This paper reports experimental measurements of the radiation characteristics of green algae used for carbon dioxide fixation via photosynthesis. Particular attention was paid to three widely used species namely Botryococcus braunii, Chlorella sp., and Chlorococcum littorale. Their extinction and absorption coefficients were obtained from normal-normal and normal-hemispherical transmittance measurements over the spectral range from 400 to 800 nm. Moreover, a polar nephelometer is used to measure the scattering phase function of the microorganisms at 632.8 nm. It was observed that for all strains, scattering dominates over absorption. The magnitudes of the extinction and scattering cross-section are functions of the size, shape, and chlorophyll content of each strains in a non-trivial manner. Absorption peaks at 435, 475, and 676 nm corresponding to chlorophyll a and chlorophyll b have been clearly identified in the three species considered. The results can be used for scaling and optimization of CO2 fixation in ponds or photobioreactors as well as in the development of controlled ecological life support systems.