Random laser-writer position errors in raster-scanned monochrome halftone images are examined by using Fourier analysis techniques. For a high-contrast, narrow-exposure latitude recording material (typically used in halftone reproduction) with medium-sized halftone dots (25–85%), a one-dimensional halftone model is developed to derive the signal power spectrum of a halftone image containing position errors in the slow-scan (page-scan) direction. The spectrum of such an image is shown to consist of a periodic component and a random component, which is a function of position error but independent of dot size. The term signal power spectrum, in the context of this paper, is the average squared modulus of the Fourier transform of an image containing these errors. A comparison is made between the percent position-error specification quoted for continuous-tone laser writing and the corresponding specification made in writing digital halftones. For a given absolute position error, the ratio of the resulting percent position error of a single pixel in continuous-tone writing to that for a halftone cell is shown to be roughly , where X is the halftone cell period and L is the pixel size used in continuous-tone reconstruction. Thus, whereas the position-error tolerance for halftone writing is almost an order of magnitude greater than its continuous-tone counterpart on a percent basis, on an absolute scale they are approximately equal. The results can be generalized to include any digitally generated halftone image based on a center-growing dot configuration and containing dot-size/shape distortions.
© 1988 Optical Society of AmericaFull Article | PDF Article
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