“Radiant Flux Explained: Radiometric Terms for UV and SWIR LEDs”

Radiant flux measures the total optical power an LED emits, in watts, with no weighting for human eye sensitivity. It is the correct metric for ultraviolet and infrared products, where lux and lumens are nearly meaningless because the emission is largely invisible. A UV source can radiate hundreds of milliwatts of optical power while producing a lumen reading close to zero — which is exactly why comparing special-purpose LEDs by lumens or by input wattage leads to specification errors.

Two Unit Systems, One Rule

Radiometric units describe physical energy; photometric units describe eye-perceived brightness. UV and SWIR LEDs live almost entirely in the radiometric world, so their datasheets should be read against the left column of this table:

The split follows a simple rule: photometric units vanish where the eye has no sensitivity. Below roughly 380 nm and above roughly 780 nm, the eye’s response is effectively zero, so a 275 nm UVC LED or a 1550 nm SWIR emitter produces zero lumens no matter how many watts it radiates. Every milliwatt of that power is invisible to a lumen meter, yet fully real to a photoinitiator or to a microorganism’s DNA.

Irradiance and Dose Do the Work

Irradiance, in mW/cm², is the optical power arriving at a work surface. Dose, in mJ/cm², is irradiance multiplied by exposure time. In UV curing, the photoinitiator in an adhesive, ink or coating must absorb a sufficient dose at its activation wavelength to complete polymerization: too little dose leaves the material tacky, and uneven dose produces patchy cure across the bond line. In disinfection, the delivered dose determines the log reduction of microorganisms, which is why germicidal systems move to the UVC band near 254–280 nm, where nucleic acids absorb most strongly. Irradiance also falls with distance — in the far field it follows the inverse-square law, so doubling the working distance cuts it to a quarter. A curing process therefore fixes the working distance, not just the exposure time. Both fields demand calibrated radiometric measurement — the electrical wattage on a datasheet says nothing about what actually reaches the target surface.

Choosing the Wavelength

Radiometric work starts from wavelength. UVA LEDs near 365 nm dominate adhesive and coating curing because common photoinitiators absorb strongly in that band. In the short-wave infrared, materials respond to specific bands: water absorbs strongly around 1450 nm and again near 1900 nm, which is why SWIR sources across roughly 970–1700 nm are used for moisture detection in food sorting, polymer identification in plastic recycling and defect inspection in semiconductor processing. The source wavelength must match the property the inspection system needs to detect, and the specification should state peak wavelength and spectral half width (FWHM), not just a band name.

Specifying Without Ambiguity

A defensible radiometric specification states the unit system, the measurement standard and the operating conditions: peak wavelength, spectral half width, radiant power at a stated drive current, and lifetime data under that same current. Confirm that the values were measured with a calibrated instrument — an integrating sphere or a calibrated radiometer — rather than derived from electrical calculations. Component makers, system integrators and acceptance testers can then all measure the same quantity.

Mixing radiometric and photometric units remains one of the most expensive specification errors in special-purpose lighting. Writing the unit system, the standard and the operating current into the specification is the simplest guard against it.

Key Takeaways

• Radiant flux is total optical power in watts — the metric that replaces lumens for UV and infrared LEDs.
• Irradiance (mW/cm²) and dose (mJ/cm²) govern UV curing and disinfection outcomes.
• Specify peak wavelength, spectral half width and radiant power at real operating current, measured with calibrated instruments.