UV-A and UV-B: broadband sensors for meteorological applications

UV-A and UV-B: broadband sensors for meteorological applications

LSI Lastem UV-A and UV-B sensors are broadband radiometers for the measurement of atmospheric radiation in the UV-A and UV-B spectrum according to the description of WMO n.8 chap.7.

The sensing element is a photodiode with an optical filter with interferential deposition, which improves the spectral transmission. A high-quality fairing and diffuser improve the cosine response for incident radiation with low angles of incidence. Since the two sensors have a broadband spectral response, which manages to cover the entire UV-A and UV-B spectrum, they are suitable for outdoor measurements, where the source is the sun, while other sensors on the market are characterized by narrower spectral response, which make them more suitable for measurements of radiation from artificial sources (indoor measurements).

The uncertainty of the instantaneous reading was defined after the

calibration of a reference sensor performed by ARPA Ivrea. Thanks to this calibration it was possible to define the daily uncertainty, obtained from the integral of the daily radiation, equal to 12% for the UV-A sensor and 15% for the UV-B. The uncertainty given by the angular response is good, equal to ± 8%, as required by the WMO (<10% for angles of incidence <60°). One of the strong points of the sensors, in addition to the documented uncertainty, is the calibration under the sun by comparison with the reference sensors calibrated at ARPA Ivrea, a process that can last from 1 to several days.

Even if UV-A and UV-B radiometers are not expressly declared instruments for measuring erythemal spectral irradiance, by measuring the two ultraviolet components, it is possible to have an estimate of the UV Index, with an error within 22%. The index is obtained through an empirical formula, derived from the formula for the UV index dictated by the World Health Organization (http://www.who.int/uv/publications/globalindex/en/), calibrated on the data of the JRC (Joint Research Center) of ISPRA (European reference for ultraviolet measurements) and inserted on board LSI Lastem data loggers. The universal index of UV radiation (UVI) indicates the level of solar ultraviolet radiation that reaches the earth’s surface in a certain area. The UV index varies from zero upwards and indicates the growing potential damage to the skin and eyes caused by sun exposure. The index was created, in order to raise awareness of the risks of solar exposure, as part of a collaboration between the World Health Organization (WHO), the United Nations Environment Program (UNEP), the World Meteorological Organization (WMO) and the International Commission for Non-Ionizing Radiation Protection (ICNIRP).

Valeria Menichini

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