Light intensity is a crucial factor that determines the growth of photosynthetic cells. This study analyzed the light distribution in a photobioreactor by processing images, captured with a digital camera, of a rectangular photobioreactor containing Synechococcus sp. PCC6801 as a model microorganism. The gray-scale images obtained clearly demonstrate the variation of the light-distribution profiles upon changing cell concentrations and external light intensity. Image-processing techniques were also used to predict the cell density in the photo bioreactor. By analyzing the digitized image data with a neural network model, we were able to predict the cell concentrations in the photobioreactor with a <5% error. (C) 2003 Wiley Periodicals, Inc.