The exact knowledge of Drop Size Distributions (DSD) plays a major role in various fields of applications to control and optimize processes, as well as reduce waste. In the microbial production of advanced biofuels, oil droplets are produced under turbulent conditions in an aqueous medium containing many surface active components, making DSD knowledge essential for process optimization. The capability of a photo-optical measurement method for DSD measurement in fermentation broth and in plate separators is illustrated aiming at cost reduction in the microbial production of advanced biofuels. Measurements were carried out with model mixtures in a bioreactor, and at the inlet and outlet of a plate separator. In the bioreactor, the method was effective in detecting a broad range of droplet sizes and in differentiating other disperse components, e.g., microbial cells and gas bubbles. In the plate separator, photo-optical measurement effectively determined the influence of the varied parameters on the separation efficiency.