Abstract: An improved apparatus and method for monitoring the characteristics of materials by their fluorescence are described. A unique design is employed in which common fields of view are provided for the optical paths for the illuminating light and the emitted fluorescence. In a preferred form they are coaxial. This permits the apparatus to be contained within a probe which may be inserted directly in a vessel containing the material to be analyzed. The device is specifically designed to withstand sterilization with steam, to operate aseptically within biological processes, and to maintain a high stability and sensitivity over extended periods of time.