Why is inverse square wrong for a UV lamp?

It describes a point source, and it applies only beyond about five source dimensions. A 10 in arc would need 50 in of standoff; UV curing works at 1 to 6 in, where a tube behaves as a line source.

How big is the error in practice?

The ratio of the two predictions is the distance ratio. A 2 in to 3 in move gives 800 mW/cm² under the line law against 533 under inverse square, and dose calculations inherit that error.

So which number should I use?

The one the page selects for your regime, treated as the middle of a bracket. A real tube sits between the laws; put a radiometer at the new distance where the decision matters.

What about focused reflector systems?

Neither law describes them near focus. Irradiance peaks at the focal plane and falls on either side, so moving closer can reduce it. Treat the stated focal distance as the operating point.

UV Curing calculator

UV Lamp Distance Intensity Calculator

Predict what moving a UV lamp does to irradiance at the part. Enter the measured baseline and its distance, the new distance and the arc length; the page picks the falloff law.

What this calculator does

What moving a lamp does to irradiance at the part, using the falloff law a tube actually follows.

Formula used

Distance ratio = reference distance ÷ new distance
Line source = baseline × ratio, falling as 1/d
Point source = baseline × ratio², falling as 1/d²
Far field starts at 5 × arc length
Prediction = point source in the far field, line source otherwise

Inputs explained

Baseline Irradiance at Reference Distance: Measured irradiance at the distance you are moving from.
Reference Distance: Lamp-to-part distance where the baseline was measured.
New Distance: Lamp-to-part distance you are moving to.
Lamp Arc Length: Illuminated arc length, which decides the falloff law.

How to use the result

Best suited to predicting a lamp raise for tall parts, bracketing irradiance where no radiometer fits, checking a standoff against a minimum irradiance.
Focused systems peak at a focal plane; neither law holds near it, and moving closer can reduce irradiance. The five-arc-length boundary is a rule of thumb, so predictions near it are least reliable. Real tubes are neither perfect lines nor points; the truth lies between the predictions.

Common questions

Why is inverse square wrong for a UV lamp? It describes a point source, and it applies only beyond about five source dimensions. A 10 in arc would need 50 in of standoff; UV curing works at 1 to 6 in, where a tube behaves as a line source.
How big is the error in practice? The ratio of the two predictions is the distance ratio. A 2 in to 3 in move gives 800 mW/cm² under the line law against 533 under inverse square, and dose calculations inherit that error.
So which number should I use? The one the page selects for your regime, treated as the middle of a bracket. A real tube sits between the laws; put a radiometer at the new distance where the decision matters.
What about focused reflector systems? Neither law describes them near focus. Irradiance peaks at the focal plane and falls on either side, so moving closer can reduce it. Treat the stated focal distance as the operating point.

Last reviewed 2026-10-01.