Intercomparison of Spectroradiometers for Global and Direct Solar Irradiance in the Visible Range

Abstract

This paper presents the results of the analysis of the spectral, global, and direct solar irradiance measurements in the visible range (400–700 nm) that were made in the framework of the first Iberian UV–visible (VIS) instruments intercomparison. The instruments used in this spectral range were four spectroradiometers: three Licor 1800s equipped with different receiver optics and one Optronic 754. For the direct solar irradiance measurements the spectroradiometers were equipped with collimators with different fields of view. Parallel studies have been carried out with the data given by the spectroradiometers with their original calibration file and with the same data that is corrected, following in situ calibration of the instruments using a laboratory reference lamp. To compare the series of spectral data the relative values of mean absolute deviation (MAD) and root-mean-square deviation (rmsd) have been used. The results obtained from the measurements of global irradiance show that the Licor 1800s presented very significant differences at the beginning and at the end of the day due to the deviation from ideal cosine response of the collection optics (i.e., cosine errors). This forced the analysis to be limited to the measurements corresponding to solar elevations higher than 30°. For this solar elevation range, the results of the intercomparison between the Licor instruments, before their in situ calibration, showed differences of about 5% in the visible range. The results from the measurements of direct irradiance show that, if correction factors are considered, these deviations are reduced to 3%, and when the Licors are compared with the Optronic, the deviations are less than 2%.

Design of a sun tracker for the automatic measurement of spectral irradiance and construction of an irradiance database in the 330–1100nm range

“…On the other hand, the authors have also estimated, using the Nakajima's SKYRAD.PACK code [26,27], the contribution of the sky radiance to the measured irradiance, due to the fact that the solid angle subtended by the collimators is higher than the solid angle subtended by the sun. The inaccuracy due to the circumsolar radiation, for 500 nm, was 0.6% assuming that the sun was perfectly centred in the instruments FOV [16,28]. When the Sun was not in the centre, the circumsolar radiation would probably be slightly less, and so too the inaccuracy due to this radiation.…”

“…The inaccuracy in the measurement of the global irradiance in the visible range is 5%, while in the measurement of the direct irradiance is below 3% [16]. Using these values, a conservative approximation for the inaccuracy in the diffuse irradiance in the visible range would be 8%.…”

“…In these campaigns the spectroradiometer Licor 1800 has been widely used [9][10][11][12][13] and its validity has been proved beyond doubt [14][15][16].…”

“…In that work they have taken into account the factors that can introduce some type of uncertainty in the experimental values-both the inaccuracies of the measurement apparatus and those due to random causes such as temperature fluctuations or calibration instrument errors. Recently Martı´nez-Lozano et al [16] have analysed the uncertainty associated with the measurements of these spectroradiometers in the visible range. The overall uncertainty (root square sum of the uncertainties) is 75.0%.…”

“…The use of the fiber optic has the important limitation that invalidates the UVB spectral range (300-320 nm). Nevertheless, we have to keep in mind that in such range the instrument shows the maximum error, around 20% [16], which makes it not suitable for the spectral UVB measurement. The radiance limiting tube, used for the direct irradiance measurements, has been designed based on the work of Cannon [23].…”