How do you calculate UV lamp intensity decay?

Divide today's irradiance reading by the original new-lamp baseline and multiply by 100. In the example, 1380 divided by 2000 times 100 gives 69% of new output.

At what percentage should you replace a UV lamp?

It depends on the process window, but many shops set the threshold between 60 and 80 percent of new. In the example the threshold is 75%, and at 69% the lamp is already below it, so headroom is only 6 points above where it would have been at threshold.

Wait, is 69% above or below a 75% threshold?

Read the headroom sign carefully. The tool reports headroom as percent-of-new minus threshold; if your reading is below threshold, headroom is negative and the lamp is due. Confirm the direction against your own reading before acting.

Why does my cure fail before the lamp hits 0%?

Cure depends on dose, and dose is irradiance times dwell time. Once irradiance decays past the point where your line speed can still deliver target dose, parts undercure long before the lamp goes fully dark.

What is a good replacement threshold to set?

Set it at the irradiance where your slowest acceptable line speed still just meets target dose, plus a safety margin. If your window is tight, a higher threshold like 80% protects you; a wide window may tolerate 60%.

Percent of new vs delivered dose, which should I trust?

Both matter. Percent of new is a fast health check on the lamp, but delivered dose in mJ/cm2 at the part is the true acceptance metric because it also reflects line speed and part distance.

UV Curing calculator

UV Intensity Decay Calculator

UV lamp and LED output falls with hours of use, and once irradiance drops far enough your process window collapses and parts start undercuring. This calculator expresses today's radiometer reading as a percentage of the new-lamp baseline and compares it to the replacement threshold your process was qualified against. Maintenance planners and process engineers use it to schedule lamp changes before quality drops rather than after a scrap event. It turns a raw mW/cm2 number into a clear go or replace signal.

What this calculator does

Calculate how much UV irradiance has dropped vs a brand-new lamp baseline, with a verdict against your minimum-acceptable percent.
Use it after every radiometer pass to trend lamp aging objectively and decide whether to swap, slow the line, or keep going.
It computes today's irradiance as a percentage of the original new-lamp baseline and reports how many points of headroom remain above your replacement threshold.

Formula used

% of new = today's reading ÷ original baseline × 100
Headroom = % of new − replacement threshold

Inputs explained

Today's irradiance reading:
New-lamp baseline irradiance:
Lamp replacement threshold:

How to use the result

Use it each time you log a periodic radiometer reading, or when deciding whether an aging lamp can survive the next production run.
Percent of new only tracks peak irradiance; it does not capture spectral shift or reflector fouling, which can degrade cure even when the number still looks acceptable.

Common questions

How do you calculate UV lamp intensity decay? Divide today's irradiance reading by the original new-lamp baseline and multiply by 100. In the example, 1380 divided by 2000 times 100 gives 69% of new output.
At what percentage should you replace a UV lamp? It depends on the process window, but many shops set the threshold between 60 and 80 percent of new. In the example the threshold is 75%, and at 69% the lamp is already below it, so headroom is only 6 points above where it would have been at threshold.
Wait, is 69% above or below a 75% threshold? Read the headroom sign carefully. The tool reports headroom as percent-of-new minus threshold; if your reading is below threshold, headroom is negative and the lamp is due. Confirm the direction against your own reading before acting.
Why does my cure fail before the lamp hits 0%? Cure depends on dose, and dose is irradiance times dwell time. Once irradiance decays past the point where your line speed can still deliver target dose, parts undercure long before the lamp goes fully dark.
What is a good replacement threshold to set? Set it at the irradiance where your slowest acceptable line speed still just meets target dose, plus a safety margin. If your window is tight, a higher threshold like 80% protects you; a wide window may tolerate 60%.

Last reviewed 2026-05-12.