Sunlight has both positive and negative effects; the sun itself is the source of life on Earth, but it can be a hazard, too. Specifically, the ultraviolet spectrum of the sun’s radiation (UV) can cause real damage to people with lighter skin types.
Image Credit: OTT HydroMet
This article outlines the energy-rich end of the sunlight and the technical challenges faced when measuring it.
Electromagnetic radiation that emanates through sunlight is filled with photons cast over a broad wavelength range. Humans can only see a small part of the sun’s spectrum (visible light spectrum) which is in the average range from 400 to 780 nm (10^-9 m). The ultraviolet (UV) part starts at much shorter wavelengths (and more energy) spanning 100 to 400 nm.
UV light has a number of beneficial effects, as it promotes the production of vitamin D, but it may also be very harmful if exposure to UV radiation exceeds “safe” limits. The UV index is a guide for measuring UV exposures as it warns against UV radiation and any potential harmful effects.
Wavelengths of Solar and Atmospheric Radiation
The meteorologically significant spectral range extends from 300 nm to 3000 nm (short-wave radiation). Around 96% of the all extra-terrestrial radiation sits within this spectral range. The maximum intensity of the solar spectrum radiation occurs at 500 nm, which is close to the blue end of the visible range.
Wavelengths of solar and atmospheric radiation. Image Credit: OTT HydroMet
The complete spectrum is made up of ultraviolet (UV), visible (Vis) and infrared (IR) wavelengths. However, the ranges of each of these wavelengths must be sub-divided depending on the specific application fields.
Amongst these are the prismatic colors, the colors of the rainbow, the most well-known visible light. IR is divided into near infrared (NIR) and far infrared (FIR).
A Look at the UV Spectrum of Light
UV is typically subdivided into UV-A, UV-B and UV-C radiation. Around 6% of all solar radiation that reaches Earth is ultraviolet. Shorter wavelengths (higher frequency) have higher energy, thus enhancing the impact on biological and chemical systems.
Kipp & Zonen offers a range of radiometers for UV measurements, which can be customized for certain parts of the spectrum.
For applications in skin health, there is a universal standard called Global Solar UV Index (UVI).
This standard was created as part of a collaborative international effort by the World Health Organization (WHO), the United Nations Environment Programme (UNEP), the World Meteorological Organization (WMO), and the International Commission on Non-Ionizing Radiation Protection (ICNIRP).
Radiation exposure categories of Global Solar UV Index. Image Credit: OTT HydroMet
To measure ‘harmful’ UV radiation levels at the Earth’s surface, an instrument with a spectral response is used that acts as a ‘standard’ for human skin affected by specific parts of the UVB and UVA spectrum. This Erythemal irradiance, UVE, is usually comprised of about 17% UVA and 83% UVB for a clear blue sky around solar noon.
UVE is measured in W/m2 by the Kipp & Zonen SUV-E radiometer and multiplied by 40 m²/W to translate the value to the UV Index.
Due to the fact no two skin types are the same, especially regarding their sensitivity to UV doses, a grouping system was established according to the skin’s ability to tan.
Skin type classification. Image Credit: OTT HydroMet
How to Calculate the UV Index
The UV Index acts as a critical guide to raise public awareness and to make people really consider the need to embrace protective measures if and when exposed to harmful UV radiation.
Using UV-E radiation measurements, the UV-Index is calculated as follows:
Extract the output from the UV-E radiometer in accordance with ISO 17166:1999/CIE S007/E-1998. Convert the output voltage to W/m² using the instrument’s sensitivity, for instance, 0.4 Volt, which refers to an erythemal radiation value of 0.0675 W/m².
Multiplying it by the factor of 40 m²/W will get the UVI value, in this instance, 2.7.
Image Credit: OTT HydroMet
As sunburn is a direct consequence of too much exposure to UV radiation, the UV radiometer should mimic that of the human skin. Therefore, a particular Erythemal action spectrum was developed, which relates to the sensitivity of the human skin on UV radiation.
The Kipp & Zonen SUV-E radiometer is fitted with special filters which equal the Erythemal action spectrum. Each model of the Kipp & Zonen SUV radiometer series is configured to measure various parts of the UV spectrum. See the complete portfolio of Kipp & Zonen SUV radiometers.
If you have any questions on UV measurement and how to incorporate it into a monitoring system, contact Kipp & Zonen experts today.
This information has been sourced, reviewed and adapted from materials provided by OTT HydroMet.
For more information on this source, please visit OTT HydroMet.