Light attenuation in purple phototrophic bacteria (PPB) has been studied. The results show that increased biomass concentrations lead to higher light attenuation, and that PPB absorb both visible and near-infrared wavelengths. Both spectrum fractions were equally absorbed at PPB concentrations above 1 g COD•L-1. A flat plate configuration showed less attenuation than top-illuminated cylindrical reactors, representative of open ponds. Neither a complex wastewater matrix nor the presence of polyhydroxyalkanoates affected light attenuation. The concentration of pigments (e.g. carotenoids and bacteriochlorophyll) had a strong effect, with much higher attenuation in the presence of pigments compared to non-pigmented biomass. In dense outdoor PPB cultures (≥1 g COD•L-1), effective light penetration is only 5 cm. This, together with the increased oxygen diffusion in large open ponds, biases design away from horizontal lagoons, and towards multi-panel systems such as flat plate reactors.