Laser Sensing for Quality Control and Classification – Applications for the Food Industry, Ecology and Medicine
Sammanfattning: Monitoring constitutes a cornerstone, both for efficient control of optimized industrial processes and for obtaining knowledge about non-controlled, natural phenomena. This thesis is all about monitoring, or measurements. In this case the primary information is gathered with light, and in a non-intrusive, and usually a stand-off or remote way. The applications of optical monitoring in this work are directed towards some main areas: atmospheric gas monitoring, the food industry, ecology, and medicine. Some of the work cannot be classified as belonging solely to one of those groups, but is instead aimed at improving optical monitoring in general. Monitoring of gases in food packaging is the primary area within the food industry explored in the work of this thesis. A current trend is that food is increasingly often packed in so called modified atmospheres, where the air surrounding the product in the package has been replaced by another gas mixture. Monitoring is currently performed by intrusive spot-checks on a small number of packages, to make sure that they are actually tight enough to maintain the modified atmosphere all the way from manufacturing to the kitchen of the consumer. The studies in this thesis aim at introducing non-intrusive methods based on diode laser absorption spectroscopy, by which the gas in the packages may be checked, and re-checked, without breaking the seal. A long-term goal that could be realized with this technique is that more packages could be checked, perhaps on the production line and, at later stages, in the store. Waste could in this way be reduced if poor packing can be avoided, or remade, and the packing process and materials improved. The results are so far very promising and one specific technique variety was commercialized during the time of this thesis work. Diode laser spectroscopy has also been used in the medical field, where the primary goal was to introduce a way to monitor the air distribution in the lungs and intestines of prematurely born children. Both of these organs are often affected by a premature birth, and our results indicate that spectroscopic gas monitoring could be helpful. Interestingly enough, the techniques used for these medical measurements and for the packaging industry applications are almost identical. Apart from the work related to the food's packaging a pair of studies were also performed on the ``food" directly, namely on apple tissue and on tea. In a study of gas exchange in apples it was found, to our surprise, that apple pieces exposed to vacuum could maintain a low internal pressure for hours after re-exposure to ambient pressure. In a study on tea, fluorescence spectroscopy was evaluated as a means to obtain information on the quality of the product with a rapid, stand-off technique. A good correlation was found between the assessment by tea tasting experts and the fluorescence technique. Fluorescence spectroscopy was also used in this work on other organic materials, in the field of ecology where damselflies and birds were studied with the light detection and ranging (LIDAR) technique. Harmless pulses of UV laser radiation were emitted in a beam out into the atmosphere. The UV radiation induces fluorescence in the feathers of the birds and the abdomen and wings of the damselflies. The spectral features of the fluorescence are characteristic for different species, genders, or other characteristics, of the animals, and the technique can therefore be used for classification. The analogy with the tea study is strong. %A variety LIDAR was also employed for monitoring of gaseous mercury and nitric oxide in the atmosphere.
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