In the experiments at small inter-spot distances, these typical overpolymerized structures, which seem to spread vertically above the focal plane, could result from "out-of-plane" overlap of the PSFs. As the light focused by the microscope objective converges to form the array of spots in the focal plane, the local light energy density increases towards the write spot locations. If the spots in the array are sufficiently separated, the energy density will be too low outside the focal plane for overlap between the light fields from the different light spots to reach the polymerization threshold. However, when the spots are closer together, at planes close to the focal plane, the converging light may concentrate sufficiently for overlap to produce polymerization (see Fig. ).