The most important parameter considered for the development and utilization of a specific type of reactor for microalgae cultivation is the light diffusion. The productivity of photo­autotrophic cultures is primarily limited by the supply of light and suffers from low energy-conversion efficiencies caused by inhomogeneous distribution of light inside the cultures (Grobbelaar, 2000). At culture surfaces, light intensities are high, but absorption and scattering result in decreasing light intensities and complex photosynthetic productivity profiles inside the cultures (Ogbonna and Tanaka, 2000). High light intensities at culture
surfaces may cause photoinhibition, and the efficiency of light energy conversion into biomass (photosynthetic efficiency) is low. An overdose of excitation energy can lead to pro­duction of toxic species (e. g., singlet oxygen) and to photosynthesis damage (Janssen, 2002)

By minimizing depth, volume is reduced or area is increased, light diffusion is maximized, and so is cell concentration. From common types of photobioreactors, light paths in open ponds are usually 10-30 cm depth, in tubular reactors ranges from 1-5 cm, and in flat panel reactors from 2-5 cm.

The light regimen itself is influenced by incident light intensity, reactor design and dimension, cell density, pigmentation of the cells, mixing pattern, and more. In outdoor photobioreactors the light regimen is also influenced by geographical location, time of day, and weather conditions. Nowadays, open paddlewheel-mixed pond is the most commonly used photobioreactor.

Some studies discuss the effect of mixing and productivity due to the "flashing-light" effect: A few milliseconds’ flashes of high light intensity followed by a several-fold longer period of darkness do not reduce culture productivity from those under constant illumination (Kok, 1953). This effect is not observed in ponds, where the light/dark period is longer. For example, although light/dark cycles of 94/94 ms were sufficiently short to increase the pho­tosynthesis efficiency in cultures of Dunaliella tertiolecta, light/dark cycles of 3/3 s were too long and the PE decreased in comparison to continuously illuminated cultures (Janssen et al.,

2001) . This refers to the theory of photosynthesis, in which carbon fixation is not dependent on the presence of light because sufficient energy has been absorbed.