Phototropic microalgae have a large potential for producing valuable substances for the feed, food, cosmetics, pigment, bioremediation, and
pharmacy industries as well as for biotechnological processes. Today it is estimated that the microalgal aquaculture worldwide production
is 5000 tons of dry matter per year (not taking into account processed products) making it an approximately $1.25 billion U.S. per year
industry. For effective observation of the photosynthetic growth processes, fast on-line sensor systems that analyze the
relevant biological and technical process parameters are preferred. The optical properties of the microalgae culture influence the
transport of light in the photobioreactor and can be used to extract relevant information for efficient cultivation practices. Microalgae
cultivation media show a combination of light absorption and scattering, which are influenced by the concentrations and the physical and
chemical properties of the different absorbing and scattering species (e.g. pigments, cell components, etc.). Investigations with frequency
domain photon density waves (PDW) allow for the examination of absorption and scattering properties of turbid media, namely the absorption
and reduced scattering coefficient. The reduced scattering coefficient can be used to characterize physical and morphological properties of
the medium, including the cell concentration, whereas the absorption coefficient correlates with the pigment content.
Nannochloropsis oculata, a single-cell species of microalgae, were examined in a nutrient solution with photon density waves. The
absorption and reduced scattering coefficients were experimentally determined throughout the cultivation process, and applied to gain
information about the cell concentration and average cell radius.