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18 | 09 | 2019


WP 1: Spectral Irradiance Traceability

The aim of this workpackage is to shorten the traceability chain of solar UV measurements in the wavelength range 300 nm to 400 nm to the SI unit and to reduce the associated transfer uncertainties. The goal is to develop uncertainties in the field comparable to those currently achieved only for primary spectral irradiance scale realisations at NMI level, i.e., at the level of ± 1 % to ± 2 % at k=2.

This workpackage will:

  • Develop a detector based spectral irradiance traceability chain at PTB. The improved European Reference spectroradiometer QASUME will be calibrated directly against a reference detector traceable to an absolute radiation standard using a tuneable laser facility. No intermediate source-based transfer standards will be required in this traceability chain (Task 1.1)

  • Develop two UV LED-based transfer standards: These are compact, stable and portable monitoring devices based on UV-LEDs, with two entrance optics geometries that will be used to maintain the spectral calibration of spectroradiometers while deployed at the UV monitoring sites (Task 1.2)

  • Develop a high output metrology (laser) light source for field calibrations with a spectral power distribution free of impurity features and spectrally flat to a level of a few percent over the solar UV range. This laser-based device has high radiation output and can be used in the far-field region (in contrast to the UV-LED devices) and thus calibrate spectroradiometers with diverse input optics (Task 1.3)

 

 

Description of Task 1.1

Detector-based traceability chain using an absolute radiometer and tuneable UV laser facility. (PTB)

Start: August 2011, End: January 2014

The aim of this task is to develop tools and methods for entirely detector-based calibration of spectroradiometers (e.g. QASUME) in the solar UV spectral range.

Description of work: PTB will develop instrumentation to calibrate the spectral irradiance responsivity of the portable reference spectroradiometer QASUME. A modified tuneable laser suite (quasi-CW tuneable laser) and a reference detector traceable to the primary standard of radiant power will be used. The target uncertainties of the method of calibration is less than 0.5 % across the wavelength range 280 nm to 400 nm.

 

Description of Task 1.2

Development of a highly stable portable transfer standard, powered by LEDs (PTB, EJPD, SFI Davos, Kipp)

Start: August 2011, End: October 2013

Background: Currently the stability of UV spectroradiometer’s responsivity is monitored using compact low-power tungsten-halogen lamps. These are fragility and their stability is affected by aging and transportation. To overcome these limitations, more robust monitoring technique will be developed. This will be based on UV LEDs, which can form the basis of a new generation of transfer standards.

The aim of this task is to develop techniques for monitoring the responsivity of UV spectroradiometers (e.g. QASUME) over extended periods of time and also under field conditions. The monitoring techniques will cover the solar UV spectral range 280 nm to 400 nm and ensure stability and reproducibility of the spectroradiometer of ± 0.3 % for a lamp-burn time of 7 hours (i.e. representing about one year of calibrations).

Description of work: Two compact, portable UV LED-based transfer standards will be designed for use with the two global entrance optics (diffusers) designed by Aalto in D4.2.2 (due July 12). Since the geometry will be "near-field", they will be used to monitor the responsivity of the spectroradiometer, while the absolute calibration will be done with far-field sources in the laboratory.

 

Description of Task 1.3

Compact laser-induced high-flux UV source as metrology light source for field applications (VSL, Kipp)

Start: August 2011, End: March 2014

The aim of this task is to evaluate the suitability of a new commercially available high-flux laser-induced UV light source for use as a metrology light source in field applications. The Laser Driven Light Source (LDLS) technology may be implemented at NMI level and has high potential to be used as a new-generation compact transfer standard for field applications in the far-field region (therefore independent of detector’s entrance optic geometry.

Background: Compared to conventional light sources, such as the deuterium or electrode-based arc discharge lamps, the lifetime and available output in the solar UV wavelength range in both radiance and irradiance mode will be inherently better. Since a LDLS operates without electrodes, long lifetime and respectively good short and long-term stability of better than ± 0.5 % per 20 hours burn time is expected. Moreover, the power spectral distribution is free of impurity features and spectrally flat to a level of a few percent over the solar UV range, whereas conventional sources show a significant change of a few orders of magnitude in that same spectral range. The expected high output and good stability at moderate costs make this source also an ideal metrology light source for field usage

Description of work: Based on an instrument selection from a range of commercially available models of award winning LDLS technology (see www.energetiq.com), VSL will implement and characterise an LDLS system that can be operated in two different configurations at NMI level, i.e., flux mode and irradiance mode. Modifications will be made to enable transportability such that the LDLS system can be used for absolute spectral irradiance calibrations under field conditions.