How to Calculate the UV Index

The sun is an essential component of all life on Earth. However, it can also cause harm: the UV rays (the sun’s radiative ultraviolet spectrum) can harm the skin, with both short-term and long-term effects.

Calculating the UV Index

Image Credit: Daniel Hansen/Unsplash

In this article, OTT HydroMet gives an overview of the energy-rich sunlight spectrum, examining ways in which it can be accurately measured.

Sunlight is comprised of electromagnetic radiation, in which photons span over a broad wavelength range. Human eyesight can only detect a minor part of the sun’s spectrum – the average spectrum that can be seen by the naked eye is within the range of 400 to 780 nm (10^-9 m). The 100 to 400 nm range encompasses the ultraviolet (UV) part with shorter wavelengths (and thus more energy).

The stimulation of Vitamin D production is only one of many beneficial effects of UV light, but this spectrum can also have harmful effects should the radiation exceed “safe” limits. The UV index is the gold standard for indicating UV exposures and warns against unsafe limits of UV and its possible detrimental effects.

A Complete Overview of the UV Index

Solar and Atmospheric Radiation Wavelengths

The spectrum range, which is the most meteorologically significant, ranges between 300 nm to 3000 nm (known as short-wave radiation). This spectral range hosts approximately 96% of the complete extra-terrestrial radiation. The maximum radiation intensity of the solar spectrum occurs towards the blue end of the visible range, at 500 nm.

Wavelengths of solar and atmospheric radiation.

Wavelengths of solar and atmospheric radiation. Image Credit: OTT HydroMet

Ultraviolet (UV), visible (Vis) and infrared (IR) wavelengths all combine to comprise the complete spectrum. However, it is essential that these wavelength ranges are further sub-divided, which is contingent on the individual application fields.

The prismatic colors of visible light are, of course, the most widely known – colloquially known as “the colors of the rainbow”. Infrared wavelength (IR) is split into near infrared (NIR) and far infrared (FIR).

Examining the UV Spectrum of Light

The split of UV is more complex: typically sub-divided into three categories of UV-A, UV-B and UV-C radiation. It is estimated that 6% of the total solar radiation which falls on the Earths ultraviolet. Higher energy is possessed by the shorter wavelengths (higher frequency), which therefore increases the effect on chemical and biological systems.

Various radiometers are on offer from Kipp & Zonen for the purpose of taking UV measurements – each of which is tailored for specific parts of the spectrum.

Radiation exposure categories of Global Solar UV Index.

Radiation exposure categories of Global Solar UV Index. Image Credit: OTT HydroMet

The Global Solar UV Index (UVI) standard is utilized for skin health applications. UVI was developed in a truly international effort by the World Health Organization (WHO), which collaborated with the United Nations Environment Programme (UNEP, the World Meteorological Organization (WMO), and the International Commission on Non-Ionizing Radiation Protection (ICNIRP).

Global Solar UV Index: Radiation Exposure Categories

An instrument with a spectral response measures the ‘harmful’ UV radiation level at the Earth’s surface, which has been specially designed to represent a ‘standard’ human skin affected by defined parts of the UVB and UVA spectrum.

Erythemal irradiance (UVE) is made up of approximately 83% UVB and 17% UVA for a clear sky around solar noon.

UVE is measured in W/m2 by the Kipp & Zonen SUV-E radiometer and multiplied by 40 m²/W to convert the value to the UV Index.

Different groups were established according to the skin’s ability to tan because skin types can differ considerably in terms of their sensitivity to UV doses.

Skin type classification.

Skin type classification. Image Credit: OTT HydroMet

Calculating the UV Index

The UV Index serves a further purpose: as an important way in which to raise public awareness, alerting the public about the importance of and vital need for the adoption of protective measures when exposed to UV radiation which can be harmful. 

The UV-Index is calculated as follows from UV-E radiation measurements:

The output must be taken from the UV-E radiometer according to ISO 17166:1999/CIE S007/E-1998. Use the instrument’s sensitivity to convert the output voltage to W/m², for instance, 0.4 Volt, which is closely linked to an erythemal radiation value of 0.0675 W/m². That then needs to be multiplied by a factor of 40 m²/W to get the UVI value, in this case, 2.7.

Calculating the UV Index

Image Credit: OTT HydroMet

The UV radiometer needs to mimic human skin because sunburn is perhaps one of the most common and widely-known consequences of excessive UV radiation exposure. This is why a special Erythemal action spectrum was outlined, which parallels the sensitivity of the human skin to UV radiation.

Special filters are attached to the Kipp & Zonen SUV-E radiometer which matches the Erythemal action spectrum.

To measure different parts of the UV spectrum, each model of the Kipp & Zonen SUV radiometer series is adjusted. The whole portfolio of Kipp & Zonen SUV radiometers can be found here.

Contact OTT HydroMet's experts today to find out more.

This information has been sourced, reviewed and adapted from materials provided by OTT HydroMet.

For more information on this source, please visit OTT HydroMet. Kipp & Zonen is one of OTT HydroMet's strong brands for professional environmental monitoring.

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