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WP 3: Improvement of Reference Spectroradiometers

Background: Currently even the best UV solar reference spectroradiometers do not provide sufficient accuracy for long-term analysis of atmospheric changes. Total uncertainty of these systems for in field measurement of solar spectral irradiance in UV reaches the level of 5 %. For monochromator-based scanning spectroradiometers a significant part of this uncertainty is due to the photomultiplier tubes (PMT). Although they offer high sensitivity and large dynamic range, they have some key issues such as non-linearity, memory effect and poor long-term quantum efficiency stability, which can range from 1 % to several percent diurnal variations during one day. In extreme cases, a continuous degradation of more than 10 % has been observed with particular PMT modules. The entrance optics for global UV measurements also need to be improved to reduce diurnal variations arising from the changing solar elevation and the ratio between diffuse and direct solar irradiance; the latter task will be accomplished in WP4.


Description of Task 3.1

Development of new detection systems for reference scanning spectroradiometers (CMI, INRIM, SFI Davos)

Start August 2011, End January 2013

The aim of this task is to study the viability and realisation of optimised, high sensitivity, large dynamic range, low noise solid-state detection system (SSDS) that may replace/outperform the photo multiplier tube (PMT) in scanning UV spectroradiometers.

The design objective for the SSDS is:

  • Linearity better than 0.1% over 5 orders of magnitude,

  • Memory effect smaller than 0.1 %,

  • Noise as low as 10 fW/Hz½ ,

  • Stable spectral responsivity at the level of 0.01 % over the period of one month,

  • Integration time for one measurement sample should be a maximum of 1 second for spectral solar UV irradiance below 0.1 mW/m2/nm and 0.1 seconds or less at irradiances above 1 mW/m2/nm,

  • Operational from 280 nm to 400 nm (minimum).

Description of Task 3.2

Validation of optimised transportable QASUME reference spectroradiometer (SFI Davos, CMI, INRIM, PTB)

Start January 2013, End: February 2014

The aim of this task is to optimise and validate the portable reference scanning spectroradiometer QASUME. The improved QASUME will be used for disseminating the improved UV irradiance scale to stakeholder instruments through the intercomparison in Davos (WP5).

Description of Task 3.3

Adaptation of a Fourier transform spectroradiometer as reference instrument for solar UV irradiance measurements (PTB, SFI Davos)

Start August 2011, End: March 2014

The aim of this task is to evaluate the suitability of this Fourier Transform Spectroradiometer (FTS) as a reference instrument for solar UV irradiance measurements. A commercially available FTS will be fitted with the global entrance optics, an absolute calibration of the instrument will be performed, and the instrument will be fully characterised.