What is UV radiation?

UV radiation is electromagnetic radiation with wavelengths below visible light. According to the International Lighting Vocabulary (CIE S 017/E:2020) it is divided into UV-A from 315 to 400 nm, UV-B from 280 to 315 nm and UV-C from 100 to 280 nm. It is invisible to the eye and can only be recorded with radiometers or spectroradiometers.

The abbreviation “UV” stands for ultraviolet radiation. This is often identified as UV light, since “light” generally sounds less dangerous than radiation. By definition we identify radiation, which is visible to the human eye, as light. So strictly speaking, UV light does not really exist.  That is an important fact for the user, as users cannot perceive the real danger. The possibly visible light is merely luminescence, which is induced by UV. This luminescence is often much weaker and less dangerous than the inducing UV radiation. For this reason, the measurement of ultraviolet radiation is important. Therefore our precise UV radiometers can be used.

Spectral ranges UV-a, UV-B and UV-C

The UV radiation is divided into three spectral ranges. UVA covers the spectral range from 315 nm to 400 nm. In US sources, the spectral range 320–400 nm is also referred to as UVA. The UVA spectral range is technically often used for the UV adhesive bonding and curing of varnishes. In addition, the luminescence and fluorescence tests are often carried out with UVA (here it is defined by the term black light).

* The limits of visible radiation are not sharply defined because the sensitivity of the eye fades out gradually at both ends. According to CIE S 017/E:2020, the lower limit is generally taken to be between 360 nm and 400 nm and the upper limit between 760 nm and 830 nm. In practice, 380 nm to 780 nm is usually used.

The range from 380 to 400 nm therefore belongs to UV-A and to visible radiation at the same time. That is not an error but the result of two different reference quantities: the UV range is defined by wavelength, visible radiation by the sensitivity of the eye – and that does not stop abruptly.

Then, in a short wave range, there is the UVB spectral range. The UVB radiation is richer in energy than the UVA radiation and can lead to skin damage during longer exposure times. Technically, the UVB radiation is used less often than the UVA or UVC radiation, however it plays an important part in the photostability, i.e. in the UV aging or weathering, since the natural sunlight contains UVB radiation to a minor degree.

UVC radiation is a radiation between 100 nm and 280 nm. Since the air absorbs UVC radiation below ca. 200 nm, this is often considered as the lower limit wavelength. UVC radiation can efficiently be generated by mercury low pressure lamps. The UV radiation yield is up to ca. 40 %, with an electrical power in the range of several hundred watts. The dominant emission is approx. 254 nm. Important applications are the disinfection of surfaces, as well as food packages and the disinfection of potable water. For the UV polymerization, the Hg medium pressure lamp with an electrical power of between ca. 400 W and 24 kW is widespread. Here, the high power allows for high speeds and process times within split seconds.

The explorer below places any wavelength between 150 and 800 nm in the ranges named above and gives frequency, photon energy and wavenumber — together with the typical source and the matching measurement equipment.

The following figure shows the different types of electromagnetic radiation and their properties:

1. Ultraviolet radiation (UV)
   • Subdivided into vacuum UV (VUV) and far UV (FUV) – together UV-C –, mid UV (UV-B) and near UV (UV-A)
   • Wavelength range: 100 nm to 400 nm
   • Photon energy: 12.4 eV to 3.1 eV
   • Typical detectors: CsI, Cs₂Te, SiC
2. Visible radiation (VIS)
   • Wavelength range: 380 nm to 780 nm
   • Photon energy: 3.3 eV to 1.6 eV
   • Typical detectors: multi-alkali photomultiplier (PMT), Si
3. Infrared radiation (IR)
   • Subdivided into near IR (IR-A, IR-B), mid IR and far IR (together IR-C)
   • Wavelength range: 780 nm to 1 mm (10⁶ nm)
   • Photon energy: 1.6 eV to 0.001 eV
   • Typical detectors: Si, Ge, InGaAs, InAs, PbSnTe, PbSnSe

Vacuum UV carries that name because air absorbs radiation below roughly 200 nm strongly: oxygen is split and ozone forms. Measurements in this range therefore require nitrogen purging or a vacuum. The part of it used in practice is the mercury line at 185 nm – the basis of UV ozone generation for surface cleaning and AOP.