Paper Surface Topography by Range from Focus.

The quality of low grade printing paper is largely determined by the physical paper properties. The surface topography is one important factor which has traditionally been measured by pointwise scanning with a mechanical or optical probe. Range camera methods provide a much faster measurement. The Range from focus method utilizes the narrow depth of field in a large aperture optical system to obtain range data. A range image is made by collecting a sequence of greyscale images at closely spaced successive focus positions, and then processing the image data to find the optimal focus position for each point in the imaged region. Here, the method is applied to microscopic images of a newsprint paper sample to obtain a topographical map of the paper surface. The resulting range image is dense and quite accurate, and the measurement is fast. There are also numerous possible ways to further improve upon the range image quality, out of which only a few have been tried.