ABSTRACT
This chapter gives an overview on the principles and applications of optical sensors, with particular emphasis given to problems associated with bioprocess control. Similar instrumentation may be used when absorbance, reflectance, or Raman scatter intensity are to be measured. A typical example is provided by the glucose detection principle: Glucose can be oxidized by the enzyme glucose oxidase to give gluconic acid (H) and hydrogen peroxide under consumption of oxygen. IV The following optical parameters have been used so far in spectroscopic sensors:absorbance, reflectance, interferometry light scattering, fluorescence intensity, fluorescence lifetime, chemilumines-cence, phosphorescence, thermoluminescence, and refraction index. Absorbance-based measurements have been applied in optical sensing much less often than reflectance or fluorescence measurements, because most real samples are too strongly colored to allow precise absorbance measurements, and because absorbance methods cannot be adapted to turbid solutions. The second (and more realistic type in case of FOCS) is diffuse reflection, where the light penetrates the medium and subsequently reappears at the surface after partial absorption and multiple scattering within the medium. Several models for diffuse reflectance have been proposed, all of which consider that incident light is scattered by particles within the medium. We have recently reported [20] on a quite new type of oxygen sensor with the fluorescence decay time as the information carrier.
