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

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

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

Solar simulators for healthy Vitamin D synthesis

Background/Aim: The angular distribution of solar radiance and its spectral characteristics is required for the determination of Vitamin D3 production in humans. Materials and Methods: The Vitamin D3 weighted exposure can be calculated by integrating the incident solar spectral radiance over all relevant parts of the human body. A novel instrument allowing simultaneous measurements of spectral radiance from more than 100 directions has been developed. A large solar simulator for controlled experiments is described. Results: In summer it is relatively easy to obtain sufficient Vitamin D because sun exposure times are short. In winter solstice Vitamin D3 cannot be obtained with realistic clothing even if the exposure were extended to all daylight hours. Conclusion: Improved and controlled experiments to determine Vitamin D3 production are required to assess the positive effects of solar UV radiation and to assess its natural variability

Global spectral irradiance array spectroradiometer validation according to WMO

Solar spectral irradiance measured by two recently developed array spectroradiometers (called UV-BTS and VIS-BTS) are compared to the results of a scanning double monochromator system which is certified as a travelling reference instrument by the Network for the detection of atmospheric composition change (NDACC) and fulfils the specifications of S-2 UV instruments of the world meteorological organization (WMO). The comparison took place between 15 and 18 May 2017 at the Institute of Meteorology and Climatology of the University of Hanover (IMuK) between 4:00 and 17:00UTC. The UV-BTS array spectroradiometer is equipped with special hardware to significantly reduce internal stray light which has been the limiting factor of many array spectroradiometers in the past. It covers a wavelength range of 200 nm-430 nm. The VIS-BTS covers a wider spectral range from 280 nm up to 1050 nm, and stray light reduction is achieved by mathematical methods. For the evaluation, wavelength integrated quantities and spectral global irradiance are compared. The deviation for UV index measured by the UV-BTS, is within ±1% for solar zenith angles (SZA) below 70° and increased to a maximum of ±3% for SZA between 70° and 85° when synchronisation between measurements was possible. The deviation of global spectral irradiance is smaller ±2.5% in the spectral range from 300 nm to 420 nm (evaluated for SZA < 70°). The VIS-BTS achieved the same deviation for blue light hazard as the UV-BTS for the UV index. The evaluations of global spectral irradiance data of the VIS-BTS show a deviation smaller than ±2% in the spectral range from 365 nm to 900 nm (evaluated for SZA < 70°). Below 365 nm, the deviation rises up to ±7% at 305 nm due to remaining stray light. The agreement within the limited time of the intercomparison is considered to be satisfactory for a number of applications and provides a good basis for further investigations. © 2018 IOP Publishing Ltd