Spectral Modelling of Light-Emitting Diodes and Atmospheric Ozone Absorption

Anna Vaskuri

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

This thesis focuses on the spectral modelling of light-emitting diodes (LEDs) and developing an uncertainty analysis in atmospheric ozone retrieval algorithms. Attention is also paid to degradation of components used in the atmospheric measurements that brings additional challenge on reliable measurements. Improved spectral models were developed for determining the junction temperatures from the experimental electroluminescence spectra of LEDs. These models consist of the effective joint density of states of conduction band electrons and valence band holes, weighted by the thermal excitation probability. It was noted that the effective joint densities of states of most common LED types made of AlGaInP and InGaN deviate significantly from each other. By fitting the models to experimental spectra, the junction temperatures of red AlGaInP and blue InGaN LEDs were estimated correctly within the standard uncertainty of 4 K. To achieve this uncertainty, the calibration of the model parameters was carried out with one reference spectrum of the same LED specimen at a known junction temperature. A Monte Carlo based uncertainty analysis was developed and integrated into a full spectrum ozone retrieval algorithm to obtain complete uncertainty budget for the total ozone columns. The uncertainty analysis accounts for possible systematic wavelength-dependent deviations in spectral data that significantly increase the uncertainty compared with the assumption that all the spectral uncertainty is white noise. The method was applied to data sets measured with three different spectroradiometers. In the case of array-based spectroradiometers, the results showed that the diurnal cycle of the total ozone columns formed an inverse U-shape arising from the stray light of the instrument. Thus, a new method was developed to reduce the effect of stray light in TOC retrievals. In this thesis, the degradation of white reflectance diffusers, polystyrene, and nickel sulphate filters used in Brewer spectrophotometers were studied. White diffuser targets are used as references in in-orbit calibrations of Earth observation satellite instruments and for characterising the ground reference test sites. In the experiments, fused-silica diffusers were more resistant to hydrocarbon contamination compared with Spectralon diffusers. When the contaminated samples were exposed to ultraviolet radiation in the air, their reflectance returned back to the original level. This is opposite to vacuum tests carried out previously by other researchers, where both hydrocarbon contamination and ultraviolet radiation decreased the reflectance of Spectralon diffusers. In the case of polystyrene, higher energy photons caused more damage and the photodegradation was non-linear with respect to radiant exposure. The spatial uniformities of old nickel sulphate solar blind filters were tens of percents worse compared with a new one.
Translated title of the contributionLoistediodien ja ilmakehän otsoniabsorption spektrinen mallintaminen
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Ikonen, Erkki, Supervising Professor
  • Kärhä, Petri, Thesis Advisor
Publisher
Print ISBNs978-952-60-8242-4
Electronic ISBNs978-952-60-8243-1
Publication statusPublished - 2018
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • spectral modelling
  • light-emitting diode
  • LED
  • atmospheric ozone
  • TOC
  • diffuser
  • material degradation

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