The past decade has seen substantial de- velopments in the fieldof indirect optical gas sensing by means of thin (5-50 μm)gas-permeable membranes incorporating dyes which undergo reversiblechanges in their visible light absorbance or fluores- cencecharacteristics on exposure to the analyte gas. The sensor membranesare typically cheap and simple to produce, and visible lightoperation allows the use of inexpensive, readily available lightsources, optics, waveguides, and photo- detectors.
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