A device (10) for energy-based skin treatment is provided. The device (10) comprises a light source (18) for providing polarized incident light (21) having an incident polarization, optical elements (11, 12, 13) for focusing the polarized incident light (21) in a focal point (22) within a collagen layer of the skin (30), a polarization-sensitive detection unit (41, 42) for selectively detecting a selected polarization component of light of a generated harmonic (23) of the polarized incident light (21) returning from the skin (30), and a processor, at least being coupled to the polarization-sensitive detection unit (41, 42), for determining a depth of the focal point (22) within the collagen layer based on the detected selected polarization component.
