Light distribution inside algal culture at different incident light intensities (I-0 = 70-350 mu mol m(-2) s(-1)) and biomass concentrations (C-b = 0.09 -1.34 g L-1) for various configurations of vessels (rectangular and cylindrical) and illuminations (single and even light directions) are investigated and discussed. At C-b > 0.5 g L-1 the results showed that the influence of I-0 is insignificant, particularly at far distances from the illuminated surface, and increasing 10 is not an efficient solution for facilitating photosynthesis in dense cultures. The influence of C-b on light attenuation is strong, especially close to the illuminated surface. Over the first 2 cm, the light intensities inside the culture decreased by 35% and 92% when C-b was 0.09 g L-1 and -1.34 g L-1 respectively. For cylindrical vessel the light distributions in dens culture are quite similar for both one-side and evenly illuminated. Furthermore, new quantitative model has been developed by adapting the Beer-Lambert model to describe light distribution inside algal cultures and predict local light intensity more accurately. The model was able to predict the sharp reduction in light intensity near the vessel surface that B-L model failed to predict; also the model was successfully simulating the experimental results under different conditions.