Laser powers were measured in front of the 50/50 beam splitters in both setups. The cumulative losses through the beam splitter and the objective (the diameter of the collimated laser beam is approx. 20% larger than the back apertures of the objectives) are estimated to be approx. 70%. As the Airy disk contains 84% of the total excitation power, the irradiance I in the diffraction limited focus with a radius of approx. λ/4 is estimated to be: