The lower the filter, the more the scatter.
The 515 filter has the most scatter and is ineffective except when treating vessels at the most superficial levels because of this scatter.
When you go above 550 the scatter drops tremendously so it is not nearly as big a problem.
The ESC tests (former owner prior to Lumenis) demonstrated that the 560 filter was the best for the hairpin loops that feed the epidermis. The 590 filter treated both the hair pin loops and the thicker feed vessels right below the hair pin loops.