We describe an enhanced spectroscopic measurement method for advanced diagnosis of phytoplankton growth for algal mass cultivation. The bulk absorption and scattering coefficient spectra of non-diluted algal samples were obtained in VIS range by using double integrating sphere measurements combined with inverse-adding-doubling computations. The theoretical optical properties were computed with Mie theory on model algal suspensions and compared to the experimental results. Our study proves the measured spectra can be analyzed to provide an extensive characterization of the culture state.