We have realized a multispectral prototype based on a wheel with eleven band pass filters to be used to carry out imaging in the UV band, MUSKY. A fisheye catadioptric imaging system is used to measure the irradiance of 2pi emissivity of the sky in the UV band. This work has been supported by the European Metrology Research Programme (EMRP) within the jooint research project ENV03 “Traceability for surface spectral solar ultraviolet radiation” (SolarUV). The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. In Fig. 1 the experimetal layout of the prototype.
Result:: Description of the prototype
The Multi-spectral UV camera is based on a UV sensitive CCD Camera equipped with a motorized filter wheel holding 11 filters having 10 nm nominal width and being uniformly distributed in the 300 – 400 nm range. See Fig. 2 for the normalized trasmittance measured with a Ocean spectrophotometer.
The objective and the camera are commercial and are designed to work in the UV. The spatial resolution is better than 1 square degree per pixel.The picture of the prototype is presented in Fig 3.
The mirror has a glass substrate and is coated with aluminum protected with quartz. The diameter of the mirror is 300 mm and the curvature radius is 262 mm.
Fig. 2. Spectral transmittance of the 11 bandpass filters mounted on MUSKY
Fig. 3. The picture of the prototype with the wheel with 11 bandpass filters, the UV objective and the camera
Result:: Calibration
The spectral response of the system depends on the reflectance of the mirror, on the trasmittance of the filters and of the objective, on the responsivity of the camera. Moreover the spectral respose of the system depends on the angle of reflection. Therefore the spectral and spatial response has to be calibrated against a reference spectrogoniometer. We calibrated it against the reference spectrogoniometer installed on the roof of Innsbruck Medicine University managed by Prof. Mario Blumthaler.
In Fig. 4 the picture of the sky reflected by the mirror and imaged on the camera.
The MUSKY has acquired several series of multispectral pictures and at the same time the reference spectrometer acquired a multitude of spectra in different portions of the sky (Fig. 5). Later the results have been elaborated by comparing the portion of the multispectral images corresponding to the portion recorded by the reference instrument at about the same time.
In Fig. 5 a measurement set made from 33 pictures. Each row is the wavelength series decreasing from left to right from 400 to 300 nm. The exposure time is respectively 1, 10 and 100 ms for the three rows.
Fig. 4. The picture of the sky reflected by the mirror and imaged on the camera
Fig. 5. Measurement set made from 33 pictures. Each row is the wavelength series decreasing from left to right from 400 to 300 nm. The exposure time is respectively 1, 10 and 100 ms for the three rows
Result:: Innsbruck
In Fig 6. Musky on the roof of Innsbruck Medicine University
In Fig 7. A picture of the team involved, from the left: Prof. Mario Blumthaler, Dr. Mario Pisani and Dr. Massimo Zucco .
In Fig 6. Musky on the roof of Innsbruck Medicine University.
In Fig 7. A picture of the team involved, from the left: Prof. Mario Blumthaler, Dr. Mario Pisani and Dr. Massimo Zucco