Talking about the defects existing in the traditional four-color printing technology
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People's recognition of natural scenery is always inseparable from shape and color. Natural scenery cannot be directly printed and reproduced. It is necessary to record the shape and color on a flat medium such as a photosensitive material by means of conversion conditions to form an analog quantity of color, or to record it in a photoelectric conversion device such as a CCD to directly generate a digital quantity of color. The color analog quantity generated on the flat medium will still need to be converted into digital quantity by means of a scanner or the like in order to be accepted by the computer software for the printing and copying process.
In terms of color reproduction, the copied content is divided into two parts, a color component and a neutral gray component. This decomposes the color, which simulates the visual perception of the color of the human eye. The rod cells feel light and dark, and the cone cells feel color. The color perceived by the external color to stimulate the human visual system is a combination of color shading and color perception. In color, the three-dimensional effect is mainly described by the neutral gray component.
The color displayed by the natural scene is perceived in the visible color gamut. When it is copied, the authenticity of the color suffers due to the limitation of the color gamut of the photosensitive material or the performance of the photoelectric conversion device. When the color digital quantity is printed and reproduced, the ink printing color gamut range will cause the color space to be compressed again, and the color separation algorithm will also change the color tone characteristic. The conventional four-color printing gamut area is less than half of the visible color gamut, so the printed color tends to be darker than the color in the visible light gamut, and the gradation compression loss is more. The print gamut range can be determined in the CIEYxy system or in the CIELab system.
Figure 1
Due to the limitations of ink coloring and production process, the spectral curves of C, M, Y, and K primary inks used for four-color printing are significantly different from those of the ideal color spectrum. The spectral reflectance curves for ideal inks and actual inks are as follows:
Figure 2
The secondary color CM, CY, MY color rendering effect of ink overprinting, especially blue and green, is much larger than the ideal color.
Figure 3
With the existing ink production process, the ink performance is greatly improved, and the space left is small. Therefore, the four-color printing color gamut is difficult to expand greatly.