The invention provides a non-invasive device comprising an optical system having a laser light source for generating a light beam, being configured such that, in use, the light beam exits the device and impinges on an outer surface of the skin to be treated； the optical system being configured to focus, in use, the light beam at a position of a treatment location inside the skin, whereby Laser Induced Optical Breakdown is induced at the position of the treatment location； an opening disposed such that, in use, light returning from the skin enters the device as a measurement light beam； and a first and second intensity detection channel configured and arranged to detect the intensity in, respectively, a first and a second bandwidth range of the measurement light beam, wherein the first and the second bandwidth ranges both comprise a wavelength of light produced in the skin as a result of Laser Induced Optical Breakdown, and the first bandwidth range comprises the Second Harmonic Generation wavelength of the laser light source, the second bandwidth range is adjacent to the first bandwidth range, and the second bandwidth range excludes the Second Harmonic Generation wavelength of the laser light source. The invention also provides a corresponding method to treat skin (30), in particular to reduce wrinkles. For the efficacy of such a treatment, two main parameters are crucial: the intensity at the position of the treatment location and the focus depth, and it is also crucial that these parameters can be measured using the same measurement system.
