Laboratory calibration for multidirectional spectroradiometers

A method for the calibration of multidirectional spectroradiometers (MUDISs) capable of the simultaneous measurement of spectral radiance at different wavelengths is presented. The calibration of the spectroradiometer is challenging and crucial for high quality measurements of the angular dependence of the radiance. The calibration device consists of an integrating sphere (also known as Ulbrichtkugel), with a diameter of 100 cm, equipped with three 100 W lamps positioned in the lower hemisphere, with the input optics of the MUDIS directed towards the upper hemisphere. The MUDIS detects radiation from 113 different directions simultaneously in a wavelength range from 300 nm to 550 nm. Due to multiple reflections within the sphere, the radiance from the upper hemisphere is nearly homogeneous with deviations of less than 3% on average. Disregarding the 3% variability and assuming a homogeneous radiance inside the upper hemisphere of the integrating sphere, the spectral responsivities of all the MUDIS channels were determined based on the measured zenith radiance, which was detected by a pre-calibrated Network for the Detection of Atmospheric Composition Change reference spectroradiometer containing a scanning double monochromator with a unidirectional input optics. The input optics of the MUDIS contains thin fibers that should not be moved to avoid changes in the instrument's responsivity. The proposed method is therefore suited to determine the absolute responsivity of the MUDIS for all directions

Characterization of the angular response of a multi-directional spectroradiometer for measuring spectral radiance

Despite its importance, few instruments are able to measure the angular distribution of the solar spectrum with a high spectral and temporal resolution. We present a novel characterization method of the multi-directional entrance optics of the AMUDIS (Advanced MUltiDIrectional Spectroradiometer) which is a multidirectional spectroradiometer based on three CCD image sensors combined with imaging spectrographs. The new type of entrance optics consists of 435 different optical fibres uniformly distributed along 145 directions covering the upper hemisphere and allowing simultaneous measurements of the radiance in the ultraviolet, visible and near infrared part of the spectrum, ranging from 280 nm to 1700 nm. The experimental setup for characterizing the multidirectional entrance optics is based on a 100 W halogen lamp and a robotic arm, which moves the lamp tangentially over the surface of a virtual sphere of 102.5 cm radius around the entrance optics. The characterization revealed misalignments in the position of the optical fibres of up to 3∘ (which can affect radiance measurements, specially under broken clouds conditions). The novel characterization method improved 3-fold the alignment up to ±0.1

Field calibration for multidirectional spectroradiometers

A mobile calibration system for a multidirectional spectroradiometer (MUDIS) to transfer the absolute radiometric calibration from the laboratory to the location of the outdoor-measurement (field calibrator) has been developed. The main part of the calibration system comprises an aluminium sphere with a diameter of 40 cm, mounting adapters and a ventilation system. The MUDIS device is capable of measuring spectral radiance from 320 to 600 nm in 113 different directions simultaneously within 1 s. When repeating radiance measurements inside the mobile field sphere, the relative standard deviation (RSD) for wavelengths between 320 and 600 nm is less than 1.8% (320 nm) for all directions with minimum RSD of 0.6% at 382 nm. The reproducibility depends not only on the wavelength but also on the individual fibre position on the hemispherical input optics, with maximum of 4.5% RSD, but most directions show a lower deviation. On average, the RSD for the channels is less than 0.9%. The calibrator enables measurements of the spectral radiance with less uncertainty than with the previous indirect calibration method, which uses measurements of a scanning reference array spectroradiometer