Why is W/in not a useful unit?

Because it is a linear density and cure responds to an areal one. W/in tells you nothing about how wide the emitting window is, and the same W/in through a half-inch window and a one-and-a-half-inch window are three times apart in irradiance. It survives as a specification unit because it lets a vendor rank their own arrays, which is a different job from telling a customer what one will do.

Is this the irradiance my part will see?

No. It is the irradiance at the emitting window, and the part is always somewhere else. LED arrays are near-field sources: output falls sharply with distance and does not follow a clean inverse-square or inverse-distance law, because the relevant geometry is the array's own size compared with the working gap. Use this page to size and compare; use a radiometer at the real gap to set a process.

What is a typical derate factor?

It depends entirely on cooling and age, which is why it is an input rather than a constant. Junction temperature is the immediate part and is fully recoverable. An array losing output to heat gets it back when the cooling is fixed. Ageing is slow and permanent. A heavy derate is worth diagnosing rather than accepting, because one of its two causes can be fixed the same day.

How do I compare an LED array with a mercury lamp?

On irradiance in the band your photoinitiator absorbs, at the same working distance, and then on the dose that produces at your line speed. Neither W/in nor installed kilowatts is comparable across the technologies, because they differ in how much input becomes UV and in how that UV is distributed across the spectrum. The dose and cure-speed pages are where the comparison becomes a decision.

Does wavelength matter here?

Enormously, and this page cannot see it. 180 W at 365 nm and 180 W at 395 nm produce identical numbers here and can differ completely in whether they cure a given material: the photoinitiator either absorbs at that wavelength or it does not. Always record the peak wavelength alongside any irradiance figure; on its own the number is incomplete.

Should I include the lens or light guide?

This page does not model optics, so the answer depends on where the quoted power was measured. If the optical power figure is measured after the optics, then the emitting aperture of the optics is the width to enter. If it is measured at the diodes, the optics will change both the irradiance and its distance behaviour, and only a measurement at the working position resolves it.

UV Curing calculator

UV LED Array Power Density Calculator: mW/cm², Not W/in

Convert a UV LED array's specification into the unit that means something. Enter the array's UV optical power, its emitting length, the thermal and ageing derate, and the width of the emitting window. The calculator returns the irradiance at the window in mW/cm², the unit a radiometer reports and a material datasheet is written in, alongside the W/in figures vendors quote. Those two are not interchangeable: two arrays at the same W/in can differ threefold in irradiance depending on how wide a window they emit through.

What this calculator does

Convert a UV LED array's optical power and geometry into the irradiance at its emitting window, in the units a radiometer and a datasheet actually use.
Use it for converting a vendor's W/in specification into a comparable irradiance, comparing two arrays with different window widths, estimating whether an array can meet a material's irradiance requirement, quantifying what an inadequately cooled array is giving up, sanity-checking a quoted irradiance against optical power and geometry.
Convert a UV LED array's optical power and geometry into the irradiance at its emitting window, in the units a radiometer and a datasheet actually use.

Formula used

Derated optical power = array optical power × derate factor
Raw density (W/in) = array optical power ÷ emitting length
Effective density (W/in) = derated optical power ÷ emitting length
Emitting area (cm²) = emitting length × emitting width × 6.4516
Irradiance at the window (mW/cm²) = derated optical power × 1000 ÷ emitting area

Inputs explained

Total UV optical power of the LED array: Radiant UV output, not electrical input. The two differ by a large factor and confusing them overstates irradiance several-fold.
Effective emitting length of the array: Length of the emitting window along the array, excluding dark end sections.
Thermal and aging derate factor: Share of rated output actually delivered at operating junction temperature and current array age. LED output falls with both.
Emitting window width: Width of the emitting aperture across the direction of travel. The input that turns a linear density into an irradiance.

How to use the result

Best suited to converting a vendor's W/in specification into a comparable irradiance, comparing two arrays with different window widths, estimating whether an array can meet a material's irradiance requirement, quantifying what an inadequately cooled array is giving up, sanity-checking a quoted irradiance against optical power and geometry.
Gives irradiance at the WINDOW, not at the part. LED arrays are near-field sources whose output falls sharply with distance, and the fall-off follows neither a point-source nor a line-source law. It depends on the optics and on distance relative to the array's own dimensions. Assumes uniform distribution across the window. Real arrays have structure at the diode pitch, which matters for a part close to the window. Says nothing about spectral distribution: 180 W at 395 nm and 180 W at 365 nm are the same number here and very different for a photoinitiator. Cannot distinguish thermal derating from ageing, which behave differently over time. Ignores any optics. Lenses, reflectors or light guides change both the irradiance and the distance behaviour.

Common questions

Why is W/in not a useful unit? Because it is a linear density and cure responds to an areal one. W/in tells you nothing about how wide the emitting window is, and the same W/in through a half-inch window and a one-and-a-half-inch window are three times apart in irradiance. It survives as a specification unit because it lets a vendor rank their own arrays, which is a different job from telling a customer what one will do.
Is this the irradiance my part will see? No. It is the irradiance at the emitting window, and the part is always somewhere else. LED arrays are near-field sources: output falls sharply with distance and does not follow a clean inverse-square or inverse-distance law, because the relevant geometry is the array's own size compared with the working gap. Use this page to size and compare; use a radiometer at the real gap to set a process.
What is a typical derate factor? It depends entirely on cooling and age, which is why it is an input rather than a constant. Junction temperature is the immediate part and is fully recoverable. An array losing output to heat gets it back when the cooling is fixed. Ageing is slow and permanent. A heavy derate is worth diagnosing rather than accepting, because one of its two causes can be fixed the same day.
How do I compare an LED array with a mercury lamp? On irradiance in the band your photoinitiator absorbs, at the same working distance, and then on the dose that produces at your line speed. Neither W/in nor installed kilowatts is comparable across the technologies, because they differ in how much input becomes UV and in how that UV is distributed across the spectrum. The dose and cure-speed pages are where the comparison becomes a decision.
Does wavelength matter here? Enormously, and this page cannot see it. 180 W at 365 nm and 180 W at 395 nm produce identical numbers here and can differ completely in whether they cure a given material: the photoinitiator either absorbs at that wavelength or it does not. Always record the peak wavelength alongside any irradiance figure; on its own the number is incomplete.

Last reviewed 2026-08-25.