Photobiotechnology is an emerging field of bioprocess engineering with a wide range of applicable phototrophic microorganisms and aimed at biomass and metabolite production for pharmaceutical, nutritional, cosmetic or even energetic purposes. Industrial-scale production calls for large photobioreactors (PBRs) but up-scaling is impeded by light as a special substrate. Photobioreactors have to meet standard requirements on bioreactor systems such as homogenization, dispersion, mass and heat transfer, and they have to enable supply of photosynthetic usable light in sufficient amounts as well. External illumination creates a light gradient within the microalgae suspension due to mutual shading. This light gradient causes a non-uniform illumination of the cell population. Moreover, mixing of the suspension generates temporal fluctuations of the light intensity. I.e., each microalgal cell experiences an individual illumination history. The construction of operational large-scale PBRs requires knowledge of the growth kinetics depending on illumination intensity and lightening regime. The effect of different light intensities and various light-dark cycles on microalgal growth has been repeatedly published but the results were contradictory due to experimental deficits. A lab-scale flat-panel photobioreactor was therefore constructed which allows cultivation of microalgae at a uniform but alterable light intensity and at changeable light-dark cycles. The homogenous illumination within the flat-panel reactor and its continuous operation simplifies the measurement of growth and metabolite synthesis. The application of LEDs allows variation of the light intensity over a wide range of illumination from the point of compensation to photoinhibition. Sophistic instrumentation (sensors for dissolved oxygen, fluorescence, and optical density) enables non-invasive online monitoring of the process. Comparative cultivation experiments were done in a conventional bubble column and in the described flat-panel photobioreactor (using Chlamydomonas reinhardtii and Arthrospira platensis as the model organisms) to demonstrate the advantages of the newly developed bioreactor system.