. This makes the study the evidentiary backbone of any utility rebate application or energy audit report.

Key Terminology

Understanding photometric studies requires fluency with a core set of terms:

Term	Definition	Unit
Illuminance	The amount of luminous flux incident on a surface per unit area	Footcandles (fc) or Lux (lx)
Luminance	The luminous intensity of a surface as seen by an observer; perceived brightness	Candela/m² (cd/m²)
Footcandle	One lumen per square foot	fc
Lux	One lumen per square meter	lx
Uniformity ratio	Ratio of average-to-minimum or max-to-min illuminance	Dimensionless
Maintained illuminance	Illuminance at end of maintenance cycle, after applying LLF	fc or lx
Light loss factor (LLF)	Multiplier accounting for depreciation; typically 0.70–0.85 for LED	Dimensionless
Calculation zone	The defined area over which photometric calculations are run	ft² or m²

Illuminance vs. luminance is a distinction that trips up many clients. Illuminance is what you design to — it’s the quantity of light hitting the ground, the wall, or the playing surface. Luminance is what the eye (or a camera) actually perceives, which depends on the surface’s reflectance. A white concrete floor at 20 fc looks much brighter than a dark asphalt parking lot at 20 fc. Roadway lighting standards often specify luminance rather than illuminance precisely because pavement reflectance varies.

How Photometric Software Works

The two most widely used professional photometric software packages are AGi32 (Lighting Analysts) and DIALux/DIALux evo (DIAL GmbH). Both follow the same fundamental workflow.

Step 1: Define the Space

The designer models the geometry — room dimensions, ceiling height, pole locations, mounting heights, aiming angles. For outdoor projects, this often involves importing a site plan (CAD or PDF) as a background and tracing key features.

Step 2: Place Luminaires with IES Files

Each luminaire in the model is assigned an IES photometric data file. The IES file (discussed in detail in our separate guide) encodes the candela distribution — the luminous intensity in every direction — as measured in a photometric laboratory. The software uses this data to calculate how much light from each fixture reaches each calculation grid point.

Step 3: Define Calculation Zones and Grid

Calculation zones are drawn over the surfaces to be analyzed. The designer sets the grid spacing and working plane height. Multiple zones can exist in one study — for example, a sports study might have separate zones for the playing field, the warning track, and the walkway perimeter.

Step 4: Run the Calculation and Review Results

The software calculates illuminance at every grid point using the inverse square law and the candela data from each contributing fixture. Results are displayed numerically and graphically, and summary statistics are generated automatically.

How to Read a Photometric Report

A properly formatted photometric report contains several sections:

1. Project Information Page: Site name, designer, date, software version, and a luminaire schedule listing each fixture type, its IES file, mounting height, wattage, and quantity.

2. Calculation Summary Table: The Eavg, Emin, Emax, and uniformity ratios for each calculation zone, typically compared against the applicable code or standard requirement.

3. Numeric Grid: The full illuminance grid with values at each point. For large sites, this is often small print, but it’s the ground truth of the study.

4. False-Color Plan: A color-coded overhead view where warm colors (red, orange) indicate high illuminance and cool colors (blue, purple) indicate low illuminance. This is the most visually intuitive deliverable and the one you’ll use to explain the design to a client or planning board.

5. Isolux (Isofootcandle) Contours: Lines connecting points of equal illuminance, similar to topographic contour lines. Useful for identifying hotspots and dark zones.

Common Applications by Sector

Parking Lots and Garages

Parking lot studies typically target Eavg of 1.0–2.0 fc for open lots, with minimums of 0.5 fc throughout and maximums at property lines of 0.1 fc. ANSI/IES RP-20-14 (Parking Facilities) provides the authoritative guidance. Uniformity ratio (Eavg/Emin) of 4:1 or better is typically required.

Sports Fields

Sports lighting studies are among the most complex, requiring separate zones for the primary playing surface, outfield/perimeter, and sometimes spectator areas. Illuminance requirements vary dramatically by class (recreational to broadcast professional) and sport. See our guide to ANSI/IES RP-6-24 for complete requirements.

Roadways

Roadway studies use maintained illuminance on the pavement surface. ANSI/IES RP-8-22 specifies target values based on roadway classification and pedestrian conflict level. Many roadway studies are also required to analyze veiling luminance (disability glare from oncoming traffic) separately.

Industrial and Warehouse

Interior warehouse and manufacturing studies focus on task-appropriate illuminance at working plane height, typically 30–75 fc for warehousing and 50–100 fc for precision manufacturing (ANSI/IES RP-7-21, Recommended Practice for Lighting Industrial Facilities). Vertical illuminance on rack faces and at pallet pick locations is also commonly analyzed.

Retail

Retail lighting design uses photometrics to balance ambient illuminance, accent lighting on merchandise, and vertical illuminance on signage. High-end retail targets 50–150 fc at display surfaces with high uniformity across sales floors (IES Lighting Handbook, 10th Edition).

Working with Illumination Pros Tools

The Illumination Pros platform provides web-based photometric calculation tools built for professionals who need results fast, without full-featured desktop software. Our tools use real IES file data, support multiple calculation zones, and produce export-ready reports in PDF and CSV formats.

Whether you’re verifying a contractor’s proposal, producing a quick feasibility study for a bid, or generating a permit-ready document, the workflow takes minutes rather than hours — with the same underlying calculation accuracy as desktop-class software.

Start with our Photometric Calculator or explore the IES File Library to find photometric data for common fixture types.