Browsing by Author "Ikonen, Erkki, Prof., Aalto University, Department of Signal Processing and Acoustics, Finland"

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  • Advances in LED Photometry and Ageing Procedures of LED Luminaires
    (2021) Askola, Janne
     School of Electrical Engineering | Doctoral dissertation (article-based)
    LED-based lighting has grown rapidly since the incandescent lamp phase out by the European Union. The increase of LED lamps in the market and the termination of incandescent lamp manufacturing introduces challenges to the calibration procedures in optical metrology. In photometry, the field of optical metrology that studies the light as human eye perceives it, move to LED based calibration sources is advantageous in many ways as most of the luminaires used for illumination already are LED based. The emitted light of white LED lies only in the visible wavelengths, and thus, when utilizing LED sources, the need for V(λ)-filter, the weighting function of the sensitivity of the human eye, is not necessary in physical measurements as the spectral responsivity difference of the detector and the human eye sensitivity can be corrected numerically. This opens the possibility to replace one of the single largest uncertainty source in primary photometric measurements with that of another type of spectral measurement which is easier to address accurately. In this thesis, various advancements in LED photometry have been pursued. The first significant finding is related to photometric measurements with silicon based unfiltered detectors utilizing nitrogen flow through the entrance aperture. The nitrogen flow, utilized for restricting dust particles and moisture entering the unfiltered detector, was found to increase the measured photocurrent. It was found out that the nitrogen flow forms a gas lens varying in length at different flow rates that effectively increases the aperture size of the detector even though the refractive index difference between nitrogen and air is only 6.2 x 10-6. Two methods to overcome the erroneous measurement results were presented: (1) adding an additional cross-flow in front of the detector and (2) using synthetic air as the purging gas. More advances were obtained in the field of ageing LED lamps and luminaires. In a study of adaptive control effect on the LED street luminaire lifetime and lifecycle costs, it was discovered that traffic-aware control can decrease the luminaire lifetime. However, the energy savings of it exceed multiple times the effect of potential lifetime reduction. In a 9-year study of retrofitted LED lamps, it was shown that the lifetime estimates at different periods during the ageing, performed according to standard, give reasonable lifetime predictions suggesting that the manufacturers should reinforce the use of current standards. An organic LED panel ageing presented a novel method with spectrally resolved ultraviolet radiation in underfilled condition that gave information of natural and accelerated ageing simultaneously at different exposure wavelengths.
  • Angle-Resolved Measurements and Modelling of Diffuse Reflectance and Luminescence
    (2017) Jaanson, Priit
     School of Electrical Engineering | Doctoral dissertation (article-based)
    This thesis covers angle-resolved spectrophotometric measurements and modelling of diffuse reflectance and luminescence. Most of the work related to diffuse reflectance is performed in view of providing SI-traceability to radiative transfer codes used in Earth observation. The angle-resolved measurements of luminescent surfaces are used to study goniometrical properties of reflectance and luminescence of reference materials used in appearance measurements. Radiative transfer models that simulate the transmission of light through atmosphere and vegetation are used to interpret the measurements of on-orbit sensors. One way of validating these models is using artificial targets that are characterised by SI-traceable measurements of reflectance and shape. The bidirectional reflectance factors of many roughened anodised and non-anodised surfaces were measured, and used to test the suitability of micro-facet scattering functions to present the scattering properties of such surfaces. Based on the results, a grooved and an identical flat target were constructed and characterised for bidirectional reflectance factors. The bidirectional reflectance factors of the flat target were used to parametrise the scattering functions for rough surfaces. The grooved target was measured for its coordinates describing shape, which were used to construct a structural model of the grooved target. The structural model, with its optical properties defined by the parametrised scattering functions, was used to simulate the bidirectional reflectance factors of the grooved target with the tested radiative transfer model. The measured and modelled bidirectional reflectance factors agreed within corresponding expanded uncertainties for most of the measured geometries, but not all. Non-Lambertian emission of luminescence from solid luminescent materials has been established, however, instrumentation for angle-resolved measurements of luminescence have not been readily available. The MIKES-Aalto goniofluorometer was extensively characterised to improve the accuracy of bispectral luminescent radiance factor measurements. In addition, a method for goniometrical measurements of quantum efficiency and quantum yield was validated against interlaboratory measurements. The improved measurement capability was used to describe absorbance dependent reflectance angular profiles in solid amorphous luminescent materials. In addition, a reference material for luminescence measurements was proposed and characterised. The new material showed more Lambertian angular luminescence emission and reflectance profiles than the conventionally used materials.
  • Applications of Interferometric Measurements and Photoacoustic Detection in Optical Metrology
    (2023) Sharma, Sucheta
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Measurement loops and detectors are irreplaceable constituents of an experimental process in optical metrology. Interferometry and photoacoustic methods can be applied as key techniques for developing such measurement and detection systems. In this thesis, two interferometric measurement arrangements are presented. The phase modulation process in interferometry is important for the measurement speed. Mechanical phase modulation, in such cases, suffers from limitations with regard to driving voltage amplitude, increased modulation frequency and system noise. In this work, the potential of electro-optic phase modulation has been assessed for developing a multi-wavelength interferometric sensor to replace the mechanical phase modulation system. The results not only show the suitability of the electro-optic sensor to improve the measurement speed of the multi-wavelength interferometer, but the sensor is also able to operate at considerably low driving voltage. Another interferometric method driven by an optical system consisting of a mirror array has been presented in this thesis to measure the surface parallelism of step gauge faces. Contact methods are commonly used for this purpose. However, the research gap lies in the available options for non-contact methods to carry out such measurements. The alignment sensitivity is a major factor that controls the accuracy of the measurement with the presented array of mirrors which are arranged as a periscope and a triangular prism reflector-type configuration. Hence, in this work, methods to monitor the alignment sensitivity and estimate corresponding corrections have been presented. The theoretical and experimental studies on the performance of the custom-built optical system show that the system has the potential to serve as a suitable tool for non-contact surface parallelism measurement of step gauge faces. For the development of detection instruments, a silicon cantilever-based photoacoustic radiation detector is presented in this work. Photoacoustic detectors are widely employed for power meter applications, however, in most available cases, the pressure sensors have limitations on the highest detectable radiation power. The photoacoustic detector presented in this work, has shown radiation detection capability with a linear dynamic range of nearly six orders of magnitude and highest detectable power in the milliwatt level which can be further extended with suitable adjustments of the detection parameters. The spectral coverage of the system was initially tested from ultraviolet to infrared region, and in the latest work the radiation sensitivity has been successfully demonstrated in the terahertz range with proper absorber materials. A numerical model for designing the cantilever pressure sensor has also been developed to improve the detection sensitivity. It is concluded that the cantilever-based photoacoustic detector can be a good solution for power measurement applications which require a broad spectral coverage and large dynamic range with robust pressure sensing element offering high damage threshold.
  • Applications of Radiometric Measurements in Non-Contact Thermometry and Mesopic Photometry
    (2016) Shpak, Maksim
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Radiometry deals with the measurement of electromagnetic radiation, its power and spectral and spatial distributions. Radiometric measurement techniques find use in many practical applications, including the measurement of visible light and its colour in photometry, measurement of temperature in pyrometry, chemical composition and physical properties in spectroscopy. The two main focus areas in this thesis are the non-contact measurement of temperature of a microscopic object and the practical application of mesopic photometry. The subject in the study of the non-contact temperature measurement was a silicon microbridge emitter. The temperature of the microbridge was determined from its radiance spectrum in the visible and near-infrared regions. In contrast to previous studies, a grey body assumption was not used, and the determination of temperature was done by modelling spectral emissivity of the multi-layered structure of the microbridge. To accurately model the emissivity, the optical properties of the silicon at high temperatures were studied, which was not previously done for silicon with high doping concentrations. The extinction coefficient was determined from the radiance of a test sample placed in a furnace. Mesopic photometry is a relatively new technique for measuring light, which takes into account the change of visual response in the overlapping region between the so-called day and night visions in the human eye. In this thesis, a novel dual channel photometer developed and characterised for the measurements in the mesopic luminance range is presented. The recommended system for mesopic photometry was published by the International Commission on Illumination (CIE) in 2010, and it provides mathematical tools for calculating mesopic quantities. The CIE mesopic system was studied in detail for this thesis and its applicability was analysed for all possible conditions in the mesopic range. Two problem areas were discovered at the edges of the mesopic range, where the mathematical model either did not converge or exhibited discontinuity. As a practical solution, a set of parameterised equations is presented that provides closed-form solutions and continuous transitions, with minimal deviation from the CIE system.
  • Characterization of Predictable Quantum Efficient Detector
    (2023) Korpusenko, Mikhail
     School of Electrical Engineering | Doctoral dissertation (article-based)
    The aim of this thesis is to develop new characterization methods for Predictable Quantum Efficient Detector (PQED) at visible wavelengths and to extend the predictable responsivity range of PQED into short visible and ultraviolet (UV) spectral ranges. The thesis presents optical studies of PQEDs made of p- and n-type photo diodes, including determination of responsivity, reflectance, spatial uniformity, and bias-voltage dependent characteristic in expanded spectral range compared to previous studies. Investigations of internal quantum efficiency of silicon detectors were done at UV spectral range. A theoretical model for quantum gain in silicon photodiodes was developed for short visible and UV spectral ranges. The n-type PQED with Al2O3 layer coating was optically characterized at short and long visible wavelengths. Its responsivity was obtained through comparison measurements against reference p-type PQED. Reflectance losses were measured and compared with simulated values and found to agree. With these measurements n-type PQED responsivity is predictable in visible range. Novel p-type PQEDs with SiNx surface layer were characterized against reference p-type PQED. It was found out that SiNx PQEDs have excellent spatial uniformity and at least as high responsivity as the used reference PQED. Responsivity of PQED and Hamamatsu trap detector was studied in UV range by comparison measurements against room temperature pyroelectric radiometer. With predicted reflectance and recombination losses, absolute value of quantum gain was retrieved from measured responsivity of PQED. Based on measured data of quantum yield, a theoretical model for quantum yield was developed for short visible and UV wavelengths. Calculated quantum gain has very good agreement with measured values at short visible wavelengths. Separation of contributions of quantum gain and reflectance in responsivity allowed to estimate recombination losses of Hamamatsu photodiodes that was not reported before. A new way of analysing PQED photocurrent dependence on bias voltage was also proposed. Such data can be used for fitting of a 3D charge-carrier transport model which was utilized in a separate work to predict fundamental parameters of PQED photodiodes and to determine spectral responsivity with an unprecedented accuracy.
  • Development and Characterization of the Predictable Quantum Efficient Detector, and Its Applications in LED Photometry
    (2017) Dönsberg, Timo
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Radiometry is the science of studying and measuring electromagnetic radiation. It has many uses ranging from industry applications to fundamental research. Recently, the Predictable Quantum Efficient Detector (PQED) was introduced as a convenient method to quantify radiant flux, i.e. optical power, in the visible wavelength range. The PQED enables absolute measurements with an uncertainty around 0.01% even at room temperature. The high quantum efficiency of the PQED is achieved with the use of custom-made photodiodes that are mounted into a trapping configuration. The PQEDs currently in use have photodiodes that have a thermally grown SiO2 layer on a p-type silicon substrate. Such detectors were characterized for spatial uniformity, reflectance, responsivity and dark current properties. The obtained results show that the PQED can serve as the primary standard of optical power and even has the potential to replace more expensive and cumbersome cryogenic radiometers in the visible wavelength range. A new type of PQED photodiode was developed, where atomic layer deposition is used to grow an Al2O3 layer on a n-type silicon substrate. Two sets of photodiodes with varying doping concentrations were manufactured. Their responsivity was modelled using a 3D model of the photodiode. A novel method to obtain input parameters for the modelling was developed. The n-type PQEDs were characterized similarly as the p-type detectors. Results indicated that the n-type PQED is a promising alternative for the p-type. Thus, the manufacturing of PQEDs is no longer dependent on the availability of particular materials and processing. The PQED can be used to calibrate conventional reference photometers. A new method for the realization of photometric units was developed, which exploits the PQED more directly. In the method, the PQED is used together with a precision aperture, but the traditionally used photometric filter is omitted. Instead, the photometric weighting is done numerically. The method is applicable to sources that emit only little light outside the visible wavelength range, such as white LED lamps. The main advantages of the new method are the reduced uncertainty and simplified traceability chain. In many of the applications, the PQED is used together with a precision aperture. A new method was developed to determine the area of an aperture that is mounted to the PQED – without dismantling the assembly. It exploits previously developed method where the aperture is scanned in front of a Gaussian laser beam and the light passing through is measured at with an integrating sphere. In the new method, PQED itself is used to measure the flux passing through the aperture. The diffraction properties of the PQED and aperture assembled were studied using numerical calculations. The results indicated negligible effects due to diffraction.
  • Improvements in spectroradiometric measurements and applications
    (2017) Pulli, Tomi
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Irradiance refers to the density of the radiant flux of electromagnetic radiation incident on a surface. Spectrally resolved measurements of irradiance are required in many fields, such as in solar ultraviolet radiation monitoring, in ozone retrieval, and in photometry. Spectral irradiance is commonly measured using scanning or array spectroradiometers equipped with a mono-chromator. In order to fulfill the accuracy requirements for a given application, several instrument properties need to be considered, including the quality of the entrance optics, the linearity characteristics of the instruments, and the stray light. This thesis describes a ray-tracing method for simulating radiation transport in the entrance optics which can be used to aid in the optimization of the angular response of the instruments. The quality of entrance optics is especially important in global measurements of solar irradiance. The thesis also presents the nonlinearity characterization results for two array spectroradiometers. Significant nonlinearities were detected for the studied spectroradiometers. Furthermore, in this thesis, the stray light properties of Brewer spectrophotometers, used globally in solar ultraviolet and ozone monitoring, were studied through direct measurements, filter characterizations, and simulations. Stray light from wavelengths outside the nominal measurement range of the instruments were found to cause significant issues in the solar ultraviolet spectral irradiance measurements.  Photometry is a field of science that focuses on measuring the brightness perception in humans caused by radiation. Spectral irradiance measurements are commonly used in photometry to determine spectral correction factors. Spectral irradiance data can also be used directly to calculate the corresponding photometric quantity, illuminance. However, this technique is rarely used in applications with high accuracy requirements, due to the uncertainties of absolute spectral irradiance determination. Instead, filtered broadband detectors, photometers, are used in the measurements. This thesis presents a method for determining the illuminance of white LED sources using a combination of relative spectral irradiance measurement and a highly accurate unfiltered broadband detector. This method can improve the uncertainty of illuminance measurements of white LEDs by eliminating the need for a separate spectral responsivity characterization of the photometer.
  • Linear and traceable scales for nanometrology
    (2014) Seppä, Jeremias
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Methods for the implementation of reliable, repeatable scales are central to all measurementtechniques. Laser interferometers and diffractometers can be used for accurate dimensionalmeasurements that are readily traceable to the definitions of the SI (Système International)units. This thesis is a treatise on the correction of periodic error in laser interferometry anddiffractometry and the application of interferometry and diffractometry for accuratemeasurements. Capacitive sensors and diffraction angles are used for measurement of periodicerror of laser interferometer, and laser diffractometer rotary table angle scale, resulting incorrections that can be used to attain picometre range uncertainties in laser interferometry anddiffraction grating calibration. Computational models resulting in sets of linear equations for linearization measurement data are formulated and used, resulting in Fourier-series type corrective terms for periodic errors. The developed symmetric differential heterodyne laser interferometer with the periodic errorcorrection using capacitive sensor is compared to an X-ray interferometer at National PhysicalLaboratory, UK. The results suggest that periodic nonlinearity error in the range of 10picometres and below is possible with the system. The lateral and vertical scales of IT-MAFM (the interferometrically traceable metrologicalatomic force microscope at the Centre for Metrology and Accreditation (MIKES)) areproduced with laser interferometry in the thesis. Grating standards calibrated with laserdiffractometer and characterized with IT-MAFM are used to characterize the measurementcapabilities of tens of research laboratories with scanning electron microscopes (SEMs) andAFMs in the Nordic-Baltic region. Stroboscopic scanning white light interferometry (SSWLI) is a method that can measuresurfaces and interfaces in e.g. oscillating objects in different phases of the oscillation. In thiswork, a SSWLI at the University of Helsinki is quasidynamically characterized with 2.3 nmuncertainty using a transfer standard with a vertically moving mirror, calibrated with laserinterferometry.
  • Measurement, reporting and applications of bidirectional reflectance distribution function
    (2022) Lanevski, Dmitri
     School of Electrical Engineering | Doctoral dissertation (article-based)
    The thesis addresses the issues of measurement, reporting and applications of bidirectional reflectance distribution function (BRDF) from the point of view of modern problems and corresponding solutions. The first part of the work considers the development of the new 3D gonioreflectometer aimed to overcome the limits of Aalto University's existing instrument by enabling out-of-plane SI-traceable BRDF measurements and allowing horizontal sample alignment needed to study reflective properties of loose samples like soil or sand. The operation principles of the new instrument as well as the design of its illumination, detection and goniometer systems are described. The uncertainty budget of the instrument is thoroughly studied and concluded in a table with numerical values assigned to different uncertainty components. The performance of the new gonioreflectometer is validated by test measurements compared with in-plane BRDF determined by Aalto University's absolute reference gonioreflectometer. The second part focuses on reporting of BRDF data and proposes a solution for BRDF data non-conformity and compatibility problems in the form of thoroughly documented universal Findable, Accessible, Interoperable and Reusable (FAIR) BRDF file format. The file format based on JavaScript Object Notation (JSON) syntaxis is carefully documented and supported by JSON schema that enables its automated validation. The uptake of the presented BRDF data format is facilitated by introduction of an open-source software build around it. The software serves as an interface between the user and JSON schema validator as well as allows to visualize and compare BRDF data stored in files. The third part considers application of BRDF for material characterization. The developed new gonioreflectometer is used to characterize BRDF of the loose ground sample collected from RadCalNet Gobabeb test site used for vicarious sensor calibration of Earth observation satellites. The peculiarities of the ground sample as well as its preparation procedures are explained. A data synthesis method for combination of BRDF data of various ground sample fractions is introduced. The results on Gobabeb ground sample BRDF from laboratory characterizations are presented, analysed and compared with in-situ measurements. The last part of the thesis demonstrates the application of BRDF for functional material design. It introduces a new method to develop reflectance reference samples with the BRDF similar to the BRDF of crumpled reflective insulation material samples. The method based on microfacet model is thoroughly described and tested using physics-based Monte-Carlo simulations. Test results as well as example designs for anisotropic and isotropic reflectance reference samples for aluminium foil are presented, explained and discussed.
  • Metrological Developments for Aerosol and Mass Measurements
    (2014) Högström, Richard
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Reliable, comparable and accurate measurement results are only achieved by having traceability to recognized primary standards. Scientific and technological progress demand for and rely on developments in already established fields of metrology, such as mass, and in new evolving fields, such as aerosol particles, where the traceability is incomplete. In this thesis, a novel device called single charged aerosol reference (SCAR) invented by Tampere University of Technology (TUT) was developed into a primary standard for particle number concentration. For this, a full metrological validation was performed for determining the uncertainty of generated particle number concentration. It was shown that the generated particles are singly charged with a standard uncertainty of only 0.16 %. As a result, SCAR is the only primary standard capable of performing calibrations in a wide size range from 10 nm to 500 nm with a relative uncertainty (k = 2) of less than 2 %. A comprehensive inter-comparison of different particle number concentration standards was performed for the first time. The particle charge concentration measurements were compared in the particle size and concentration ranges 20 nm to 200 nm and 0.16 × 10-15 C cm-3 to 2.72 × 10-15 C cm-3 (equivalent to 1000 cm-3 to 17 000 cm-3 singly charged particles), respectively. An overall agreement of ±3 % was achieve with a few exceptions at low concentrations. As a result, a solid metrological basis for particle number concentration measurements worldwide was established for the first time. A significant source of error in gravimetric measurements of vehicle particulate exhaust is the so called filter artefact, i.e. adsorption of gas phase compounds onto the sampling filter. An experimental setup based on mixing hydrocarbon vapour and soot particles was constructed for studying the effect of the filter artefact in a systematic way. Studies with teflon coated filters and pentadecane (C15H32) vapour show that both the particle concentration and the filter soot load influence the filter artefact, such that lower particle concentrations and soot loads yield more adsorption. Instability of standard weights caused by sorption effects introduces uncertainties in the realization of the unit of mass, which is the ultimate basis for traceability for most particle measurements. Atomic force microscopy (AFM) was applied for studying changes in surface contamination of stainless steel weights upon ultrasonic cleaning in ethanol and vacuum exposure. An image processing method for increasing the sensitivity of detecting changes was developed. Results show that grooves are preferential sites for adsorption of contaminants.
  • Metrology for III-V Optosemiconductors
    (2017) Baumgartner, Hans
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Light-emitting diodes (LEDs) are III-V compound semiconductors manufactured by combining elements from group III (such as Al, Ga, and In) with elements from group V (such as N, P, As), forming compounds like GaN, GaInAs, and AlInP. High-power white LEDs are manufactured by coating a blue or an ultraviolet (UV) LED with phosphor, absorbing part of the blue/UV photons and emitting them at higher wavelengths. High-power LEDs are typically packaged to form an optosemiconductor device with a power rating between 1 and 5 W. In addition to light-emitting devices, III-V optosemiconductor devices can be utilized as photon absorbing, multi-junction solar cells. The materials and manufacturing processes for LEDs and III-V solar cells are the same, making their electrical and optical properties similar. In this thesis, measurement setups and novel analysis methods have been developed for luminous efficacy, lifetime, and band gap energy of LEDs and multi-junction solar cells manufactured using III-V materials. The lifetime of high-power LEDs was studied by aging different types of LED lamps at room temperature and at elevated temperatures of 45 °C and 60 °C. The aging was measured as the change of luminous flux over time. The aging accelerated on the average by factors of 1.35 and 2.36 when aging the LEDs at the elevated temperatures. The lifetime of high quality LEDs was shown to be more than 50 000 hours and projected with a new method to exceed 100 000 hours. Despite the high efficiency of LEDs, most of the consumed electrical power is still wasted in the form of heat, weakening the lifetime and optical characteristics of the optosemiconductor devices. To study the effect of temperature on the optical characteristics of high-power LEDs and multi-junction solar cells, a temperature controller based on liquid cooling and resistive heating was designed and built. A novel model for the emission spectrum of an LED was developed to determine the band gap energy of the device under tests. The method was shown to work in determining the alloy composition of III-V LEDs. The method was tested and utilized to determine the band gap energies of III-V multi-junction solar cells as well. The developed method can be used to determine temperature-invariant band gap characteristics of all III-V optosemiconductor devices.
  • Predictable Quantum Efficient Detector
    (2013) Sildoja, Meelis-Mait
     School of Electrical Engineering | Doctoral dissertation (article-based)
    This thesis gives an overview of the Predictable Quantum Efficient Detector designed to measure optical radiation with theoretical relative uncertainty of 1 ppm (parts per million). The device is based on two custom made large area induced junction silicon photodiodes arranged in a wedged trap structure. High internal quantum efficiency (IQE) of the photodiodes is achieved by means of low doping concentration and usage of the reverse bias voltage. The IQE is predicted to be improved furthermore using low operating temperature close to 77 K. The losses due to reflected light are minimized by multiple reflections between the photodiodes. Low losses allow the PQED to work as an ideal quantum detector whose spectral responsivity is determined purely by the fundamental constants h, c, e and vacuum wavelength lambda. The remaining minor charge carrier losses are predictable using physical modelling whereas fractional reflectance losses can be measured. These properties classify the PQED as an absolute detector which does not require calibration against any other radiometric primary standard. The prototype PQED was compared against present primary standard - the cryogenic radiometer – at the wavelengths of 476 nm, 532 nm and 760 nm at room temperature and at liquid nitrogen temperature. Comparisons showed that the predicted external quantum deficiency of the PQED agreed with the measured external quantum deficiency within the expanded uncertainty of 60 ppm to 180 ppm determined by the cryogenic radiometer at both temperatures. These results indicate that the responsivity of the PQED is highly predictable and its uncertainty is comparable with the uncertainty of the conventional cryogenic radiometer. Such data provide evidence that the cryogenic radiometer operated close to 10 K temperatures may be replaced by a PQED operated even at room temperature. The advantage of the PQED is its simple operation which is comparable with any other silicon based photodetector whereas its optical radiation detection uncertainty is comparable with expensive and sophisticated cryogenic radiometer.
  • Spatial and Spectral Corrections for Integrating Sphere Photometry and Radiometry
    (2019) Kokka, Alexander
     School of Electrical Engineering | Doctoral dissertation (article-based)
    The energy efficiency of lighting is expressed in terms of luminous efficacy. It is the ratio of the visible light emitted by the source to the power consumed in the process. The total amount of useful light produced by a source is described as luminous flux, which is often measured using integrating sphere photometers. These instruments consist of a hollow sphere and a detector whose output signal is proportional to the luminous flux emitted by the device under test. Non-ideal characteristics of photometers and integrating spheres induce measurement uncertainty on the luminous flux measured – and thus the resulting energy efficiency. With solid-state lighting superseding the conventional incandescent and energy-saving lamp technologies, the associated measurement techniques need to be revised as well. As integrating sphere photometers are generally calibrated using incandescent light sources, the measurement uncertainty is increased when determining the luminous efficacy of solid-state lighting products such as LEDs. This uncertainty can be reduced by employing correction factors that take into account the imperfections of the measurement system. In this dissertation, a method based on a fisheye-lens camera was developed to reduce measurement uncertainty due to spatial non-uniformities of integrating spheres. In order to calculate the spatial correction factor, the relative angular intensity distribution of the lamp under test is required. Traditionally, obtaining such a distribution has involved time-consuming and resource intensive goniophotometric measurements. With the fisheye camera method, the distribution can be measured in seconds using a fisheye camera installed into a port of the integrating sphere. To reduce the measurement uncertainty due to differences in the spectra of the calibration source and the device under test, a new LED-based reference spectrum was developed for calibrating photometers. The reference spectrum is based on one of the eight LED illuminants that were developed in the same study to be employed in colorimetry. For the LED products and photometers tested, the new reference spectrum reduced the average spectral mismatch errors by a factor of two, when compared with an incandescent calibration source.
  • Spectral Modelling of Light-Emitting Diodes and Atmospheric Ozone Absorption
    (2018) Vaskuri, Anna
     School of Electrical Engineering | Doctoral dissertation (article-based)
    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.