This project will develop and test a chlorophyll florescence sensor for ecological studies. The investigators will measure chlorophyll florescence in the oxygen A band, where atmospheric oxygen spectrally excludes incident sunlight. A cell containing low pressure oxygen will be used as a narrow band filter to absorb chlorophyll florescence photons at the A band line centers, and later re-emit them for detection. This non-invasive technology has potentially wide commercial use in agriculture and surveys of stressed vegetation.