In the solar spectrum, the UV section is represented by the ranges of wavelength from 100 to 280 nm (UVC), 280 to 315 nm (UVB) and 315 to 400 nm (UVA). An estimated 90% of UVB and almost all of the UVC from the sun is absorbed by the atmosphere of the Earth.
At the Earth’s surface, UVA is typically 15 to 20 times higher than UVB.
Image Credit: OTT HydroMet
The World Health Organization and the World Meteorological Organization determine the partition between UVA and UVB as 315 nm. Certain organizations, especially in the USA, continue to utilize the older limit definition of 320 nm.
This represents a large variation in the quantity of UVB measured and should be considered when contrasting data from various UV sensors and different published data. At present, UV irradiance is consistently measured in W/m².
UVB and UVA testing are predominantly used to investigate and monitor the impact of solar UV radiation on animals and plants. In the testing of materials, key topics are the aging consequences of outdoor UV exposure that lead to a reduction in quality, for example, in discoloration and brittleness.
Tests are also performed in controlled specifications in environmental test chambers through the use of artificial sources of UV light. The UVS-B-T, UVS-AB-T and UVS-A-T are the most practical for these applications.
Erythemally Active UV irradiance (UVE) is the term for UV that is quantified to determine an equivalent response to human skin. Historically, this ‘harmful UV’ was tested with various specific response functions, and many countries had an individual UV Index scale.
Measurement terms like exposures in MED/hr and ‘Minimum Erythemal Dose’ (MED) were frequently employed, but these were not standardized or properly defined.
Organizations in the United Nations worked together to create the Global Solar UV Index (UVI) to provide clarity on this issue. This has now been internationally adopted as the foundation for public health data. ISO: 17166:1999 / CIE S 007/E-1998 defines the erythemal spectral response function.
The multiplication of the UVE radiation value in W/m² by 40 m²/W provides the Global Solar UV Index. For instance, a UVE value of 0.25 W/m² can be converted to a UV Index of 10. UVE cannot be appropriately measured by a UVB radiometer. The SUV-E UVE radiometer is specially tailored to this application.
The quantity of UVB and UVE radiation that reaches the ground is heavily reliant upon the height of the sun in the sky, the amount of Ozone in the atmosphere, cloud cover and altitude.
Kipp & Zonen SUV radiometers are calibrated for Ozone column concentration and a typical air mass (solar zenith angle).
The exclusive UVIATOR software from OTT HydroMet delivered with the SUV series further enhances the measurement precision by correcting for the amount of solar elevation and the level of Ozone in the atmosphere.
This information has been sourced, reviewed and adapted from materials provided by OTT HydroMet.
For more information on this source, please visit OTT HydroMet.