Details:
Summary:
Most process printing ink sets, including those used by color office printing equipment, use yellow inks which have very little absorption in the greenred wavelength spectral region. Since these inks are also quite transparent, the green-red spectral reflectance of media coated with these inks is determined, in large part, by the media reflectance. Reflectance measuring errors can occur when the measuring system collection optics fail to observe all of the reflected portion of instrument illumination which was assumed in the instrument design. These errors have been termed lateral diffusion errors (LDE) because they result from lateral diffusion of light within the sample.
The excellent transmission and low scattering of dye based yellow process printing inks result in the green-red reflectance LDE of the solid printed areas being almost as great as the LDE of the unprinted paper. However, since the LDE also reduces the reflectance difference between the red and blue ends of the spectrum, there is also a reduction in the CL*a*b* C* (chroma or color purity) which is often two or more times larger than the CL*a*b* L* (lightness-darkness) error.
Yellow and magenta process inks on high quality paper typically exhibit peak reflectances of 95% or more in the red spectral region. In an evaluation of eight ink-jet systems, cyan inks printed on good paper had a peak reflectance ranging from 62 to 89 %, dependent on the printer system, and, additionally, all exhibited lower reflectance in the green and blue regions of the spectrum. The illuminating light and the reflected light both pass through the ink layer prior to being detected by the instrument. This dictates that at the very least that the LDE should be proportional to the square of the sample reflectance. Also, the LDE of the paper layer at the cyan wavelengths is typically half that of the LDE at the red wavelengths. These factors greatly reduce the lightness and chroma errors experienced with cyan sample relative to those experienced with red and yellow samples.
Many of the BCRA tiles, which are made by firing a transparent colored glaze over a ceramic tile base, also show chroma errors similar those experienced with exhibits made with red and yellow process inks printed on paper. The coatings used in the MacBeth Color Checker(r) are relatively opaque and generally do not exhibit LDE induced chroma errors similar to those experienced with process printing inks.