The luminostat regime has been proposed as a way to maximize light absorption and thus to increase the microalgae photosynthetic efficiency within photobioreactors. In this study, simulated outdoor light conditions were applied to a lab-scale photobioreactor in order to evaluate the luminostat control under varying light conditions. The photon flux density leaving the reactor (PFD(out)) was varied from 4 to 20 mu mol photons m(-2) s(-1) sand the productivity and photosynthetic efficiency of Chlorella sorokiniana were assessed. Maximal volumetric productivity (1.22 g kg(-1) d(-1)) and biomass yield on PAR photons (400-700 nm) absorbed (1.27 g mol(-1)) were found when PFD(out) was maintained between 4 and 6 mu mol photons m(-2) s(-1). The resultant photosynthetic efficiency was comparable to that already reported in a chemostat-controlled reactor. A strict luminostat regime could not be maintained under varying light conditions. Further modifications to the luminostat control are required before application under outdoor conditions. (C) 2011 Elsevier Ltd. All rights reserved.