An inertial impactor is used for recent deposition experiments with PSL spheres onto a silicon nitride surface. Therefore, an aerosol flow is accelerated and deflected at an impaction plate. Spheres exceeding a certain size cannot follow the streamline at high flow speeds anymore due to their inertia, hit the impaction plate and deposit there. The flow speed of the aerosol is a crucial parameter of the size dependent deposition efficiency at the impaction plate. So far, it is assumed that the particle velocity and the flow speed of the aerosol are in a comparable range. In this work we present a method that can be used to determine the velocity of PSL spheres right before impacting the surface of a silicon nitride layer. Approaching PSL spheres interact with the evanescent field of light that is guided through the silicon nitride layer. The temporal behaviour of these interactions is studied with an optical detector.