For raster-written images, the modulated laser exposing beam is scanned across the photosensitive material in a line-by-line configuration. Image noise can be introduced by the writer directly, for example, by the granularity of the photosensitive materials and indirectly, for example, by beam position errors. For analysis of the effect of position errors they must first be related to their resultant exposure fluctuations. Here, position errors are addressed via models of image writing that include several spot/pixel writing schemes. Three types of image noise due to page-scan position error are examined. The effect of low-frequency position errors is described. Exposure fluctuations due to broadband stochastic errors are then addressed. For laser writers using a rotating polygon for beam deflection, the effect of stochastic facet-angle errors is repeated down the image; this results in periodic exposure fluctuations and is the third type of image noise analyzed. Expressions are given for the mean and variance of the exposure error in terms of the statistics of the position error, writing spot profile, and raster sampling distance. The analytical models are then compared with the results of an image simulation calculation. In this way, the exposure error fluctuations are described by their noise power spectra as functions of spatial frequency. After consideration of the sensitometry of the hardcopy recording materials, the exposure errors are then related to the corresponding output density fluctuations.
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