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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.
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