How do you calculate UV irradiance loss with distance?

Multiply baseline irradiance by the inverse-square distance factor, which is (old distance / new distance) squared. With 1200 mW/cm2 and a 0.444 factor you get about 532.8 mW/cm2 at the new distance.

Why does UV intensity fall off so fast with distance?

A near-point UV source spreads its energy over an area that grows with the square of distance, so irradiance drops with the square of distance. Doubling the gap cuts irradiance to roughly a quarter.

What does a negative gap to minimum mean?

It means the estimated irradiance is below your minimum acceptable value. In the example the gap is about -267.2 mW/cm2, so the new distance under-cures and you must move the lamp back or add power.

How do I find the distance intensity factor?

Square the ratio of the current working distance to the new working distance. If you move from 10 mm to 15 mm, the factor is (10/15)^2 which is about 0.444, meaning irradiance drops to roughly 44% of baseline.

Does inverse-square apply to LED UV arrays?

Only loosely. Collimated or lensed LED arrays and focused reflectors fall off more slowly than a bare point source, so inverse-square gives a conservative (worst-case) estimate. Always confirm with a radiometer reading.

How do I recover irradiance lost to distance?

Move the lamp back toward the work, increase lamp power or drive current, add a focusing reflector, or slow the line to lengthen dwell so total dose stays on target despite lower irradiance.

UV Curing calculator

UV Lamp Distance Intensity Loss Calculator

The UV Lamp Distance Intensity Loss calculator estimates how much irradiance you lose when a lamp is moved farther from the cure surface, and whether the result still clears your minimum. UV process and maintenance engineers use it when repositioning lamps, changing fixtures, or accommodating taller parts. It matters because UV irradiance falls roughly with the square of distance: a small increase in gap can drop irradiance below the level the adhesive or ink needs, silently causing under-cure. By comparing the estimated new irradiance against your minimum acceptable value, the tool tells you immediately whether the move is safe or whether you need to compensate.

What this calculator does

Estimate how UV irradiance changes when the working distance from a lamp, spot cure head, or LED array changes.
Use it when raising a lamp for taller parts, changing a fixture height, comparing spot-cure standoff distances, or troubleshooting a dose drop after a mechanical change.
It multiplies baseline irradiance by an inverse-square distance factor to estimate irradiance at the new distance, then subtracts your minimum to show the gap.

Formula used

Distance intensity factor = (current working distance ÷ new working distance)²
Estimated irradiance at new distance = baseline irradiance × distance intensity factor
Gap to minimum = estimated irradiance - minimum acceptable irradiance

Inputs explained

Baseline irradiance at reference distance:
Inverse-square distance factor:
Minimum acceptable cure irradiance:

How to use the result

Use it before moving a lamp, changing focal height for a taller part, or swapping to a fixture that increases the lamp-to-work gap.
The inverse-square model assumes an effectively point source; focused reflectors and collimated LED arrays fall off more slowly, so treat the estimate as conservative and verify with a radiometer.

Common questions

How do you calculate UV irradiance loss with distance? Multiply baseline irradiance by the inverse-square distance factor, which is (old distance / new distance) squared. With 1200 mW/cm2 and a 0.444 factor you get about 532.8 mW/cm2 at the new distance.
Why does UV intensity fall off so fast with distance? A near-point UV source spreads its energy over an area that grows with the square of distance, so irradiance drops with the square of distance. Doubling the gap cuts irradiance to roughly a quarter.
What does a negative gap to minimum mean? It means the estimated irradiance is below your minimum acceptable value. In the example the gap is about -267.2 mW/cm2, so the new distance under-cures and you must move the lamp back or add power.
How do I find the distance intensity factor? Square the ratio of the current working distance to the new working distance. If you move from 10 mm to 15 mm, the factor is (10/15)^2 which is about 0.444, meaning irradiance drops to roughly 44% of baseline.
Does inverse-square apply to LED UV arrays? Only loosely. Collimated or lensed LED arrays and focused reflectors fall off more slowly than a bare point source, so inverse-square gives a conservative (worst-case) estimate. Always confirm with a radiometer reading.

Last reviewed 2026-05-12.