How to Read a Point-by-Point Footcandle Plan

What the numbers mean

Each number on a point-by-point plan represents calculated illuminance at a specific point on a defined surface. In U.S. projects, the unit is often footcandles. One footcandle equals one lumen per square foot. Other reports may use lux, where one lux equals one lumen per square meter.

The grid is not a picture of visible brightness. It is a calculation sample. The fixture photometry, geometry, aiming, mounting height, reflectances and grid placement all affect the values.

• Average: the arithmetic mean of all included calculation points
• Minimum: the lowest included value
• Maximum: the highest included value
• Average-to-minimum ratio: average divided by minimum
• Maximum-to-minimum ratio: maximum divided by minimum

1. Confirm the calculation surface

Before interpreting any value, find the height and orientation of the calculation plane. A floor-level horizontal grid, a 30-inch workplane, a parking-lot pavement grid and a vertical wall grid answer different questions. Comparing them directly can be misleading.

Also check whether the grid excludes objects, property boundaries, curbs or unoccupied zones. A report average is meaningful only for the set of points included in it.

2. Confirm units and grid spacing

Verify whether the values are footcandles or lux. Then find the grid spacing. A coarse grid can miss localized peaks and dark areas, while an unnecessarily fine grid can make a preliminary report harder to read. The required calculation spacing may be defined by the project or applicable practice.

3. Compare average, minimum and maximum

The average describes the group but does not show distribution by itself. The minimum indicates the lowest sampled point, and the maximum helps identify hot spots or unusually high values. Always look at their locations on the plan.

Example: a plan with 40 average, 8 minimum and 72 maximum footcandles has an average-to-minimum ratio of 5:1 and a maximum-to-minimum ratio of 9:1. Whether that is acceptable depends on the task and project criteria. The ratios simply expose the range.

4. Read the spatial pattern

Trace values between fixtures, at room edges, around poles and near walls. Repeating low bands may indicate excessive spacing. A bright spot directly below a luminaire can signal that the chosen optic, mounting height or output produces a concentrated distribution. At property lines or boundaries, review the exact points required by the project.

A heatmap can make the pattern faster to recognize, but the numeric grid remains important. Color scales can change between reports and may exaggerate or hide differences if read without the legend.

5. Check uniformity correctly

Uniformity must be labeled because different ratios are not interchangeable. Average-to-minimum, maximum-to-minimum and minimum-to-average express different relationships. Confirm which ratio the requirement uses before comparing the result.

6. Verify the model assumptions

A clean-looking plan can still be wrong if the inputs are wrong. Confirm the fixture catalog number and IES file, light-loss assumptions when applicable, mounting height, fixture orientation, room or site dimensions, reflectances and calculation-plane location.

For outdoor work, check pole placement and aiming. For indoor work, check surface reflectance and workplane height. For wall-mounted fixtures, check the actual mounting surface and orientation.

Questions to ask before approving a plan

• What exact surface and height were calculated?
• Are the values footcandles or lux?
• Which fixture file, wattage and optic were used?
• Where do the minimum and maximum occur?
• Which uniformity definition applies?
• Are obstructions and boundaries represented appropriately?
• Has a qualified person reviewed the layout for its intended use?