Wavelength is the first thing asked about a UV application – and the first place misunderstandings arise. 254 nm and 265 nm are eleven nanometres apart and come from entirely different sources; 395 nm and 405 nm are separated by even less, and yet one falls in the UVA range and the other already in the visible. This explorer places any wavelength between 150 and 800 nm and gives the quantities that appear in data sheets and standards.
The second part puts the common wavelengths side by side. The interesting column there is photons per joule: it explains why a process release cannot simply be transferred from one wavelength to another. A photochemical reaction responds to absorbed photons, not to joules – and at the same dose, 405 nm delivers about 60 percent more photons than 254 nm.
Which wavelength is the right one is decided in the end by the absorption spectrum of the substance, not by photon count alone. And how much irradiance actually arrives is decided by the measurement. Opsytec Dr. Gröbel develops, manufactures and calibrates the necessary sensors, radiometers and spectroradiometers in its own accredited laboratory.
The conversions between wavelength, frequency, wavenumber and photon energy apply in vacuum; in a medium with refractive index n, wavelength and propagation speed change while frequency and photon energy do not. Spectral ranges follow CIE S 017 (ILV); the formerly used DIN 5031-7 has been withdrawn. The sources and applications named are typical examples, not an exhaustive list; they replace neither a system design nor a risk assessment. The measurement with calibrated instruments is what counts.
Fundamentals
Wavelength, frequency, wavenumber and photon energy describe the same radiation from four points of view and convert into one another without any further information. The relations are ν = c / λ, σ = 1 / λ and E = h · c / λ; with the wavelength in nanometres the last one shortens to E[eV] = 1239.842 / λ[nm]. Wavenumber in cm⁻¹ is customary in spectroscopy because it is directly proportional to energy — unlike wavelength. Molar photon energy in kJ/mol, finally, relates the radiation to reaction enthalpies and says whether it can break a given bond at all.
Under DIN 5031-7, UVC runs from 100 to 280 nm, UVB from 280 to 315 nm and UVA from 315 to 400 nm.
| Range | Wavelength | Typical sources |
|---|---|---|
| VUV | 100–200 nm | xenon excimer 172 nm |
| UVC | 200–280 nm | low-pressure mercury 254 nm, KrCl 222 nm, UVC LEDs 265 and 280 nm |
| UVB | 280–315 nm | XeCl excimer 308 nm, UVB fluorescent lamps 311 nm |
| UVA | 315–400 nm | UVA fluorescent lamps 340 nm, UVA LEDs 365 to 395 nm |
| VIS | 380–780 nm | violet and blue LEDs from 405 nm |
Between 380 and 400 nm, UVA and the visible range overlap — the standard assigns this section to both. Radiation there is perceptible as a faint violet and is still assessed as UVA. That is why a 405 nm source is called a UV source or a light source depending on the point of view.
A dose is an energy per unit area. Divide it by the energy of a single photon and you get the number of photons per unit area — and that is what photochemistry and photobiology respond to. Because photon energy falls as wavelength rises, the same dose contains more photons at longer wavelengths. At 254 nm there are about 1.28 · 10¹⁸ photons per joule, at 405 nm about 2.04 · 10¹⁸. Anyone converting a system from a mercury lamp to UVA LEDs shifts not only the spectrum but also the ratio of energy to photons.
Wavelength alone says nothing about whether an application will work. What matters is whether the substance absorbs at that wavelength: a photoinitiator absorbing in the UVA responds to 365 nm and remains largely unaffected at 254 nm, even though the individual photons there carry more energy. Nor does placing a wavelength replace the safety assessment — the hazard depends on the action spectrum, not on the name of the range.
Frequently asked questions
Under DIN 5031-7: UVC from 100 to 280 nm, UVB from 280 to 315 nm, UVA from 315 to 400 nm. The visible range already begins at 380 nm and therefore overlaps the upper UVA. Other bodies of rules put the boundary between UVA and visible at 400 nm — in borderline cases it is worth checking which standard is being applied. More under spectral ranges and standards.
Through E[eV] = 1239.842 / λ[nm]. 254 nm gives 4.88 eV, 365 nm 3.40 eV, 405 nm 3.06 eV. The number is the product of the Planck constant and the speed of light, divided by the elementary charge.
Wavenumber σ = 1/λ, given in cm⁻¹, is directly proportional to energy. In spectroscopy the distance between bands is therefore quoted in wavenumbers rather than nanometres: a gap of 4670 cm⁻¹ means the same energy difference everywhere, a gap of 75 nm does not.
254 nm is the resonance line of the low-pressure mercury lamp — a very narrow line from a very efficient source. 265 nm lies closer to the absorption maximum of DNA and is produced by UVC LEDs, which typically have a bandwidth of 10 to 12 nm. Achieving the same germicidal effect therefore requires different doses; converting between them needs the action spectrum.
Not without testing. The photoinitiator has to absorb at the new wavelength, and the number of photons per joule changes with it. In practice the process release has to be repeated, with a dose measurement at the location of the workpiece. The calculation is in the UV dose calculator.
Below 200 nm a VUV sensor with nitrogen purge; in the UVC a spectrally weighted sensor — DVGW-compliant for disinfection systems; in the UVA a UVA sensor or a radiometer with the matching head; in the visible a photometer or a spectroradiometer. The explorer names the matching solution for every wavelength; the quantities behind it are explained under radiometric quantities.
Author: Dr. Mark Paravia
Dr.-Ing. Mark Paravia is the managing director of Opsytec Dr. Gröbel GmbH in Ettlingen and heads the accredited calibration laboratory. Following his research on pulsed xenon excimer discharges at the Institute of Lighting Technology at KIT, his current focus is on optical radiation measurement technology. He is vice-chair of the DIN Standards Committee FNL 7 “Optical Radiation” and a member of the DVGW Project Group on UV Disinfection.
