Why Understanding Photometrics Prevents Under- or Over-Lighting
Photometric reports translate lighting design intent into measurable performance. For parking lots and warehouses, these reports are often generated from IES files supplied by fixture manufacturers and processed through lighting software. While many facility owners rely on designers or contractors to interpret the data, understanding the fundamentals allows stakeholders to verify compliance, avoid over-lighting, and prevent costly redesigns.
This guide explains how to read a standard photometric report, focusing on foot-candles, uniformity ratios, and how these values influence safety, efficiency, and code compliance.
What an IES File Represents
An IES file is a standardized digital file that describes how a lighting fixture distributes light. It does not describe brightness in isolation; instead, it maps light output across three-dimensional space.
| IES Component | What It Defines |
|---|---|
| Luminous intensity | How light is distributed in each direction |
| Total lumens | Overall light output of the fixture |
| Optical geometry | Beam shape and cutoff behavior |
The IES file becomes meaningful only when applied to a specific layout, mounting height, and spacing.
Understanding Foot-Candles and Illuminance
Foot-candles (fc) measure illuminance—how much light reaches a surface. One foot-candle equals one lumen per square foot.
| Application | Typical Target (fc) |
|---|---|
| Parking lots | 0.5–2.0 avg |
| Warehouse aisles | 20–30 avg |
| Open warehouse floors | 15–25 avg |
Average foot-candles alone do not guarantee visual comfort or safety; distribution consistency is equally important.
Uniformity Ratios (Max/Min and Avg/Min)
Uniformity ratios describe how evenly light is distributed across the area. Two ratios are commonly reported:
- Max/Min: Compares the brightest point to the darkest point
- Avg/Min: Compares average illuminance to the darkest point
| Ratio | Lower Is Better | Typical Target |
|---|---|---|
| Max/Min | Yes | ≤10:1 (parking) |
| Avg/Min | Yes | ≤4:1 (warehouse) |
High ratios indicate harsh contrasts, which reduce visibility and increase eye fatigue.
Reading a Parking Lot Photometric Layout
Parking lot reports typically include a calculation grid overlaid on the site plan.
- Check minimum foot-candles at property lines
- Confirm uniformity ratios meet local ordinances
- Verify optics prevent light trespass beyond boundaries
Designs that focus only on average foot-candles often fail inspection due to poor uniformity or spill light.
Reading a Warehouse Photometric Layout
Warehouse photometrics prioritize vertical and horizontal illumination consistency.
- Review aisle centerlines for minimum fc
- Confirm beam angles align with racking heights
- Evaluate uniformity to reduce shadowing between fixtures
For high-bay layouts, incorrect spacing often causes low-min values even when average levels appear adequate.
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Learning to read photometric reports empowers facility owners and managers to validate designs, ensure compliance, and make informed lighting decisions without relying solely on third-party interpretation.
Brandon Waldrop
As the lead technical specialist for our commercial lighting technical operations, Brandon Waldrop brings over 20 years of industry experience in product specification, outside sales, and industrial lighting applications.
His career began in physical lighting showrooms, where he focused on hands-on product performance and technical support. He later transitioned into commercial outside sales, working directly with architects, electrical contractors, and facility managers to translate complex lighting requirements into energy-efficient, code-compliant solutions.
Today, Brandon applies that industry experience to architect high-performance digital catalogs and technical content systems, helping commercial partners streamline the specification process and deploy lighting solutions with total technical confidence.
