Direct answer
A calculated footcandle value is a software prediction from stated assumptions. A field-measured value is a meter reading taken at a specific point, on a specific date, under specific conditions. They are different categories of information and they routinely differ. The difference is caused by aiming, voltage, temperature, surface reflectance, meter and procedure, luminaire age, and any deviation between the modelled and installed geometry — not necessarily by a defect.
Who this is for
Owners, athletic directors, municipal staff, consulting engineers, and contractors who are writing acceptance criteria, commissioning a new installation, or investigating why a field reading does not match a design report.
Why the two numbers differ
A photometric calculation assumes an idealised installation: every luminaire at the modelled position, aimed exactly as documented, operating at nominal voltage and rated output, over surfaces of assumed reflectance, in an environment with no unmodelled obstruction.
A field measurement records what actually exists on the day. The gap between the two comes from identifiable causes, and naming them is what turns a dispute into a resolution.
Aiming. Small angular deviations at height translate into large positional shifts on the surface. Aiming is the most common single cause of divergence.
Installed geometry. Poles set a few feet off the drawing, at a slightly different height, or with a different setback, change the result.
Voltage and drive current. Supply voltage variation and driver settings change output.
Temperature. Ambient temperature affects LED output. A measurement in cold weather and one in summer are not equivalent.
Surface reflectance. Modelled reflectance for turf, clay, acrylic, or synthetic surfaces is an assumption. Wet surfaces, worn surfaces, and seasonal colour change alter interreflection.
Luminaire age and cleanliness. A new installation measured on day one should read above its maintained calculated value, because the maintained value includes depreciation that has not happened yet. This is the single most misunderstood point in commissioning.
Meter, procedure, and point locations. A different meter, a different cosine correction, a different height above surface, or measurement at points that are not the calculation grid points will produce different numbers.
Unmodelled obstructions. Netting, windscreens, banners, scoreboards, trees, and adjacent structures block or reflect light that the model did not account for.
Stray light. Adjacent facilities, streetlights, and parking lighting contribute to a field reading and do not appear in a calculation of the subject installation alone.
What gets evaluated, and how each figure is labelled
Figure | Label | Meaning |
Design average maintained illuminance | Calculated | Prediction including light-loss factor. |
Design initial illuminance | Calculated | Prediction excluding depreciation. |
Grid point values in the design report | Calculated | Predictions at modelled points. |
Meter reading at a stated point | Field-measured | An observation with a stated meter, date, time, weather, and procedure. |
Average of field readings across a stated grid | Field-measured | Only comparable to a calculation using the same points. |
Luminaire output at test conditions | Reported | Laboratory result on a sample, not on the installed unit. |
As-built pole positions and heights | Field-measured | Survey result; the basis for any re-calculation. |
As-built aiming angles | Field-measured | Recorded during commissioning. |
How to write acceptance criteria that work
Acceptance criteria that say only "the installation shall achieve 30 footcandles" will produce a dispute. Criteria that survive contact with a light meter state all of the following:
1.Which value is being tested — initial or maintained. If maintained, state how a day-one measurement will be interpreted, since depreciation has not yet occurred.
2.The measurement grid — the same points as the calculation, or a stated subset, with spacing and height above surface.
3.The instrument — meter type and calibration status and date.
4.The conditions — minimum warm-up time, acceptable ambient temperature range, supply voltage tolerance, surface condition, and whether adjacent lighting is on or off.
5.The procedure — who takes the readings, in what order, and how they are recorded.
6.The tolerance — an explicit allowance for the difference between calculation and measurement, agreed in advance.
7.The remedy path — what happens if readings fall outside tolerance: re-aim, re-measure, re-calculate on as-built geometry, or a defined corrective scope.
Agreeing tolerance in advance is the difference between a commissioning step and a claim.
Retrofit versus new construction
New construction gives you as-built survey data and a fresh installation, so divergence is usually traceable to aiming or construction tolerance and is usually correctable by re-aiming.
Retrofit divergence is more often geometric. Existing poles may not be where the record drawings say they are, may lean, may have settled, or may be at heights that were never recorded accurately. If a retrofit field measurement diverges, survey the as-built geometry before assuming a product problem, then re-run the calculation on surveyed geometry. Comparing a measurement against a calculation built on assumed pole positions proves nothing about either.
What can change the answer
•Re-aiming after measurement changes field results without changing the design report.
•A survey showing as-built geometry differs from the model requires re-calculation before any comparison is valid.
•Measuring at a different time of year, or at a different ambient temperature, changes readings.
•Turning adjacent site lighting on or off changes readings.
•Replacing a failed driver or luminaire changes the population being measured.
•A change of meter or calibration status changes the readings' basis.
Verification checklist
1.Confirm whether the target under test is initial or maintained, in writing.
2.Confirm the measurement points are the calculation points, or a stated defensible subset.
3.Record meter model and calibration date.
4.Record date, time, ambient temperature, and weather.
5.Record supply voltage at the panel during measurement.
6.Record warm-up time before the first reading.
7.Record whether adjacent lighting was on or off.
8.Record surface condition, including moisture.
9.Verify aiming against the aiming report before concluding anything from readings.
10.Survey as-built pole positions and heights if divergence exceeds tolerance.
11.Re-calculate on as-built geometry before attributing divergence to product performance.
12.Record every figure with its label — calculated or field-measured — in the commissioning file.
Duvon's role and limits
Duvon Lighting supplies products and calculated layouts, and states the assumptions behind every calculation we issue. If a field measurement diverges from a layout we prepared, we will review the aiming report, the as-built geometry, and the measurement conditions with you, and re-run the calculation on surveyed geometry where that is the right step.
We do not present calculated results as measured results, and we do not offer light-level guarantees. Independent field measurement, where an owner requires it, should be performed by a party independent of the supplier.
Next step
If you are writing acceptance criteria or investigating a divergence between a design report and a field reading, send us the layout, the aiming report, and the measurement record. We will review the assumptions and tell you what the numbers can and cannot support.