The spot size has been measured positioning a fluorescent YAG screen at the sample position and taking an image with CCD camera and a calibrated magnification optic placed at about 300 mm (S. Kubsky, Surface Laboratory).

3D representation of the photon beam intensity

Comparison of the image obtained by ray tracing calculation(left) and of the measured focal spot (right) for the same field of view: 300μm x 300μm.