Browsing by Author "Ikonen, Erkki, Prof., Aalto University, Finland"

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Characterization of Light Emitting Diodes and Photometer Quality Factors
(2012) Poikonen, Tuomas
School of Electrical Engineering | Doctoral dissertation (article-based)
Light-emitting diodes (LEDs) are used in various applications due to their small size, durability and energy efficiency. The introduction of white high-brightness LEDs has changed the lighting, electronics and automobile industries for good. Incandescent lamps are being replaced by solid-state lamps (SSLs) for better energy efficiency. However, the optical and electrical properties of LEDs and SSLs differ from incandescent lamps, and characterization of these new light sources requires careful evaluation of the measurement methods for obtaining low measurement uncertainty. In this thesis, measurement setups have been developed for luminous flux and luminous efficacy measurements of LEDs and SSLs. The photometer heads utilized in the measurements have been characterized for the spectral and directional responses. Methods for analyzing the uncertainties of the photometer spectral and directional quality indices f1’ and f2 have been developed using Monte Carlo simulation. A multifunctional 30-cm integrating sphere has been constructed with custom ports and baffles for total and partial luminous flux measurements of both low- and high-power LEDs. The spatial corrections obtained are less than 0.2 % for typical directional LEDs. The expanded uncertainty (k = 2) of the measurement setup varies between 1.2 % and 4.6 %, depending on the color and the angular spread of the LED. For luminous efficacy measurements of SSLs, a measurement setup based on a compact goniospectrometer and a 1.65-m integrating sphere has been constructed. Test measurements of 25 different SSLs showed large differences between the lamps, especially in the luminous efficacies and the quality of the built-in power supplies. It was also found that the self-absorptions of SSLs have spectral dependence due to the materials used in the lamps. The luminous efficacy of a typical SSL with stable electronics can be measured with 1.2 % (k = 2) expanded uncertainty. The uncertainties of the spectral and directional quality indices f1’ and f2 of photometers have been investigated using Monte Carlo simulation with biased and random error models. The results show that simple random variation of the individual data points of the response data may lead to underestimated uncertainty of the quality index. For proper estimation of the uncertainties, physical models of the characterization measurements are needed. The developed methods give, for the first time, a solid basis for the uncertainty analysis of the photometer quality indices, which manufacturers typically report without uncertainties.