This contribution presents a novel approach to investigate entrainment in distillation and absorption columns. An image-based probe allows precise droplet detection at various radial and axial positions above trays. Validations achieve an average error of 6.4 % (monospheres 9.2-114.4 mu m) and 3 % (monodisperse droplet stream up to 19 m s(-1) and 74.5 mu m). Experiments in a DN 450 cold flow test rig show an increasing (decreasing) share of larger droplets with higher gas (liquid) loads. Locally measured droplet sizes depend on probe position as well as tray design and enable an extrapolation to integral entrainment rates.