New methods in stray light correction and multi-imaging spectroradiometry

Abstract

Especially for measurements in the UV spectral range the internal stray light is often a crucial contribution to the measurement uncertainty for array spectroradiometers. In this work a technical procedure for significant stray light correction for the application mentioned has been developed. With a measurement device based on this procedure deviations in the UV index of less than 1% and vertical ozone thickness of less than 2%, compared to established double monochromator reference systems, have been achieved. The detection limit of the spectral irradiance of solar radiation was reduced by about two orders of magnitude, 5 · 10−5 Wm 2nm-1 compared to 2 · 10−3 Wm-2nm-1, the level of a typical array spectroradiometer. As a result, the achieved detection limit is significantly closer to the recommended level of a double monochromator (S-2 instrument according to WMO UV instrumentation specifications) with 1 · 10−6 Wm 2nm-1. The total measurement time of a typical solar spectral irradiance measurement is about 8 seconds. This is significantly shorter than that of double monochromators, which typically require 16 minutes for a measurement in comparable spectral range and resolution. It should be mentioned that changes in the spectral irradiance within the stated 8 seconds can lead to increased measurement uncertainties in measurement intercomparisons. A comparatively precise time synchronization of individual wavelength measuring points, such as with a double monochromator (own time stamp per wavelength step), is not possible. This approach was developed to achieve a significant improvement in the stray light correction of array spectroradiometers in order to be able to use their temporal advantages in terms of total measurement time as well as other advantages such as the smaller size compared to established double monochromators. This was achieved by a spectral pre-filtering of the optical radiation to be measured by means of various optical bandpass and edge filters, which are moved in the optical path with a filter wheel. Due to the optical pre-filtering, the in-coupled optical radiation can be spectrally limited to the spectral range to be measured, parts of the spectral range or the spectral range not to be measured. Consequently the potential optical radiation, which is able to generate stray light in the device, is significantly reduced or serves to characterize the stray light. In this way different filter measurements can be combined with each other in order to obain a stray light corrected measurement of the entire spectral range. The duration of the several individual measurements is decisive for the total measurement time achieved.

For the simultaneous measurement of radiance distributions, also called "spectral snapshot imaging", a further optical approach was developed as part of this work. The resulting prototype allows by use of the complete field of view the simultaneous measurement of 196 measurement channels (14 x 14). For the proof of feasibility, a laboratory measurement of a 10 mm x 10 mm measuring field with these 196 measurement channels in the spectral range from 380 nm to 800 nm with 10 nm optical bandwidth (FHWM) was performed. A channel crosstalk of 4% was achieved. In addition, a spatially resolved measurement with the same amount of measurement channels of the zenith radiance of the atmosphere at a FOV of 0.5 °, in the spectral range 280 nm to 470 nm at 3 nm optical bandwidth (FWHM) was accomplished. Here a suboptimal deviation of -30% to UVSPEC modeled data was determined. However, according to other publications, this deviation is within the expected deviations between independently calibrated measuring systems and model data. Especially for a prototype that is not temperature stabilized and has to be optimized substantial with regard to mechanical implementation and light-tightness. It has to be mentioned that the achieved 3 nm optical bandwidth leads to an increased measurement uncertainty especially in the UVB spectral region for solar spectral radiance measurements. Supplementary, the feasibility of a partial hemispherical field of view with solar zenith angles from 17 ° to 74 ° was demonstrated by means of a specially designed additional entrance optics developed by a colleague. The number of usable measurement channels was reduced by this entrance optic due to mechanical shading and incomplete use of the field of view to 120. For all measurements, the total measurement time was in the range of a few seconds. It is important to note, that the results achieved by the prototype in this work are to be assessed as a feasibility and potential study of the new procedure. The motivation for developing this novel measurement system was to reduce the number of optical components significantly compared to existing systems and to use benefits such as possible adaption of different FOVs, ideally with the same or higher number of measurement channels. For the analysis of rapidly changing radiance distributions, it is advantageous to be able to carry out a spectral radiance measurement simultaneously and with a high spatial resolution (number of measurement channels). All scanning technologies (spectral or spatial scanning) have the common disadvantage that time-resolved effects can only be analyzed if they are significantly slower than the scan itself. A complex and rapidly changing spectral radiance distribution for instance often occurs in the presence of broken clouds. In recent years some measurement concepts have already been presented. For example, the MUDIS measurement system with 113 measurement channels in the spectral range 250 nm to 600 nm and its further developed system AMUDIS, which is at the time of publication of this work in the test phase with 150 measuring channels in the spectral range of about 300 nm to 1700 nm. The approach is based on a special designed input optics, which is adapted to a line scanning spectrometer. This input optics produces multiple images of the radiance distribution to be measured, by using a facet mirror based optics. A different part of the radiance distribution to be measured can then be selected from each of these images by apertures and coupled into a spectrometer for further spectral analysis.