Surface waves on a falling liquid film were visualized using shadowgraphy. A method of image processing was developed to extract images of wavefronts from a shadowgram. The wavefront image data was used to calculate the wave velocity and wavelength as a function of vertical and horizontal coordinates on the film surface. This method allows measurement of a newly defined variable, the wave distortion ratio which expresses the extent of distortion from a straight, horizontal line. The abundant two-dimensional results show the advantages of this image-processing method for investigations of surface waves on a liquid film.