<title>Periodic image artifacts in digital halftone prints</title>

Abstract

In digital halftone printers, image artifacts (unwanted texture ) are produced by laser writer position errors. Examples of this type of error are errors induced by imperfections in M-sided rotating polygon mirrors, errors induced by pitch errors in lead screw positioners .The determination of the tolerable position error requires a calculational method for estimating the visibility of the position error induced texture. The method for estimating the visibility of halftone dot texture proposed by Nasanen [1] is applied in this paper to position error induced artifacts. In order to simplify the analysis, the response characteristics of the material are considered to be binary. This assumption greatly simplifies the mathematical description of the halftone dot (or pel as it is sometimes called) since the transmissivity is now a constant over the written region and writing only occurs when a threshold is exceeded .( This is the model used by Melnychuck and Shaw [ 2] .) In Section II the contrast detection model proposed by Quick [ 3 and applied to digital halftones by Nasanen [ 1 1 .is discussed. In order to compare different halftone patterns, the dot visibility calculation is used to select a repeat size (spatial period ) so that the halftone dots are not detectable when there is no positioning error. In Section III, the same model is then applied to calculate the visibility of sinusoidal position errors. For raster scanned,continuos tone images , Bestenreiner et al. [4] and Schubert[5] have shown that the visibility of the error depends on the frequency of the position error , and the ratio of the magnitude of the error to the repeat size. The results presented below indicate that , for digital halftone printing, the visibility of the error depends also on the pattern used if the dot size exceeds the step size ( or the distance between addresable dots. ) Thus , a maximum acceptable position error can be calculated for given printing conditions and in some cases shown to be less than that calculated for raster scanned (contone) images [4],[5]. For position errors that are described by a spectrum rather than a single frequency, some of the simplifying assumptions used by Nasanen[1] have to be reexamined. An extension of the contrast response model to the case in which the position error is described by a frequency spectrum or a power spectral density is proposed in SectionlV.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.