A method of detecting contaminants on a window surface of a viewing system comprises the steps of: reflecting light off of contaminants on the window surface; capturing the reflected light in an image; converting the image into image data; and processing the image data to detect the contaminants on the window surface. Also, apparatus for detecting contaminants on an external surface of a window of a viewing system comprises: at least one light source for reflecting light from contaminants on the external surface; an imager for capturing an image of the reflected light and converting the image into image data; and a processor for processing the image data to detect the contaminants on the external surface. The apparatus may be embodied in a viewing system wherein the imager also captures images of a scene of the viewing system both including the light reflected from the contaminants and not including light reflected from contaminants; and the processor processes both of image data of the scene excluding reflected light from the contaminants, and image data of the scene including reflected light from the contaminants to detect the contaminants on the external surface.