Proceedings Article | 28 January 2008
KEYWORDS: Printing, Calibration, Visibility, Eye, Nonimpact printing, Environmental monitoring, Scanners, Modulation, Image processing, Colorimetry
Fine-pitch banding is one of the most unwanted artifacts in laser electrophotographic (EP) printers. It is
perceived as a quasiperiodic fluctuation in the process direction. Therefore, it is essential for printer vendors to
know how banding is perceived by humans in order to improve print quality. Monochrome banding has been
analyzed and assessed by many researchers; but there is no literature that deals with the banding of color laser
printers as measured from actual prints. The study of color banding is complicated by the fact that the color
banding signal is physically defined in a three-dimensional color space, while banding perception is described
in a one-dimensional sense such as more banding or less banding. In addition, the color banding signal arises
from the independent contributions of the four primary colorant banding signals. It is not known how these four
distinct signals combine to give rise to the perception of color banding. In this paper, we develop a methodology
to assess the banding visibility of the primary colorant cyan based on human visual perception. This is our
first step toward studying the more general problem of color banding in combinations of two or more colorants.
According to our method, we print and scan the cyan test patch, and extract the banding profile as a one
dimensional signal so that we can freely adjust the intensity of banding. Thereafter, by exploiting the pulse
width modulation capability of the laser printer, the extracted banding profile is used to modulate a pattern
consisting of periodic lines oriented in the process direction, to generate extrinsic banding. This avoids the
effect of the halftoning algorithm on the banding. Furthermore, to conduct various banding assessments more
efficiently, we also develop a softcopy environment that emulates a hardcopy image on a calibrated monitor, which
requires highly accurate device calibration throughout the whole system. To achieve the same color appearance
as the hardcopy, we perform haploscopic matching experiments that allow each eye to independently adapt to
different viewing conditions; and we find an appearance mapping function in the adapted XYZ space. Finally, to
validate the accuracy of the softcopy environment, we conduct a banding matching experiment at three different
banding levels by the memory matching method, and confirm that our softcopy environment produces the same
banding perception as the hardcopy. In addition, we perform two more separate psychophysical experiments
to measure the differential threshold of the intrinsic banding in both the hardcopy and softcopy environments,
and confirm that the two thresholds are statistically identical. The results show that with our target printer,
human subjects can see a just noticeable difference with a 9% reduction in the banding magnitude for the cyan
colorant.
