Until recently, the combination of flow and optogenetics, or flow with any photo-regulated cell sample, was impossible. Most people were and still are using fluorescent microscopy. This makes perfect sense for all assays that require subcellular resolution. But for all other assays, flow offers a high-content, high-throughput alternative. Moreover, there are no limitations to the choice of fluorophore. The photostimulation happens in a different place and time from the analysis, so you can use the whole spectrum of fluorophores for your experiments.