The optical calibration facility at IZO has been developed within the framework of the Specific Agreement of Collaboration between the University of Valladolid and the IARC-AEMET: “To establish methodologies and quality assurance systems for programs of photometry, radiometry, atmospheric ozone and aerosols within the atmospheric monitoring programme of the World Meteorological Organization”. The main objective of the optical calibration facility is to perform Quality Assurance & Quality Control (QA/QC) assessment of the solar radiation instruments involved in the ozone, aerosols, radiation, and water vapour programs of the IARC.
The set-ups available are the following:
- Set-up for the absolute radiance calibration by calibrated integrating sphere. The system is traceable to the AERONET standard at the Goddard Space Flight Center (Washington, USA). This set-up is mainly used by Cimel sun-photometers, but other instruments are also calibrated. At the end of 2017, in the framework of a competitive scientific infrastructure call of the National Plan for Research, Development and Innovation of Spain, a new integrating sphere was installed at Izaña for AERONET-Europe absolute radiance calibrations as well as optical tests required for QC/QA of reference instruments. The integrating sphere has a 20 inches diameter, an 8 inches aperture. This system is also traceable to the AERONET standard at the Goddard Space Flight Center.
- Set-up for the alignment of the Brewer spectrophotometer optics. It is suitable to perform adjustments of the optics without sending the instrument to the manufacturer.
- Set-up for the slit function determination. The characterization of the slit function is performed illuminating the entrance slit of the spectrophotometer with the monochromatic light of a VM-TIM He-Cd laser. The nominal wavelength of the laser is 325 nm, its power is 6mW, and its beam diameter is 1.8 mm.
- Set-up for the angular response calibration. It is used to quantify the deviations of the radiometer’s angular response from an ideal cosine response. The relative angular response function is measured rotating the mechanical arm where the seasoned DXW-type 1000 W lamp is located. The rotation over ±90° is controlled by a stepper motor with a precision of 0.01° while the instrument is illuminated by the uniform and parallel light beam of the lamp.
- Set-up for the absolute irradiance calibration by calibrated standard lamps in a vertical oriented position suitable for relatively large spectrophotometers. The basis of the absolute irradiance scale consists of a set of DXW-type 1000 W lamps traceable to the primary irradiance standard of the PTB (Physikalisch-Technische Bundesanstalt).
- Set-up for the absolute irradiance calibration by calibrated standard lamps in a horizontally oriented position suitable for instruments different.
- Set-up for the spectral response calibration. It is used to quantify the spectral response of the radiometer. The light is scattered by an Optronic double monochromator OL 750 within the range 200 to 1100 nm with a precision of 0.1 nm. An OL 740-20 light source positioned in front of the entrance slit acts as radiation source and two lamps, UV (200-400nm) and Tungsten (250-2500nm) are available.
