The invention provides a non-invasive skin treatment device (100) comprising: a light source (10) constructed and configured for generating linearly polarized probe light (12) and linearly polarized treatment light (22), a polarization modulator (30) constructed and configured for controlling a polarization direction of the probe light and a polarization direction of the treatment light, a polarization sensitive sensor (40) constructed and configured for sensing a level of depolarization of the probe light by sensing an intensity of back-scattered probe light (42) from the target position (210) in a predefined polarization direction of the polarization sensitive sensor, and a controller (60) being configured for scanning the polarization direction of the probe light over a predefined range while receiving the measurement signal (Sm) and for selecting an optimum polarization (P1) direction for which the depolarization of the probe light is at a minimum. The invention further provides a computer program product for controlling the skin treatment device.