Florescent lamps, halogen lamps, optical fibers, neon tubes, light emitting diodes all giving photosynthetically active radiation (PAR) are possible artificial light sources for photofermentation. One of the main important parameters deciding system productivity is the light conversion efficiency (g):

[33.61 + pH x VH21 g = H H2 x 100

I x A x t

where VH2 is the volume of the produced H2 in l, qH2 is the density of the produced hydrogen gas in g/l, I is the light intensity in W/m2, A is the irradiated area in m2 and t is the duration of hydrogen production in hours [94]. The light conversion efficiencies depend on the strain and the substrate used. Generally varying between 1 and 6% but a 9.23% light conversion efficiency was achieved by Rhodobacter sphaeroides using lactate as carbon source and a tungsten lamp with a light intensity of 200 W/m2 [123]. The photobioreactor can use the sunlight alone or it can be combined with artificial lights. Light intensity may be measured by either W/m2 or lux. The conversion between these two units depends on the wavelength but it can be assumed as 1 W/m2 is equal to 30-100 lux [5]. For photofermentation experiments it is important to decide the best light intensity value before starting the large-scale experiments. Especially in large-scale application the shading effect of organisms could affect the system performance. At this point internal illumi­nation could be a good option. Different kinds of light sources were combined in terms of increasing the hydrogen productivity and the photobioreactor was illu­minated by combining light sources, including an internal illumination with optical fiber excited by solar energy (OF(sunlight)) as well as external irradiation of tungsten filament lamp (TL). A 138 and 136% increase in cumulative hydrogen production achieved by combination of OF (sunlight)/TL was found to be more effective than TL/TL combination [14].