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18 | 09 | 2019


WP 2: Array Spectroradiometer Characterisation

The aim of this workpackage is to develop new characterisation techniques for UV array spectroradiometers addressing the most relevant sources of errors.

Background: Measuring the UV part of the solar spectrum requires a high dynamic range due to the sharp cut off of the solar spectrum in the UVB-range by ozone absorption. Usually the resolution of array spectrometers is 12-14 bit, which is not sufficient for solar UV measurements. Additionally array spectroradiometers are designed for operation in a laboratory, and not in a continuous mode. Therefore the current limitations of array spectroradiometers make them unsuitable for solar UV measurement.

The main effects to consider when characterising UV array spectroradiometers are spectral stray light, bandwidth, wavelength accuracy and linearity. Quantifying these effects is not only fundamental for calculating the measurement uncertainty but it also allows correcting for the instrument imperfections and thus decreases the overall uncertainty of measurements

The workpackage will:

  • Identify requirements of array spectroradiometers for solar UV measurements in the range 300 nm to 400 nm, and the best measurement procedures. This will be encapsulated in “A guide to measuring solar UV spectra using array spectroradiometers” (Task 2.1)

  • Development of a complete model for uncertainty calculation of array spectroradiometers. It will include all relevant parameters including effects of correlations among spectral values (Task 2.2)

  • Characterise and correct UV array spectroradiometers for in-range and out-of-range stray light effects (Task 2.3)

  • Develop an algorithm to transform measured solar spectra onto a uniformly spaced wavelength grid with a nominal spectral resolution. It will allow correcting band pass effect of even low-resolution instruments by using a high-resolution solar reference spectrum (Task 2.4)

  • Develop two wavelength characterisation devices for array spectroradiometers.

  1. A Fabry-Perot multilayer structure working in the UV range. It will enable the characterisation of the wavelength scale at the ± 0.01 nm uncertainty level (Task 2.5)

  2. A polarisation gradient filter concept usable for field applications.

  • Compare three linearity characterisation methods based on complementary techniques (Task 2.6)

 

 

Description of Task 2.1

Requirements of array spectroradiometers for solar UV measurements (REG(IMU), PTB, SFI Davos)

Start: August 2011, End: December 2011

Background: Array spectroradiometers have generally been designed for operation in a laboratory, and not in a continuous mode. To enable array spectroradiometers to be used for solar UV measurements, improvements and adaptations are required particularly in the 300 nm to 400 nm range.

The aim of this task is to specify and report on the requirements of an array spectroradiometer, suitable for outdoor solar UV measurements.

 

Description of Task 2.2

Uncertainty estimation of array spectroradiometers (LNE, All)

Start: January 12, End: December 2012

The aim of this task is to develop guidelines for estimation of uncertainties in array spectroradiometer measurements of solar UV irradiance including effects of correlations among spectral values.

Background: Uncertainty of solar UV irradiance measurement data using array spectroradiometers depends on a number of device characteristics, such as linearity, stray light suppression, wavelength calibration, angular response, instrument stability, as well as calibration and measurement conditions.

A complete treatment of measurement uncertainties requires identification and inclusion of correlations among measured spectral values. Correlations are introduced via calibration of the spectroradiometer as well as instrument characteristics manifested in e.g. LSF characterisation results. Nevertheless, uncertainty propagation in solar UV irradiance measurements using array spectroradiometers so far tends to be accomplished without taking into account correlation effects.

 

Description of Task 2.3

Stray light characterisation and correction methodologies (PTB, EJPD, SFI Davos, REG(IMU))

Start: March 2012, End: July 2013

The aim of this task is to develop a stray light characterisation methodology for array spectroradiometers and eventually correction algorithms for stray light.

Background: Stray light can originate from inside the spectral range of the solar UV spectroradiometers, or outside of the working spectral range of the instrument (but still within the sensitivity range of the sensor, typically up to 1100 nm for a silicon CCD detector). Spectral stray light effects caused by the light within the spectral range of an instrument are usually corrected based on line spread function (LSF) characterisation results and a simple matrix method. For the solar UV measurements, however, such a correction is not enough due to a significant contribution to the stray light by the radiation at longer wavelengths than the working range of the instrument but still within the sensitive range of the silicon CCD sensor.

 

Description of Task 2.4

Bandwidth and wavelength correction methodologies applied to solar spectra (SFI Davos, REG(IMU))

Start: November 2011, End: October 2013

Background: Measured solar spectra are used by end-users to calculate biological weighted quantities such as the erythemal weighted irradiance, as well as quantifications of irradiance changes at specific wavelengths. Therefore nominal wavelengths grids and a constant nominal bandwidth over the whole measured spectrum are often required.

The aim of this task is to create an algorithm, which converts measured solar spectra onto a uniformly spaced wavelength grid with a nominal spectral resolution. The key feature of this algorithm will be its ability to cope with arbitrary spaced wavelength grids and spectrally varying slit functions which are both specific to array spectroradiometers. The application of such an algorithm to solar spectra requires the use of a high-resolution solar reference spectrum due to the Fraunhofer structure and the low resolution of the array spectroradiometers.


Description of Task 2.5

Development of two wavelength scale characterisation devices (EJPD, SFI Davos, VSL, REG(IMU))

Start: August 2011, End: January 2013

Background: High spectroradiometer wavelength calibration accuracies are key parameter for solar spectral measurements. Typically, spectroradiometers are calibrated with spectral discharge lamps, providing distinct and well-defined spectral emission lines. However, for small spectral ranges, where only one or two lines are present, the calibration becomes inaccurate. Furthermore, the accuracy of the calibration may locally vary according to the number of nearby calibration lines and their different intensity and the nonlinearity relationship between wavelength and recording pixel, or grating drive. This task will investigate the concept of broadband source filtering, direct laser source referencing and traceability.

The aim of this task is to develop improved wavelength characterisation devices for diode array spectroradiometers, and scanning spectroradiometers. The target expanded uncertainty in wavelength is ± 0.01 nm at k=2 over the wavelength interval 280 nm to 400 nm. Two methods are to be developed and compared.


Description of Task 2.6

Linearity of array spectroradiometers (PTB, Aalto, EJPD, VSL)

Start: January 2013, End: May 2014

Background: Solar UV irradiance measurements are carried out over a large dynamic range (up to 5 orders of magnitude between 300 nm and 400 nm). At a manufacturer’s site, linearity of the CCD instruments is typically characterised by varying the integration time of the array spectroradiometers. This is a simple but by far not complete characterisation method since it accounts mostly for the linearity of the signal processing electronics. In principle, varying the spectral irradiance level should test the linearity of such devices. Residual deviations from the linear regime will yield errors both in absolute values as well as in relative spectral distribution of the measured solar UV irradiance.

The aim of this task is to develop procedures for linearity characterisation of array spectroradiometers used for solar UV irradiance measurements. The objective is to determine the linearity of the array spectroradiometers over 5 orders of magnitude representative for local noon solar UV irradiances (between 10 µWm-2nm-1 and 1 Wm-2nm-1). Three alternative methods will be developed and evaluated.