Abstract: Measurement of sample surface flatness of a continuously moving sample. A conveyor continuously conveys a sample beneath a grating disposed at a non-zero angle with respect to the plane of conveyance. The relative distance between the sample and the angled grating changes with the horizontal translation of the sample. A camera disposed above the sample and the grating captures, at constant time intervals, a sequence of images, each image comprising a shadow moiré fringe pattern that is indicative of the sample's surface flatness. The continuous change in relative distance between the sample and the grating introduces a known or unknown phase step between the shadow moiré fringe patterns of each successive image. A computer associated with the camera processes the images to determine phase values of pixels at, and the relative height of the sample surface at, selected pixel locations of the sample.