Continuous, homogeneous lighting conditions are fiction. The form of the culture vessel, as well as the light source and the culture density, influence the light field within a culture. What actually happens is due to the alteration of light within an optical lens. This also applies to the usual exposition devices used for studying primary production. In the worst case, with a point source and without any precautions, as much as 10% of the lens (vessel) volume is shaded by optical effects, whereas light is concentrated along a focus within the lens. Stirring of algal cultures introduces time, so focusing is transformed into flickering. A solution is to immerse the vessel in a water bath of rectangular shape.