May 2006Analytical Instrumentation & Environmental Monitoring
The LP UVB 02 radiometer measures the global irradiance in the UVB spectral region on a plane surface (W/m2). The instrument's spectral sensitivity is centred at 305 nm with a 5 nm bandwidth (FWHM). The global irradiance is the sum of the direct solar irradiance and the sky diffuse irradiance on a surface parallel to the ground. It should be noted that in contrast to the visible spectrum, where the direct component prevails over the diffuse component, in the UVB spectral region light is strongly diffused by atmosphere and thus the two components are equivalent. Therefore it is of primary importance for the instrument to be capable of measuring both components accurately. This instrument is typically used in the following sectors:
* Monitoring the ozone layer: The radiation around 295 nm to 315 nm is particularly well absorbed by the ozone located in the stratosphere, so increases and decreases in the UV radiation reaching the ground give an indication of the condition of the ozone layer.
* UVB radiation measurement in work spaces - where over-exposure to UVB radiation may be possible.
The LP UVB 02 radiometer requires power to function, as the weak signal from the photodiode needs to be amplified by the built-in current/voltage amplifier (transimpedance amplifier). The need to use sophisticated filters (unavoidably causing some attenuation of the wanted signal) and the relatively weak sun-produced irradiation in this spectral area (in the best case) causes the photodiode-generated current to be only in the order of hundreds of picoamperes. For this reason the signal must be amplified at its source.
The LP UVB 02 is robust and is designed to operate for long periods without maintenance, making it suitable for meteorological sites. A platinum-resistance thermometer (Pt100) is inserted inside the LP UVB 02 in order to monitor its temperature, which must remain within quoted limits in order to achieve the accuracy quoted in the specifications. (Exposure to temperature higher than +60°C can alter the interferential filters spectral characteristics.)
Working principle
The LP UVB 02 radiometer is based on a solid state photodiode, the spectral response of which was adapted to that desired by using special interferential filters. The chosen photodiode and its associated filters exhibit stable characteristics, both over the operating temperature range and over long time periods. This permitted the design to be implemented in a manner that did not require heating, thus reducing energy consumption.
Particular attention has been given to filter design so as to make the instrument essentially blind to wavelengths outside the desired band of interest. (Bear in mind that the solar energy within the 302 nm to 308 nm spectral band is only 0,01% of the total energy from the sun reaching Earth's surface.) The instrument features a refillable silica gel cartridge to absorb any trapped moisture within the device and prevent any condensation on the inside of the sensor's dome window - as this would result in reading errors.
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