Abstract: A fast scan error measuring system of a raster output scanner (ROS) is provided in order to determine whether the laser beam position in the fast scan direction is within acceptable operational parameters. A detector, for example, a linescan or area array camera, is used to acquire multiple successive images from each facet of the ROS as the unit is operating. For each rotating facet the position of a beam spot (centroid) at the end of scan line is measured. A sufficient number of centroids are collected so that a Fourier transform of the centroids will produce an accurate spatial frequency spectrum measured in cycles per millimeter. This spectrum can be easily compared to the product's fast scan error specification limits.