A degradation model that describes many image degradations produced by desktop scanning is used to study the edge noise that is present in bilevel document images. The standard deviation of the additive noise does not adequately describe the noise present after the image is converted to a bilevel image. A measure of noise called Noise Spread is developed which describes the edge noise and is a function of the scanner parameters. If phase effects are removed this Noise Spread quantity is directly proportional to the expected value of the Hamming distance between scans with and without edge noise. The Noise Spread has also been correlated with the ability to accurately estimate edge locations. A simple method to estimate this quantity is proposed.