Ultraviolet (UV-A, UV-B, UV-C, VUV, EUV)
Radiation from about 10 to 400 nm, divided by the CIE into UV-A (315–400 nm), UV-B (280–315 nm) and UV-C (100–280 nm), with vacuum UV below 200 nm and extreme UV from about 10 to 121 nm. Its 3 to 124 eV photons break chemical bonds; EUV lithography runs at 13.5 nm.
Ultraviolet light begins just beyond the violet end of the visible, at about 400 nm, and extends to about 10 nm, where it merges with soft X-rays. Its photon energies, from 3.1 eV to 124 eV, are high enough to excite and break chemical bonds, which makes the ultraviolet useful for curing, lithography, sterilization and spectroscopy, and harmful to skin, eyes and many materials.
Bands
| Band | Range | Notes |
|---|---|---|
| UV-A | 315–400 nm | Passes most window glass; blacklights, curing, 365 nm mercury i-line |
| UV-B | 280–315 nm | Causes sunburn; mostly absorbed by stratospheric ozone |
| UV-C | 100–280 nm | Germicidal (254 nm mercury line); fully absorbed by the atmosphere |
| Vacuum UV (VUV) | 10–200 nm | Absorbed by oxygen, so optics must be in vacuum or purged with nitrogen |
| Extreme UV (EUV) | 10–121 nm | Absorbed by almost everything; reflective optics only |
UV-A, UV-B and UV-C are the CIE's photobiological bands. The VUV and EUV limits vary between sources; ISO 21348 places EUV at 10 to 121 nm. Photon energies at the boundaries are 3.10 eV at 400 nm, 3.94 eV at 315 nm, 4.43 eV at 280 nm and 6.20 eV at 200 nm.
Sources and uses
Semiconductor lithography has moved down the spectrum to print smaller features: the mercury i-line at 365 nm, KrF excimer lasers at 248 nm (5.0 eV), ArF at 193 nm (6.4 eV), and EUV at 13.5 nm (91.8 eV), generated by tin plasma and imaged with multilayer Mo/Si mirrors because no material is transparent there. AlGaN and GaN LEDs now cover the UV-A and the long-wavelength end of the UV-C, near 255 to 280 nm, and are replacing mercury lamps for curing and disinfection. Frequency-tripled and quadrupled solid-state lasers at 355 and 266 nm are used for micromachining and inspection.
Materials and detection
Ordinary optical glass such as N-BK7 starts to absorb below about 350 nm and is nearly opaque by 300 nm; UV-grade fused silica transmits to about 180 nm, calcium fluoride and magnesium fluoride further into the VUV. Many coatings, adhesives and plastics degrade under UV exposure. Silicon photodiodes need UV-enhanced versions below about 400 nm, and photomultipliers with suitable windows, silicon carbide and AlGaN photodiodes, and phosphor-coated cameras are used in the deep and vacuum UV. UV exposure limits for skin and eyes are set in the ICNIRP guidelines and IEC 62471.
References: CIE S 017:2020, ILV: International Lighting Vocabulary; ISO 21348:2007, Space environment: Process for determining solar irradiances; ICNIRP, Health Phys. 87, 171 (2004).