Luca Egli

Stray light correction of array spectroradiometers for solar UV measurements.

An approach is presented to characterize and correct stray light in spectra measured with array spectroradiometers and caused by out-of-spectral range radiation. A prerequisite for out-of-range stray light correction is knowledge of the spectral irradiance not measured by the instrument itself. A way of solving this problem for solar UV measurements is shown. The effect of out-of-range stray light is especially important for solar UV spectroradiometers typically having a spectral range narrower than that of the silicon detectors in use. Two different types of instruments used for solar UV measurements were characterized and corrected for out-of-range and in-range stray light. As a hardware solution to the out-of-range stray light problem, a bandpass filter was fitted in one array spectroradiometer. Results of test measurements using this modified instrument are also shown.

A guide to measuring solar UV spectra using array spectroradiometers

Array spectroradiometers are cost-effective instruments allowing fast measurement sequences to monitor the high variability of solar radiation. From the spectra any desired biologically weighted doses within their operational spectral range can be derived by post-processing. Therefore, array spectroradiometers have the potential to replace filter radiometers currently used in UV monitoring networks. However, they suffer significantly from stray light. Thus for routine operation it is important to follow strict procedures for characterisation, operation and data post-processing, which are suggested in these guidelines. The most important characteristics of array spectroradiometers are wavelength calibration, slit function, stray light properties, spectral structure of the dark signal, linearity, noise equivalent irradiance and spectral responsivity. For routine operation, temperature stabilization is absolutely necessary as well as automated dark signal measurements. Integration time and number of repetitions control the noise level and they have to be defined according to the measurement requirements. Data post-processing includes consideration of nonlinearity, dark signal with its spectral structure, integration time, stray light correction, spectral response function and weighting function.

Monte Carlo analysis of uncertainty of total atmospheric ozone derived from measured spectra

We present a Monte Carlo based model to study effects that possible correlations in spectral irradiance data may have on the derived total ozone column values. Correlations may produce systematic errors in the spectral irradiance which behave differently from uncorrelated data. The effects are demonstrated by analyzing the data of one day’s measurements.

Solar ultraviolet irradiance measurements in Aosta (Italy): An analysis of short- and middle-term spectral variability

A general algorithm to estimate atmospheric and environmental parameters from solar spectral irradiance was developed. Total ozone amounts retrieved from a high-quality series of global solar ultraviolet irradiance spectra show large correlation (ρ=0.99) to the values measured by a MKIV Brewer spectrophotometer. The effective albedo also retrieved by the algorithm was compared to MODIS satellite estimates giving good agreement (correlation index ρ=0.63). Moreover, the influence of snow cover on the UV irradiance at ground was assessed. Finally, a new extraterrestrial spectrum was tested and systematic differences between measured and modeled irradiance spectra are discussed.

New technologies to reduce stray light for measuring solar UV with array spectroradiometers

Measurements of solar UV irradiance with array spectroradiometers (ASRM) are susceptible to errors in particular in the wavelength regions with low irradiance levels. This is due to the impact of stray light in the often compact single grating configuration of ASRMs. However, a significant advantage of ASRMs, in contrast to traditional scanning spectroradiometers, is their ability to detect one entire solar UV spectrum quasi instantaneously. This study aims to evaluate three different concepts and respective technologies to physically reduce the impact of stray light in conjunction with ASRM. 1. The concept of modulating the incoming solar radiation to reduce the dynamic range of the solar UV spectrum using the Digital Light Processing (DLP ®) technology. 2. The concept of pre-selecting a range of wavelength before guiding the incoming radiation to an ASRM. The corresponding technologies for this pre-dispersing instrument are based on tunable MEMS gratings. 3. The high dynamic range of the solar UV light ...