Abstract: A system of measuring oxygen levels based on the intensity of a sensor material. Through use of intensity-based measurement techniques and improved light rejection, less excitation light and system gain are required as compared to certain conventional lifetime/phase shift techniques. One embodiment applies an intensity-based measuring system for taking measurements, and on an intermittent basis, using a lifetime/phase shift-type sensor in order to automatically calibrate the intensity-based measuring system in situ. A single sensing membrane may be provided while still allowing the system to be used in both the lifetime/phase-shift mode and the intensity mode. The system of the present invention may also include use of a variable thickness sensor for an oxygen sensing system, such as a portable handheld sensor and/or a sensor or controller at a fixed location.