Relationship between Color Measurement Density Method and CIE L*a*b*

3. Test data contains information

The Epson stylus Pro 7600 inkjet printer was used to print a 13-step OMYK four-color ruler on a coated paper, and then the density and CIE L*a*b* were used to measure the samples, respectively, and the results were compared.

Fig. 3(A) is a trapezoid diagram of CMYK four colors, Fig. 3(B) is a result of a density test, and Fig. 3(C) is a CIE L*a*b* test established by X-Rite colorShopX software. Result graph. Among them, Figure C provides information that is less noticeable in Figure B, such as the degree of color cast on the black scale. Since the human eye is very sensitive to gray, even the slightest color cast can be distinguished, so the correct reproduction of neutral colors becomes very important. The density map does not indicate whether this gray level is neutral or contains color shifts. However, in the CIE L*a*b* diagram, it can be clearly seen that the gray scale does not change straight up the middle axis but spirals, which indicates different color shifts in the bright, middark, and dark tone portions.

Observe the magenta ruler in the CIE L*a*b* graph and find that when the color darkens, the color sig- nal goes downward from the vertex, as expected. But at this time the magenta is marked with a spiral, which indicates that the magenta is reddish in the dark tone region. The higher the percentage of magenta outlets, the more pronounced the magenta bias.

In addition, as can also be seen from the three-dimensional chromaticity diagram, as the yellow steps change, the arrangement of color sample points forms a straight line that is approximately perpendicular to the luminance axis. This shows that when the density of yellow changes, the yellow saturation changes, but the brightness is almost constant. It can be seen that for yellow inks, changing the density value will have an effect on the color saturation of the reproduced image, and the correct analysis cannot be obtained by the density rule. Cyan is between yellow and magenta.

Why does this difference exist between the density method and the CIE L*a*b*? Because the densitometer measurements are made through separate color filters, the data is not merged. While measuring CIE L*a*b*, the hardware and software of the measurement equipment perform a large amount of data integration and post-processing operations. These mathematical calculations and three-dimensional representations make CIE L*a*b* the first choice for evaluating printed matters.

Conclusion

The current density method is still relatively common in the color measurement of the printing industry. However, with the continuous introduction of new technologies and the continuous emergence of new types of graphics acquisition methods, such as digital cameras, digital cameras, screen soft proofing, and digital proofing, printing operators need to master new methods of measuring and controlling color. Compared to the density method, CIE L*a*b* is more advanced and has visually relevant characteristics. However, from the point of view of the quality and selectability of current color measuring devices, it is not yet fully time to start using CIE L*a*b* to control the color of digital color images.

(College of Publishing and Printing, University of Shanghai for Science and Technology, Sun Hao Tian Quanhui)

<Printing Magazine>