Color separation method and printed product of the method

#The present invention is directed to a method of making color separations from an original color image so that an ultimate printed image is tolerant of some misregistration of the individual color printing plates. The invention also encompasses the ultimate printed product made using the method.

#Color printing by lithography goes back to the early years of the nineteenth century. Multiple stones were used, one for each ink printed, and the print went through the press as many times as there were stones. The problem was then, as it is today, to keep these individual images in proper register. Considerable skill was required on the part of the printer to make sure that each color would be in the correct position and that the overlying colors would merge correctly. The process left much to be desired and up to the middle of the century the highest quality works were colored by hand. High costs severely limited availability of color illustration. The development of a number of photomechanical processes near the turn of the nineteenth century reduced the level of skill needed and greatly expanded the use of printed illustration. Processes such as photolithography, photogravure, photoengraving, and others, allowed the image to be photographically transferred from an original photograph or drawing to a printing plate. These processes initially were useful only for black and white illustration. The later invention of the trichromic halftone process was a major breakthrough in printing color images. Improved photographic films with wide spectral sensitivity allowed the use of individual camera filters which removed all colors except the red, green and blue spectral region of the original. This allowed the preparation of a series of halftone plates that printed the images in complementary cyan, magenta and yellow inks. So-called process printing, using subtractive primary Cyan, Magenta, and Yellow inks, along with blacK ink (CMYK) is now in wide use. While equal amounts of superposed transparent cyan, magenta, and yellow inks would nominally produce black, the result is more often a muddy brown. The separate black ink is used to overcome the spectral impurities of the three subtractive primaries. With standard process printing, four image separations are needed, one for each ink color. A separation is a monotone (grey scale) image that indicates how much of a given color ink needs to be printed at a given location. For higher quality printing where a wider color gamut might be needed, other processes such as Hexachrome® can be used. Hexachrome is a registered trademark of Pantone, Inc, Carlstadt, N.J. This process adds orange and green to the basic CMYK colors. A black separation is also normally used to provide a wider range of image luminance and render better neutral greys and shadow density.

#The basic trichromic halftone process remains the standard for both large and small run print reproduction using traditional (non-digital) presses. Keeping the colors in register during printing was an early problem and remains so today.

#This patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee.

#FIG. 1 is an illustration of contrast modulation as a function of sinusoidal test patterns of varying spatial frequencies.

#FIG. 2 is a plot of the chromaticity and luminosity modulation transfer functions of the human visual system.

#The method of invention exploits the unique characteristics of the human visual system. A brief review of the human visual system, specifically the acuity characteristics of black & white (luminance) and color vision (chrominance), is useful to understanding the present invention. A large body of literature in the field of vision research has shown that contrast sensitivity to luminance and chrominance are considerably different. Objects can be better distinguished from each other or their background, if the difference in luminance or chrominance is large. Of these two factors, however, luminance plays the most important role. Furthermore, it isn't the absolute difference in luminance, which matters but the relative difference. Like most imaging systems, the human visual system attenuates (reduces) image contrast while removing (filtering out) high spatial frequencies in a scene. The term “contrast” is defined as the difference between two luminances divided by their sum. Objects that have small contrast with respect to their background are difficult to distinguish. The reciprocal of the minimum contrast needed for detection of an object is referred to as “contrast sensitivity”. Just as temporal frequency refers to how rapidly a signal is vibrating or oscillating with time, i.e. cycles per second, the term “spatial frequency” refers to how an optical signal is varying with distance, i.e. the contrast of a black and white signal in space. Spatial frequency is measured in cycles per mm or equivalently, cycles per degree (cpd) of viewing angle, a viewing-distance independent relative measure.

#Investigations of the contrast sensitivity of the human visual system are typically done using test patterns. Typically, sinusoidal patterns are used, as illustrated in FIG. 1. For a sinusoidal luminance pattern, contrast is defined as the amplitude of the black and white sinusoidal variation divided by its average luminance (mean greyness). This expression is often called “modulation depth” or simply “modulation”. The minimum modulation required for the detection of this pattern is referred to as the “modulation threshold”. By modulating sinusoidal gratings of varying spatial frequencies (as shown in FIG. 1) and recording what the modulation threshold is at each frequency, the contrast sensitivity function or its equivalent, the modulation transfer function can be determined. Knowledge of the contrast sensitivity function or modulation transfer function is vital in that it not only fully characterizes, but determines the performance and capabilities of the human visual system.

#FIG. 2 is a plot of Granger's chromaticity and luminance modulation transfer functions for the human visual system (Edward Maurice Granger, Ph.D. Dissertation, Specification of Color image Quality, University of Rochester (NY), 1974). The modulation transfer function of the black and white channel (curve labeled “A”) which characterizes luminance acuity is essentially bandpass (dashed box). This pass band is only about two-octaves wide, ranging roughly between 2-8 cycles per degree (cpd) of viewing angle. The modulation transfer functions for the red-green and yellow-blue opponent channels (labeled “T” and “D”, respectively) characterize the human visual systems chrominance acuity. Chrominance acuity can be modeled as low pass (dashed box). While red-green acuity is slightly more sensitive to high spatial frequencies than yellow-blue, the difference is small. In essence, the cutoff frequency for chrominance acuity is roughly one cpd and there is little significant frequency overlap between the chrominance and luminance modulation transfer functions. From FIG. 2 it is evident that chromatic frequencies beyond the cutoff frequency of 1 cpd cannot be seen. However, achromatic frequencies above 1 cpd are perceptible. Given this processing dichotomy, the removal or absence of high spatial frequencies in the chrominance channels should not be readily discernable to the human eye provided the luminance channel conveys the full spatial frequency content of the original image. In other words, one can filter the frequency content of an image's luminance channel to correspond to the band-pass characteristics of the human visual systems luminance modulation transfer function. The same can be done with the frequency content in the chrominance channels to correspond to the low-pass characteristics of the chrominance modulation transfer functions.