An optical sensor is provided with reduced sensitivity toward external light influences, fluorophores, and radiation, more particularly gamma radiation. The sensor is suitable for determining at least one parameter in a medium. The sensor has a matrix that contains a fluorescent dye. The matrix is supported by a transparent substrate and has a precious metal layer on the side facing the medium. The precious metal layer provides protection against photobleaching and radiation. The optical sensor is suitable for implementation in containers and laboratory products that are sterilized by gamma radiation, such as disposable bioreactors.