Calibration of an all-sky camera for obtaining sky radiance at three wavelengths

Abstract

Abstract. 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.

Coupling sky images with radiative transfer models: a new method to estimate cloud optical depth

“…Other ground-based sky imaging designs have also been developed (Seiz et al, 2007;Souza-Echer et al, 2006;Calbo and Sabburg, 2008;Cazorla et al, 2008;Heinle et al, 2010;Román et al, 2012;Gauchet et al, 2012) with the most dissimilar design consisting of a downward-pointing camera capturing the sky from a reflection off a spherical mirror (Pfister et al, 2003;Kassianov et al, 2005;Long et al, 2006;Mantelli et al, 2010;Martínez-Chico et al, 2011). Most ground imaging devices follow a relationship between the camera's signal and radiance similar to Eq.…”

Development of a sky imaging system for short-term solar power forecasting

“…The sensors used are often full frame charge-coupled devices (CCDs) because of the high quantum efficiency and fill factor, but require mechanical shuttering which limits the frame rate of the system. One system similar to the USI from the astronomy field is the All Sky Infrared Visible Analyzer (ASIVA, Klebe et al, 2014;Sebag et al, 2008) with a dual camera system that captures both visible and LWIR images. It is one of the few LWIR dioptric (refraction-based) whole-sky designs (catadioptric (reflection and refraction) designs similar to the TSI are more common).…”

Validation of spectral sky radiance derived from all-sky camera images – a case study

“…The spectral skylight from images using a linear pseudo-inverse has been presented (López-Álvarez et al, 2008). Recent research by Román et al (2012) investigated the spectral sky radiance at three effective wavelengths from hemispherical sky images. A matrix calibration described the relationship between the output signal of images and reference values of the simulated sky radiance were proposed.…”

“…In contrast to Román et al (2012), we propose a method to obtain the spectral sky radiance distribution from all-sky images for the complete spectrum in the visible part (380-760 nm) using the non-linear regression technique. The instrumentation and the methods for retrieving the spectral radiance will be introduced in the next section, followed by spectral and spatial comparison of the sky radiance distributions derived by the hemispherical sky imager (HSI) system with the measured radiance distributions obtained by a CCD spectroradiometer.…”