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| Titel |
Calibration of an all-sky camera for obtaining sky radiance at three wavelengths |
| VerfasserIn |
R. Román, M. Antón, A. Cazorla, A. Miguel, F. J. Olmo, J. Bilbao, L. Alados-Arboledas |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 5, no. 8 ; Nr. 5, no. 8 (2012-08-21), S.2013-2024 |
| Datensatznummer |
250003053
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| Publikation (Nr.) |
 copernicus.org/amt-5-2013-2012.pdf |
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| Zusammenfassung |
This paper proposes a method to obtain spectral sky radiances, at three
wavelengths (464, 534 and 626 nm), from hemispherical sky images. Images are
registered with the All-Sky Imager installed at the Andalusian Center for
Environmental Research (CEAMA) in Granada (Spain). The methodology followed
in this work for the absolute calibration in radiance of this instrument is
based on the comparison of its output measurements with modelled sky
radiances derived from the LibRadtran/UVSPEC radiative transfer code under
cloud-free conditions. Previously, in order to check the goodness of the
simulated radiances, these are compared with experimental values recorded by
a CIMEL sunphotometer. In general, modelled radiances are in agreement with
experimental data, showing mean differences lower than 20% except for the
pixels located next to the Sun position that show larger errors.
The relationship between the output signal of the All-Sky Imager and the
modelled sky radiances provides a calibration matrix for each image. The
variability of the matrix coefficients is analyzed, showing no significant
changes along a period of 5 months. Therefore, a unique calibration matrix
per channel is obtained for all selected images (a total of 705 images per
channel). Camera radiances are compared with CIMEL radiances, finding mean
absolute differences between 2% and 15% except for pixels near to the
Sun and high scattering angles. We apply these calibration matrices to three
images in order to study the sky radiance distributions for three different
sky conditions: cloudless, overcast and partially cloudy. Horizon
brightening under cloudless conditions has been observed together with the
enhancement effect of individual clouds on sky radiance. |
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