A system mounted on a vehicle for detecting an obstruction on a surface of a window of the vehicle, a primary camera is mounted inside the vehicle behind the window. The primary camera is configured to acquire images of the environment through the window. A secondary camera is focused on an external surface of the window, and operates to image the obstruction. A portion of the window, i.e. window region is subtended respectively by the field of view of the primary camera and the field of view of the secondary camera. A processor processes respective sequences of image data from both the primary camera and the secondary camera.