Photometric analysis is a lighting calculation process that uses fixture photometric data and a model of the project to predict illuminance. A photometric study records the inputs, assumptions, and results; the photometric plan presents the layout and calculated values for review.
In this guide, we will explain what photometric analysis is, how it works, and why it matters in professional lighting design.
What information is needed for a photometric study?
- A scaled site or floor plan and the exact calculation areas.
- Fixture models, optics, lumen packages, and matching IES files.
- Mounting heights, tilt, aiming, and relevant obstructions.
- Calculation-plane heights, grid spacing, and light-loss assumptions.
- The owner’s criteria and any applicable review requirements.
To inspect fixture distribution before building the model, use Stetra IES Studio. Compare the file and model inputs with the actual fixture submittal.
What is Photometric Analysis and Why is It Important?
Photometric analysis is the process of evaluating light distribution in a given area. It provides a detailed photometric plan that includes:
- Illumination levels (foot-candles or lux)
- Uniformity ratios
- Light distribution patterns
A useful study makes its assumptions explicit so the team can compare the results with the agreed criteria. Calculations support design decisions; they do not guarantee approval, energy savings, or as-built performance.
Initial and maintained results
A report should identify whether its light levels are initial or include light-loss factors for the stated maintenance and operating assumptions. Compare layouts on the same basis. The IES definition of light loss factor and AGi32 light-loss documentation explain the purpose of these adjustments. Select and document assumptions for the actual fixture and project rather than applying one multiplier to every study.
How to Create an Accurate Photometric Plan
1. Understanding the Space
Before performing a photometric analysis, it is essential to understand the space. Consider the following:
- Size and layout
- Required light levels
- Outdoor vs. indoor conditions
For outdoor lighting, tools like Google Earth help determine site dimensions.
Example: Parking Lot Lighting Design
For a 1.5-acre church parking lot, several factors must be considered:
- Pole placement for even lighting
- Mounting height, aiming, and shielding selected for this layout and checked from relevant viewing positions.
- Property-line calculation points evaluated against the limit and measurement plane specified for the project.
The illustrated layout is a project example. Its fixture count, wattage, distribution, and mounting assumptions must be checked against its calculation report before being reused. Do not size another 1.5-acre site from this example alone. Our parking lot photometric plan guide explains the project-specific review items.
2. Determining Lighting Requirements
The next step involves setting key lighting specifications:
Compliance with energy and safety codes
Color temperature (warm, neutral, or cool)
Light intensity based on use (task lighting, security, general illumination)
3. Choose the Right Fixtures
Selecting the correct fixtures is crucial for achieving balanced illumination. Key considerations include:
- Fixture shape and size
- Beam angle and light distribution
- Photometric properties (candela values, cutoff angles, efficiency)
Choose optics for the actual coverage area and mounting locations. Type III can suit some perimeter layouts, while other distributions may suit different positions. Verify coverage, uniformity, and spill light with the intended fixture’s IES data.
4. Using Photometric Analysis Software
To ensure accuracy, designers use specialized software for photometric analysis. These tools help simulate real-world conditions and refine fixture placement.
Common programs include:
- DIALux EVO – Free, user-friendly, and ideal for 3D rendering
- AGi32 – Advanced calculations with in-depth reporting
Use the calculation results to compare fixture locations, optics and aiming before installation. Evaluate energy use and the applicable review requirements separately; a software model alone does not prove a saving or guarantee approval.
5. Review the Photometric Analysis
Once the photometric analysis is complete, the results must be analyzed. The plan should:
Check the model inputs, calculation coverage, light levels and uniformity against the agreed criteria. Document unresolved issues and the revisions needed before the report is issued.
For outdoor projects, check the property-line values on the specified calculation plane against the adopted review criteria. Confirm how the reviewer expects limits and rounding to be reported; a displayed 0.0 is not proof of absolutely zero incident light.
For the step-by-step drawing review, see how to read a photometric plan. The overview below identifies the outputs of the calculation process.
How to Read a Photometric Plan
A photometric plan consists of multiple data points that help evaluate lighting effectiveness. Key components include:
1. Lighting Layout
The first page of a photometric plan typically provides an overview of the site, displaying:
✔ Fixture placement and mounting heights
✔ Light output levels in foot-candles (fc)
✔ Area boundaries and compliance zones
2. 3D Lighting Simulation
A 3D photometric analysis offers a visual representation of how the space will be illuminated.
- False Color Rendering makes it easy to identify dark spots or over-lit areas.
3. Calculation Points
These points indicate foot-candle levels at specific locations, ensuring balanced lighting.
Example: gas station canopy lighting. Evaluate the canopy, fueling positions, circulation, and site edges as defined in the project scope. Select maintained light levels and uniformity from the applicable criteria rather than applying one range to every station. See our gas station photometric plan guide for the application-specific workflow.
Our outdoor lighting foot-candle guide explains how application and calculation area affect the criteria you should review.
4. Calculation Surface
The calculation surface is a critical component of photometric analysis, as it evaluates light distribution, uniformity, and overall illumination quality within a given area. This section helps determine whether the lighting system meets design goals and regulatory requirements for visibility, safety, and energy efficiency.
Key metrics used to assess the calculation surface include:
- Ē (Average Foot-Candle Level) – Represents the overall lighting performance by calculating the average illumination across the entire surface. A well-balanced Ē ensures that spaces receive adequate lighting without excessive brightness or dark spots.
- Emax (Maximum Foot-Candle Value) – The highest calculated illuminance on the selected grid. It helps locate hot spots; it is not a direct measure of glare.
- Ē/Emin (Average/Minimum Ratio) – Measures the uniformity of light distribution by comparing the average illumination to the lowest recorded value. A lower ratio indicates more consistent lighting across the space, reducing areas of under-illumination.
- Emax/Emin (Maximum/Minimum Ratio) – Evaluates the balance of light distribution by comparing the brightest and darkest points within the space. A lower Emax/Emin ratio is desirable, as it prevents stark contrasts between different areas, ensuring a more comfortable and evenly lit environment.
Review the minimum, average, maximum, and ratios together. Compare alternative layouts on the same surface, grid, and operating assumptions. Glare, energy use, and compliance require their own relevant checks; none is established by a uniformity ratio alone.
Why is Photometric Analysis Important?
✔ Energy Efficiency – By preventing over-lighting, it helps reduce costs and minimizes unnecessary energy consumption.
✔ Safety Compliance – Proper lighting ensures visibility for both pedestrians and vehicles, reducing accident risks.
✔ Improved Visual Comfort – Glare reduction enhances aesthetics and creates a more pleasant environment.
✔ Light Pollution Control – Meeting dark sky regulations prevents unwanted spillover and maintains responsible outdoor illumination.
How Long Does It Take to Complete a Photometric Plan?
The timeframe depends on the size and complexity of the project:
✔ Small projects (e.g., single-room lighting) – a few hours
✔ Mid-sized commercial spaces – 1-2 days
✔ Large-scale outdoor lighting projects – several days
Stetra Lighting has more than 10 years of experience preparing photometric plans. Send the site plan, fixture data, and review criteria so we can confirm the calculation scope and delivery schedule. For budgeting factors, see the photometric plan cost guide.
For a quick estimation of your indoor lighting project try our free Lumen Calculator.
Conclusion
A useful photometric analysis connects the selected fixtures and project model with clear calculation results. Record the assumptions, review the weak points as well as the average, and compare the proposed layout with the selected criteria before installation.
Need help with the calculations? Request a photometric plan and include your site plan, fixture information, and project criteria so we can review the scope.