A simultaneous external and internal illumination method is proposed for cylindrical airlift photobioreactors in this paper, in which multiple light sources are placed around the downcomer region, to achieve a more homogeneous light distribution. Internal light sources are mounted near the wall of the draft tube and external light sources near the wall of the photobioreactor. The light distribution in an airlift photobioreactor under simultaneous external and internal illumination (APER-SEII) with various biomass concentrations and radii is investigated using the two-flux model. This model is able to obtain reasonable prediction compared with the simulation of more rigorous DO models, and is suitable for real-time control of microalgae cultivation in photobioreactors. Light attenuation profiles are compared with those of only externally illuminated airlift photobioreactors (APER-EI), showing that the new illumination method can increase the size of the illuminated zone in photobioreactors and enable a more uniform light distribution in the downcomer region. The calculation results also show that the annular distance of the downcomer region (Delta R) is more critical than the radius of the photobioreactor (R) to light distribution and therefore the design of photobioreactors. Asymmetry of light distribution in an airlift photobioreactor with the same radiation flux on the two walls is pronounced for low biomass concentrations and small radii. (C) 2012 Elsevier Ltd. All rights reserved.