Why is W/in not a useful unit?

It is a linear density, and cure responds to an areal one. W/in says nothing about window width, and the same W/in through half-inch and one-and-a-half-inch windows is three times apart in irradiance.

Is this the irradiance my part will see?

No, it is at the emitting window and the part is somewhere else. LED output falls sharply with distance in a way no simple law captures, so measure at the real gap.

What derate factor should I use?

It depends on cooling and age, which is why it is an input. Junction temperature losses are recoverable by fixing cooling; aging is permanent. Diagnose a heavy derate rather than accept it.

Does wavelength matter here?

Enormously, and this page cannot see it. 180 W at 365 nm and 180 W at 395 nm are identical here but can cure differently. Record the peak wavelength with every irradiance figure.

UV Curing calculator

UV LED Array Power Density Calculator

Turn a UV LED array's optical power, length, derate and window width into the irradiance at the emitting face. W/in ranks arrays; mW/cm² is what cure responds to and what datasheets quote.

What this calculator does

Convert LED array optical power and window geometry into the irradiance at the emitting face.

Formula used

Derated power = array optical power × derate factor
Raw density (W/in) = array optical power ÷ emitting length
Effective density (W/in) = derated power ÷ emitting length
Emitting area (cm²) = length × width × 6.4516
Window irradiance = derated power × 1000 ÷ emitting area

Inputs explained

Array UV Optical Power: Radiant UV output, not electrical input.
Effective Emitting Length: Illuminated window length, excluding dark end sections.
Thermal and Aging Derate: Share of rated output at operating temperature and age.
Emitting Window Width: Width of the emitting aperture across travel.

How to use the result

Best suited to converting a vendor's W/in figure into irradiance, comparing two arrays with different window widths, quantifying what poor cooling is giving up.
Gives window irradiance, not part irradiance; LED fall-off follows no simple law. Ignores spectral distribution; 180 W at 365 nm and 395 nm are different. Cannot separate thermal derating from permanent aging.

Common questions

Why is W/in not a useful unit? It is a linear density, and cure responds to an areal one. W/in says nothing about window width, and the same W/in through half-inch and one-and-a-half-inch windows is three times apart in irradiance.
Is this the irradiance my part will see? No, it is at the emitting window and the part is somewhere else. LED output falls sharply with distance in a way no simple law captures, so measure at the real gap.
What derate factor should I use? It depends on cooling and age, which is why it is an input. Junction temperature losses are recoverable by fixing cooling; aging is permanent. Diagnose a heavy derate rather than accept it.
Does wavelength matter here? Enormously, and this page cannot see it. 180 W at 365 nm and 180 W at 395 nm are identical here but can cure differently. Record the peak wavelength with every irradiance figure.

Last reviewed 2026-10-01.