The WCC-UV hosts a UV laboratory consisting of two solar UV spectroradiometers, a spectral and an angular response facility. For absolute calibrations and for irradiance calibrations dedicated setups are available. The total ozone column and aerosol optical depth in the UV range are recorded continuously.
The traveling spectroradiometer Qasume
installed at El Arenosillo, Spain.
The instrument of choice for the measurement of absolute spectral solar radiation is a well characterized spectroradiometer. At PMOD/WRC the reference instrument is the transportable reference spectroradiometer Qasume. The instrument consists of a Bentham double monochromator DM-150. The irradiance is sampled by a diffusing head built by CMS Schreder. A 6 m fiber cable connects the input head to the entrance of the monochromator.
This reference instrument was validated in the European project Q.A.S.U.M.E.. The aim of the project was the development of a new approach for facilitating Quality Assurance of solar Spectral Ultraviolet irradiance Measurements carried out in Europe (Qasume). The reference spectroradiometer (commonly called Qasume) is made available by the Joint Research Centre of the European Commission in Ispra under the cooperation agreement 2004-SOCP-22187.
The second spectroradiometer of the WCC-UV is the double monochromator Brewer #163. This instrument is mainly designed to measure the total ozone column. It is also capable of recording spectral direct solar UV irradiance and global solar irradiance in the wavelength range from 285 to 365 nm. The instrument was equipped with a diffusing entrance optic. During 2008 the Brewer #163 will be modified to measure absolute and polarized solar sky radiance.
response measurement setup
of the WCC-UV
The relative spectral response facility consists of a Bentham double monochromator DM-150. The wavelength can be selected within the range 250 to 500 nm with a precision of 0.1 nm and the slit width was chosen to yield a nearly triangular slit function with a FWHM of 1.92 nm. A 150 W Xenon lamp positioned in front of the entrance slit acts as radiation source.
The wavelength scale of this monochromatic source facility was determined by measurements of selected spectral emission lines of a mercury lamp. In addition we characterized the source in its usual operating state using the Qasume reference spectroradiometer as reference detector.
The angular response
setup of the WCC-UV
The angular response function (ARF) of a radiometer is measured on a 3 m long optical bench. A 1000 W Xenon lamp mounted at one end of the optical bench serves as radiation source. The detector is mounted on a goniometer at the other end of the optical bench. The resolution of the rotation stage is 29642 steps per degree, or 0.12 arcseconds.
The setups for the irradiance calibration are shown in the pictures below. The basis of the irradiance scale consists of a set of 1000 W FEL lamps traceable to the primary irradiance standard of the "Physikalisch-Technische Bundesanstalt (PTB)", Germany. This irradiance reference has become the de-facto standard for spectral UV measurements in Europe. DXW-type 1000 W lamps (vertical calibration, middle picture) or any other transfer standards can be calibrated relative to the Qasume irradiance reference using the Qasume spectroradiometer. An example is shown in right picture using the transportable calibration unit from CMS Schreder loaded with a 250 W lamp.
The absolute calibration of a broadband filter radiometer should be performed with a radiation source having a similar spectral distribution to the radiation for which the radiometer will be used. For the case of solar measurements this requirement implies that the calibration has to be done using the sun as a source since the spectral distribution and variability of the surface solar radiation cannot be simulated in the laboratory with the required accuracy. The platform for the absolute calibration at the PMOD/WRC is setup up on the roof of the institute. The following pictures give an impression of an outdoor calibration with over 30 broadband radiometers in summer 2007.
One of the UV-PFR installed
on a Brusag-tracker on the roof of PMOD/WRC
Two UV Precision Filter Radiometers (UV-PFR) with narrowband interference filters are used to measure direct solar irradiance at fixed wavelengths. The radiometers are built at PMOD/WRC for very accurate aerosol optical depth measurements (see WORCC at the PMOD/WRC). The instrument has four channels, three in the UV-B range with center wavelengths at 303, 310 and 319 nm and one channel in the UV-A range at 340 nm with the respective bandwidths of 1, 2, 2 and 4 nm.