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Certain kinds of cellulose, such as a hydroxyethylcellulose, swell in a humid atmosphere and show a decrease in refractive index. Using this effect, a simple optical humidity sensor with fast response and high sensitivity, which is based on the plastic optical fibre structure change from leaky to guided, was easily fabricated and could be used for real-time humidity monitoring.
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Muto et al. (2003) studied this question.
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