Browsing by Author "Poikonen, Tuomas, Dr., VTT MIKES, 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.