Background In this study, a simple, non-invasive, non-destructive digital imaging method was developed to determine the cell biomass concentration and distribution in photobioreactors. For this purpose, images of a panel type photobioreactor containing Rhodobacter capsulatus as a model photosynthetic microorganism were captured with a digital camera and processed to obtain RGB color information. Results Acquired photobioreactor images were processed to quantify cell concentration, blue color component was shown to be suitable for estimating the cell concentration accurately. In a batch reactor, the biomass change over time was tracked efficiently by the proposed method as verified by mass and spectrophotometric analysis. Cell distribution profile in the photobioreactor was revealed by further analyses of the selected captured images. It was shown that cells were denser at the bottom part of the photobioreactor, and the concentration gradient increased over time during the batch runs. Conclusion This research suggests that image analysis technique may be used as a simple method for continuous and online monitoring of cell concentration in photobioreactors. The research also demonstrated that a cell distribution map in the photobioreactor can be constructed to reveal non-homogeneity. (c) 2012 Society of Chemical Industry