Choosing the Right UV Radiometer
Published: Friday, July 19, 2013
The UV part of the solar spectrum covers the wavelength ranges 100 to 280 nm (UVC), 280 to 315 nm (UVB) and 315 to 400 nm (UVA). Almost all UVC from the sun, and approximately 90 % of UVB, is absorbed by the Earth’s atmosphere. UVA radiation at the Earth’s surface is normally 15 to 20 times greater than UVB.
The World Meteorological Organisation and the World Health Organisation define the boundary between UVA and UVB as 315 nm. However, some other organisations, particularly in the USA, still use the older boundary definition of 320 nm. This makes a significant diffrence to the amount of UVB measured and must be taken into account when comparing data from different published information and from different UV sensors. Nowadays UV irradiance is always measured in W/m².
UVA and UVB measurements are mainly used to monitor and investigate the effects of solar UV radiation on plants and animals. In materials testing important issues are the ageing effects of outdoor UV exposure that cause degradation, such as brittleness and discolouration. Measurements are also carried out under controlled conditions in environmental test chambers using artificial UV light sources. For these applications the UVS-A-T, UVS-B-T and UVS-AB-T are most suitable.
UV measured with a similar response to the human skin is termed Erythemally Active UV irradiance (UVE). In the past this ‘harmful UV’ was measured with a number of different response functions and several countries had their own UV Index scales. Measurement terms such as ‘Minimum Erythemal Dose’ (MED) and exposures in MED/hr were often used but these are not well defined or standardised.
To avoid this confusion, United Nations organisations combined to produce the Global Solar UV Index (UVI). This is now accepted world-wide as the basis for public health. information. The erythemal spectral response function is defined by ISO: 17166:1999 / CIE S 007/E-1998. The Global Solar UV Index can be calculated by multiplying the UVE radiation value in W/m² by 40 m²/W. For example, 0.25 W/m² of UVE represents a UV Index of 10. A UVB radiometer is not ideal for the measurement of UVE. The UVS-E-T and UVS-AE-T radiometers are specifically designed for this application.
The United Nations ‘Practical Guide to the Global Solar UV Index’ can be found at:
www.unep.org/pdf/Solar_Index_Guide.pdf
The amount of UVB and UVE radiation reaching the ground is strongly dependent upon altitude, the height of the sun in the sky, the amount of Ozone in the atmosphere and cloud cover. UVS radiometers are calibrated for a typical air-mass (solar zenith angle) and Ozone column concentration. Our unique UVIATOR software supplied with the UVS further improves the accuracy of the measurements by correcting for the amount of Ozone in the atmosphere and the solar elevation.