1. A method of identifying intended response marks in an optically scanned image of a response sheet having response targets printed in areas of the sheet apart from a section of the sheet containing key marks, the key marks and response targets being printed in the same color, the method comprising the steps of: (a) providing a digital imaging scanner having a light source and sensor for detecting light reflected off the sheet, the scanner being capable of assigning pixel darkness values so that each pixel is assigned a gray scale value; (b) providing a color filter in the color of the key marks and response targets, (c) arranging the filter in relation to the sheet as it passes through the scanner such that light reflected from the section containing key marks is not filtered and light reflected from the response targets is filtered, (d) scanning the sheet with the digital imaging scanner; (e) using the gray scale values to locate key marks in the captured image, (f) calculating from the location of the key marks in the image an expected location of response targets in the image, (g) using the gray scale values to identify an intended response mark at the expected location of a target.