Browsing by Author "Ikonen, Erkki, Prof., Aalto University, Department of Electrical Engineering and Automation, Finland"

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Improved Calibration and Uncertainty Estimation Methods for Optical Radiometry
(2024) Maham, Kinza
School of Electrical Engineering | Doctoral dissertation (article-based)
This thesis focuses on developing and characterizing detectors, measurement setups, and methods tailored for optical radiometry. Additionally, emphasis is placed on the importance and methods of uncertainty estimation in metrology, aiming for reliable measurements. The predictable quantum efficient detector (PQED) ensures traceability of optical power to the SI system, making it a promising primary standard for detector calibration. The PQED is used to calibrate standard trap detectors, achieving an expanded uncertainty of 0.05% in the visible region. A comparative analysis with a cryogenic radiometer validates the PQED as a primary calibration standard. To extend calibration capabilities from the visible region upto the wavelength of 2000 nm, a portable tunable laser line setup is built. As a reference, the setup uses InGaAs and Ge detectors that are characterized against a pyroelectric radiometer. The tunable laser's increased spectral power reduces the expanded uncertainty from 4% to 2.2% - 2.6% across 820 nm - 1600 nm compared to older calibration methods at Metrology Research Institute (MRI). This thesis also discusses estimation of uncertainties and their correlations within the spectral mismatch factor of solar cells using Monte Carlo analysis. Our study assesses the uncertainties linked with the spectral mismatch factor (SMM) across various scenarios. We examine three scenarios: the worst-case scenario, where components are assumed to have severe correlation; the average scenario, where partial correlation is presumed; and the best-case scenario, where components are considered uncorrelated. The resulting expanded uncertainties (with a coverage factor k = 2) for these scenarios are 1.26%, 0.44%, and 0.06%, respectively. These figures represent the spectrum of potential uncertainties based on the assumed correlation conditions among the components involved in the SMM correction factor. The new method for calculating uncertainties benefits from prior knowledge of correlations. This knowledge is sought by analysing Consultative committee for photometry and radiometry (CCPR) key comparisons of radiometric quantities spectrally. This analysis provides insights into spectral correlations, aiding in the quantitative assessment of uncertainties in spectral integrals. Leveraging these insights, the uncertainty estimation methods in optical radiometry, including SMM and color-correlated temperature (CCT), are improved.
Investigations and Applications of Angle-Resolved Measurements of Spectral Reflectance and Transmittance
(2024) Aschan, Robin
School of Electrical Engineering | Doctoral dissertation (article-based)
This doctoral thesis explores the quantification of a material's appearance through angle-resolved measurements of spectral reflectance and transmittance. The appearance of a material often determines the quality of a product across various sectors of industry, entertainment, and research. Therefore, there is a need for accurately characterized reference standards, traceable to the international system of units (SI). Accordingly, this thesis focuses on bidirectional reflectance distribution function (BRDF) and bidirectional transmittance distribution function (BTDF) measurements of diffuse material in the visible and near-infrared wavelength range. Moreover, the thesis presents the development of an SI-traceable facility for the accurate determination of angle-resolved scattering quantities. One application of BRDF includes the verification of proper operation of ray tracing models (RTM) using calibrated reference standards. The first part of the thesis introduces an SI-traceable 3D gonioreflectometer for BRDF measurements of an artificial sample. To find a suitable sample for the verification process, the 3D instrument systematically measured several material candidates, evaluating their BRDF characteristics. Subsequently, the instrument's uncertainty in BRDF measurements is characterized, and the reliability of RTM is checked by comparing simulated and measured BRDF data of an artificial sample based on the best material candidate. The following sections of the thesis are dedicated to exploring the practical applications of BTDF. The sections present the development of a facility for SI-traceable BTDF measurements, featuring an absolute gonioreflectometer extended to BTDF measurements in the visible and near-infrared wavelength range. The instrument specializes in measurements of matte surfaces that uniformly transmit light in all directions. Validation of the absolute instrument's capabilities in BTDF measurements was conducted through a detailed uncertainty evaluation and also a comparison measurement with a commercial spectrophotometer. Furthermore, the thesis discusses the adaptation of the 3D instrument for out-of-plane measurements of BTDF, recognizing the utility of materials that have anisotropic scattering distributions. The fourth part of the thesis offers considerations on the definition of BTDF in the context of thick sample measurement. The thesis presents a method for correcting measured BTDF as a function of viewing zenith angle based on the geometry of the facility. Furthermore, the method was validated by using two instruments exhibiting varying degrees of sensitivity to sample thickness. This work provides tools for unambiguous treatment of the BTDF of thick samples.