OPTISCHER SENSOR

In an optical sensor used mainly for determining the partial pressure of O2 or CO2 in respiratory gas includes a sample measuring chamber through which sample medium to be analyzed flows or into which it is fed, a reference measuring chamber containing a reference gas or a reference liquid of constant composition, a light source, a unitary excitation filter positioned between the light source and first sides of both the sample measuring chamber and the reference measuring chamber, a single fluorescent sensor element on the second sides of both the sample measuring chamber and the reference measuring chamber, and first and second detectors for detecting the fluorescence radiation from the fluorescent sensor element respectively based on the light passing thereto from the sample measuring chamber and the reference measuring chamber.