UV Calculations

How to Calculate UV Dose, Irradiance, Belt Speed, and Cure Margin

Work through the core UV curing formulas with real units and numbers: dose from irradiance and time, distance falloff, dwell time, belt speed, and cure margin.

UV cure hinges on one equation: dose equals irradiance times time. Dose is in millijoules per square centimeter (mJ/cm2), irradiance in milliwatts per square centimeter (mW/cm2), and time in seconds. Because 1 mW equals 1 mJ per second, the units cancel cleanly: 850 mW/cm2 held for 1.8 seconds delivers 850 times 1.8, or 1,530 mJ/cm2. That is the number the UV Dose and Cure Margin calculator returns. Everything else in this category feeds one of two inputs, the irradiance that actually reaches the surface or the time the surface spends under useful intensity. Get both honest and the dose is trustworthy.

Start with irradiance at the part, not the lamp rating. A lamp specified at 1,200 mW/cm2 at its focal distance delivers far less a few millimeters away, because a focused reflector behaves roughly like an inverse square between reference and actual working distance. UV Irradiance at Part and UV Lamp Distance Intensity Loss handle this: if you measured 1,000 mW/cm2 at 50 mm and the part sits at 65 mm, intensity scales by (50/65) squared, about 0.59, giving roughly 590 mW/cm2. That 15 mm lift cut usable intensity by 41 percent. Always take the radiometer reading at the true cure surface in the wavelength band your chemistry absorbs.

Dwell time is the second input, and on a conveyor it is not a free parameter. Dwell equals the effective cure-zone length divided by belt speed, with units reconciled. A 2 foot (610 mm) high-irradiance zone at a belt speed of 20 feet per minute gives dwell equal to 2 divided by 20 minutes, or 0.1 minute, which is 6 seconds. UV Dwell Time Under Lamp does exactly this conversion. Use the effective zone where irradiance is high enough to count toward cure, not the full housing length, or you will overstate dwell and quietly undercure. For flood or spot cures, dwell is simply lamp-on time at the part.

Combine the two and you have delivered dose at production speed. With 590 mW/cm2 at the part and 6 seconds of dwell, dose is 590 times 6, or 3,540 mJ/cm2. Compare that to a coating that requires 1,200 mJ/cm2 and you have real headroom. UV Dose and Cure Margin reports margin as delivered divided by required times 100: 3,540 over 1,200 is 295 percent, or roughly 2.95 times the requirement. Margin below 100 percent means undercure. A practical floor is 120 to 150 percent so that lamp aging and small distance changes do not push you under the cure threshold mid-run.

To find the fastest belt speed that still cures, invert the dose equation. Required dwell equals required dose divided by irradiance at the part: for 1,200 mJ/cm2 at 590 mW/cm2, that is 1,200 over 590, or 2.03 seconds. Maximum belt speed then equals cure-zone length divided by required dwell: 610 mm divided by 2.03 seconds is 300 mm per second, about 59 feet per minute. UV Conveyor Belt Speed runs this directly. Every recipe has a ceiling speed set by irradiance and required dose, and running above it drops you below target no matter how the line feels.

Lamps fade, so today's dose is not next month's. UV Intensity Decay vs New compares current radiometer irradiance to the new-lamp baseline: a reading of 720 mW/cm2 against an 1,000 mW/cm2 baseline is 72 percent output, meaning 28 percent decay. Feed that into UV Lamp Life Remaining, which projects hours until output falls below the level that still delivers required dose at your set speed. If a mercury lamp starts at 1,000 mW/cm2, decays roughly linearly, and your recipe needs 590 mW/cm2 at the part to hold margin, you have usable life until output crosses that floor, not until an arbitrary hour count.

Uniformity is the trap that averages hide. Dose across a wide part varies from center to edge, and cure is set by the lowest point, not the mean. UV Dose Uniformity and UV Dose Mapping Spread quantify the high-to-low ratio: if center reads 3,540 mJ/cm2 but an edge reads 2,100 mJ/cm2, the spread is 2,100 over 3,540, about 59 percent, and the edge is what must clear 1,200 mJ/cm2. It does here, but a tighter recipe could pass at center and fail at the edge. UV Shadowing Risk Score flags geometry that blocks direct exposure, where local dose can drop to near zero.

One correction keeps the whole chain honest: radiometer band and calibration. A meter reading in the wrong wavelength window, or drifted since its last calibration, poisons every number downstream. UV Radiometer Reading Calibration Correction adjusts a field reading back to a reference so your irradiance input matches the material's absorption band. Before trusting any dose result, confirm the reading is in the correct band (often UVA around 365 nm for many LED chemistries, or the mercury bulb's H, V, or D spectrum), taken at the part surface, with a meter calibrated within its interval. The math is only as good as that single measured milliwatt figure.

Published 2026-07-01.