1. A method for sensing moisture (20) on the exterior surface (22) of a sheet of glass (14) comprising the steps of; positioning an imaging lens (16) adjacent to the interior surface (18) of the sheet of glass (14), passing light rays from moisture (20) on the exterior surface (22) of the sheet of glass (14) through the imaging lens (16) and producing first and second successive images of the moisture (20) on the glass (14), and successively directing the first and second images from the lens (16) onto a focal plane detector (12), dividing each of the images into a plurality of elements (24), said method characterized by scanning the elements in a first scan and producing an initial reference value for each element in response to an initial image in that element, scanning the elements in a second scan and producing a subsequent value different from the initial reference value in response to an initial image in that element, developing a correlation coefficient value from the reference and subsequent values, and producing a wiper signal in response to the correlation coefficient value deviating from a predetermined value.