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WP 4: New Technologies

 The aim of this workpackage is to develop new techniques, technologies and equipment that can be used to improve the uncertainties of solar UV measurements.

State-of-the-art: Currently the commercially available cost-effective spectroradiometers have a number of limitations. They do not achieve the required sensitivities; deviations of at least 20 % in solar UV irradiance measurements at 310 nm are currently observed and this is thought to be due to:

  • Global entrance optics currently have angular responses which deviate significantly from the nominal cosine response, producing systematic deviations of up to 10 % between instruments measuring global solar UV irradiance.

  • Inadequate stray light rejection in the UV wavelength range and particularly in the UV-B range

This WP aims to resolve these issues by:

  • Developing a hyperspectral sensor for the determination of spectral UV sky radiance. This will be used as ancillary instruments to correct spectral measurements from current spectroradiometers with traditional (non-optimised) global entrance optics and by spatial integration to provide also the diffuse spectral irradiance as a traditional imaging spectroradiometer.

  • Developing new global entrance optics (diffusers) with improved angular responses.

  • Developing array spectroradiometers with optimised stray light rejection of 106 (for measuring solar UV irradiance). Two different approaches to stray light rejection will be investigated; Task 4.3 investigates the use of adaptive optics, whilst task 4.4 investigates filtering, both will be thoroughly characterised by the tools developed in WP1 and WP2.



Description of Task 4.1

Realisation of a UV hyperspectral camera (INRIM, EJPD, REG(IMU))

Start August 11, End October 2013

Background: At present only few hyperspectral imaging devices in the UV have been realised as research prototypes or commercial products (e.g. Headwall Photonics). All are based on the use of dispersive gratings or tuneable acousto-optic band pass filters. Both approaches have the disadvantage of being slow and expensive. INRIM have successfully developed two hyperspectral imaging devices for use in the visible and short wave infrared regions, but not yet in the UV. This JRP will develop a hyperspectral camera based on UV sensitive sensors, a UV compatible Fabry-Perot cavity and UV fisheye optics (that require development from scratch). The planned system would be a breakthrough in hyperspectral imaging; allows faster imaging and lower cost realisation.

The aim of this task is to develop a novel type of hyperspectral imaging device (or imaging spectroradiometer) for the measurements of diffuse sky UV radiance. The instrument allows measurement of the UV spectrum of each pixel of an image (about 300 kpixels) in a single shot, where each pixel is associated to a portion of the sky. The device will utilise fish-eye UV collection optics to get an image of a wide portion of the sky. This high-resolution image will be used to improve cosine correction methods of existing solar UV spectroradiometers with conventional entrance optics, as the picture gives exact knowledge on the spectral sky radiance distribution.

Description of Task 4.2

Realisation of improved entrance optics for global solar UV spectroradiometers (Aalto, CMS, Kipp)

Start August 11, End: July 2013

Background: One problem in manufacturing diffusers with a lambertian angular response is the translucency of diffusing materials. In addition to the desired diffuse transmittance, there is a specular component in the transmitted beam. The spatial transmittance profile can be accounted for in the design, but diffuser materials vary from material to material, although the trade name could be the same.

The aim of this task is to develop two improved global entrance optics (diffusers) for solar UV spectroradiometers, to better measure spectral global solar irradiance. One general-purpose diffuser connected to an optical fiber will be manufactured by CMS Schreder, while the second diffuser will be made specifically for the Brewer spectrophotometer by Kipp.

Description of Task 4.3

Array spectroradiometer with improved stray light rejection using adaptive optics (CMI, PTB, SFI Davos)

Start August 11, End June 2014

The aim of this task is to design and develop a pre-dispersing optical device used in conjunction with a standard array spectroradiometer. The design objective is to reach a stray-light rejection ratio of 106 or better in the combined system. The specifications of a solar UV array spectroradiometers will be set in Task 2.1, which leads to the deliverable 5.1.6 (Guideline document). Two alternative methods will be investigated and the most promising will be selected.


Description of Task 4.4

Array spectroradiometer with improved stray light rejection using band pass filters (LNE, PTB)

Start August 11, End June 2014

The aim of this task is to develop spectrograph specifically tailored for solar UV measurements. The device will be based on a commercially available diode array spectroradiometer (a Jobin Yvon VS140). This will be modified by the incorporation of a tailored band pass filter to reduce stray light error. The target value for the stray light rejection is 106.