Photobiological hydrogen production is a sustainable alternative to thermo-chemical and electrolytic technologies with the added advantage of carbon dioxide mitigation. However, the scale-up of this technology from bench top to mass culture in industrial photobioreactors suffers from low solar-to-chemical conversion efficiency and irradiance transmittance limitations. In order to overcome these challenge, algae such as Chlamydomonas reinhardtii, can be genetically engineered to have reduced pigment concentrations in their photosystems. Thus, algae do not absorb more light than they can utilize. Moreover, reducing pigment concentration will enable solar radiation to penetrate deeper in the photobioreactor thus facilitating greater productivity in the high density mass culture. The objective of this study is to experimentally measure the radiation characteristics of the unicellular green algae Chlamydomonas reinhardtii CC125 and its truncated chlorophyll antenna transformants tla1 and tlaX. Their extinction and absorption coefficients are obtained from normal-normal and normal-hemispherical transmittance measurements over the spectral range from 300 to 1,300 nm. Moreover, a nephelometer with a He-Ne laser source is used to measure the scattering phase function of the microorganisms at 632.8 nm. Results indicate that the mass absorption cross-sections decrease for the truncated chlorophyll transformants while the single scattering albedo increases. Therefore, light scattering becomes more important in photobioreactors using the genetically engineered strains. The results reported can be used with the radiative transport equation (RTE) to accurately predict and optimize light transport in photobioreactors for photobiological hydrogen production and/or carbon dioxide mitigation using genetically engineered microorganisms.