What a Photometric Report Actually Tells You

A photometric report translates a fixture’s IES file into a prediction of real illuminance on a real site. It’s the document that answers whether a lighting design will actually perform the way the layout suggests, and it’s usually the deciding factor in whether a submittal gets approved or sent back. 

Most reports look denser than they are. Once you know what each section is answering, they read less like a wall of numbers and more like a checklist working through fixture selection, layout, and compliance in sequence. 

Start With the Fixture Schedule, Not the Numbers

It’s tempting to jump straight to the calculation grid, but the numbers there only mean something once you know what’s producing them. The fixture schedule is where that context lives: tags matched to fixture type, wattage, photometric distribution, and color temperature.

Mounting height matters just as much and gets skipped just as often. A fixture rated for strong coverage at 25 feet performs very differently at 30 feet, so even small mounting-height errors compound quickly across a layout.

A few things worth confirming before moving past the schedule:

• Fixture tags on the schedule match the tags shown on the calculation plan
• Mounting height and any pole or ceiling assumptions are stated explicitly, not implied
• Distribution type is appropriate for the application, since a type III distribution behaves very differently than a type V in the same layout

Reading the Calculation Grid 

The calculation grid is a set of points across the site or space where the software has calculated footcandle values based on the fixture data and layout geometry. For exterior sites, the work plane is set at grade, since that’s where people actually walk and drive. For interior spaces, it’s typically set 30 inches above the floor to represent a working surface.

Isolux, or isofootcandle, contours group points of similar illuminance together and are often faster to scan than the raw grid. A cluster of tight contour lines usually flags a sharp transition between bright and dark areas, which is exactly the kind of spot worth a second look before a project gets built.

Why Uniformity Ratios Matter More Than the Average 

Average footcandle is the number most people look at first, and it’s often the least useful one on its own. A design can achieve a strong average while still leaving dark pockets that fail an inspection or make a walkway feel unsafe. Minimum footcandle and uniformity ratios are usually what code and inspection outcomes actually hinge on.

Two ratios do most of the work here:

• Avg/Min ratio, which shows how far the darkest point falls below the average
• Max/Min ratio, which shows the full spread between the brightest and darkest points in the calculation zone

A lower ratio generally means more even lighting. A high ratio, even alongside a strong average, is a signal to look more closely at fixture spacing before signing off.

Checking Property Line and Light Trespass Values 

Exterior photometric reports often get flagged during review for a subtle setup error rather than a design flaw: the calculation zone stops exactly at the property line, rather than extending beyond it. That shows the illuminance value right at the line but doesn’t reveal how light actually spills onto the adjacent property.

Requesting that the calculation grid extend a short distance past the property line provides a more accurate picture of light trespass. Many jurisdictions set tight maximum illuminance thresholds at property lines adjacent to residential zones, and this is one of the more common reasons a submittal gets sent back for revision.

A Photometric Report Is Not a Fixture Test Report 

It’s easy to conflate a project-specific photometric report with a manufacturer’s fixture test report, but they answer different questions. A fixture test report, based on the IES LM-79 measurement standards, documents how a single fixture performs in isolation under laboratory conditions.

A photometric report takes the same fixture data and applies it to an actual layout at actual mounting heights across an actual site or space. One tells you what the fixture does. The other tells you what the project will look like once that fixture is installed. Knowing which document you’re holding changes what questions it’s reasonable to ask of it.

Getting the Read Right the First Time 

A photometric report rewards a specific reading order: fixture schedule first, then the calculation grid, then uniformity, then property line conditions, and finally a check on which type of report you’re actually looking at. Most submittal delays trace back to skipping one of those steps rather than any flaw in the design itself.

Crown Lighting Group works alongside lighting designers, architects, and engineers to both produce photometric reports and review photometric submissions before they become a problem in the field. If you need photometric design assistance or help evaluating your site or space plan through photometric review, our team is glad to work through it with you.