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At EI 2007 in San Jose, California detailed
physical models for monochrome and color
electro-photographic printers were presented.
These models were based on computer
simulations of toner-dot formation for a variety
of halftone structures. The optical interactions
between the toner-dots and the paper substrate
were incorporated by means of an optical
scattering function, which allowed for the
calculation of optical dot-gain (and physical
dot-gain) as function of the halftone structure.
The color model used simple red-green-blue
channels to measure the effect of the
absorption and scattering properties of the
cyan, magenta, yellow and black toners on the
final half-tone image. The new spectral model
uses the full absorption and scattering
spectrum of the image toners in calculating the
final color image in terms of CIE XYZ values
for well-defined color and gray patches. The
new spectral model will be used to show the
impact of halftone structure and toner-layerorder
on conventional dot-on-dot, rotated dot
and error diffusion color halftone systems and
how to minimize the impact of image toner
scattering. The model has been expanded to
use the Neugebauer equations to approximate
the amount of cyan, magenta, and yellow
toners required to give a "good" neutral in the
rotated dot halftone and fine tuning is achieved by adjusting the development threshold level for each layer to hold a good neutral over the full tonal range. In addition to the above fine-tuning, cyan, yellow and magenta offsets are used to find an optimum use of the halftone dither patterns. Once a "good" neutral is obtained the impact on dot gain, color reproduction and optimum layer order can studied with an emphasis on how the full spectral model differs from the simpler three-channel model. The model is used to explore the different approaches required in dot-on-dot, rotated dot and error diffusion halftones to achieve good results.
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