Technical Information

In offset printing of spot colors, the general process is that the colorist first matches the customer's spot color sample to a Pantone color number and then uses their color matching experience to mix the spot color. Next, a test print is done on the press. The press operator then adjusts the ink keys on the printing press based on the depth of the printed spot color sample. After the spot color ink depth is adjusted, the color difference is measured to assess whether the color meets the customer's requirements. This process of on-press adjustment takes considerable time and ink and paper costs. If there were a way to determine whether the spot color ink meets the standard in the initial stage of the press run, it would greatly reduce the time and cost of on-press adjustments. The best match function can solve this problem.

Best match is a method of calculating color difference based on ink thickness or concentration. This method can measure the actual color difference between the printed spot color and the target color, the best match color difference, and how to achieve this best match color difference by adjusting the ink thickness or concentration.

In the figure below (Figure 1), the red dot represents the position of the target color in the CIEL*a*b* chromaticity diagram, and the green dot on the right represents the position of the sample in the chromaticity diagram. The red dashed line represents the actual color difference, which is relatively large. In this case, many operators may consider re-mixing the ink. However, as the green density increases, the sample's position in the color space will continuously move. The solid green line is the L*a*b* color space trajectory of the sample as the density increases. We can see that when the trajectory moves to the position where it intersects the normal to the target color, the distance is short, i.e., the color difference is small. This intersection point is the best match point. The solid red line represents the best match color difference that can be achieved by adjusting the ink density or depth. Therefore, in this case, the operator does not need to remix the spot color ink, only increase the ink density to the best match point to obtain the desired small color difference.

(Figure 1: Best Match Principle Diagram)

So, how much ink density needs to be increased to achieve the best match color difference? Let's take the best match function of X-Rite's classic eXact spectrophotometer as an example, as shown in Figure 2:

(Figure 2: Best Match Test Diagram)

The green on the left is the standard color, and the green on the right is the sample color. We use the best match function of the eXact spectrophotometer to measure, and the results are shown in Figure 3:

(Figure 3: eXact Best Match Function Diagram)

The current color difference is 4.19, and the best match color difference is 0.65. This means that only by adjusting the ink density or thickness can a color difference of less than 1 be achieved. ΔC +0.31 indicates that increasing the ink amount in the current basis, so that the density of C increases by 0.31 is sufficient. Of course, there are also cases where the best match color difference is relatively large, such as Figure 4:

(Figure 4)

In this case, the best match color difference reaches 5.85, indicating that even adjusting the ink depth is difficult to achieve the ideal color difference. The ink hue is incorrect, and the operator should not waste time adjusting the ink thickness. Remixing the spot color ink is the correct choice. The best match function can be applied not only to offset printing but also to gravure or flexographic printing of liquid inks, such as Figure 5:

(Figure 5)

The figure shows that the best match color difference is 0.82. Only by reducing the concentration of the spot color green ink by 15% can the ideal color difference be achieved, which means only adding some thinner ink is needed, greatly reducing the difficulty of color adjustment for the operator. Many X-Rite software have the best match function, and the application scenarios are also different:

1) ColorCert Scorecard System. ColorCert is a software for scoring print quality. The score will affect the evaluation of suppliers by brand customers. In the figure below (Figure 6), the actual black density is 1.51, and the actual color difference is 2.57, which will inevitably affect the score. According to the best match function, only by reducing the black density to 1.37 can the theoretical color difference be reduced to 0.12, and the score will definitely improve.

(Figure 6: ColorCert Best Match Function Diagram)

2) IFS Ink Formulation Software. IFS is X-Rite's computer color matching software. Although this software can calculate ink ratios and fundamentally solve the problem of spot color mixing, the best match function is still very important and can help speed up ink mixing, as shown in Figure 7 below:

(Figure 7: IFS Best Match Function Diagram)

The actual color difference obtained from the IFS software is 1.2. The colorist encounters a problem: should they continue to correct according to the software's color correction function, or can they go directly to the printing press without correction? At this time, check the best match function in the IFS chart analysis. The color difference in the trough is very small, about 0.4, indicating that there is no need to correct the ink ratio. Only reduce the ink thickness to about 0.9 corresponding to the horizontal axis coordinate during printing, reducing the number of adjustments and improving color matching efficiency.

3) Intellitrax 2 and eXact Auto Scan scanning spectrophotometers. These help the press control the ink keys to control the printing color. The best match function is also very useful. For print preparation, it can quickly determine whether the current paper and ink combination meets international standards or specifications. The software will also recommend the "best density" that can achieve the target L*a*b* chromaticity data to guide printing production, as shown in Figure 8:

(Figure 8: ITX2 Best Match Function Diagram)

For the C, M, and Y colors, the average actual color difference is above 2. However, through the software's best match function, it is known that as long as the density is reduced to the recommended value, a color difference of less than 0.5 can be obtained. Therefore, it can be quickly determined that the combination of the four-color ink and paper can achieve the target standard by adjusting the ink thickness. This function plays a very important role in print certification or the Finger Print of the printing press.

Different industries have different requirements for color difference. Normally, a total color difference of about 0.8 can distinguish the color difference. In the printing industry, because color is relatively difficult to control, the color range requirements are relatively relaxed, and anything within the range of 1.5-3 is considered normal. In the electronics industry, the color difference is required to be controlled within 0.5. In the textile industry, the color difference requirement is within 2.0, which is also a relatively large color difference range, for reasons similar to the printing industry. The color difference in the plastics and coatings industries is required to be controlled within 1.0. If it exceeds this range, the product is undoubtedly unqualified. Therefore, when using a color difference meter, you need to determine whether the measured color difference is normal based on the specific industry.

Before understanding its meaning, it is recommended to first understand the color space. The so-called color space refers to a three-dimensional space established with color elements as coordinates. In this space, different colors have corresponding parameters, and each parameter corresponds to only one color, making color quality control simpler.

Current color difference meters do not simply judge colors based on Lab values, because judging solely based on this is not very meaningful. Therefore, when comparing, the Lab values of the color difference meter are often used to judge their color difference. L represents the brightness of the color, a represents the red-green value (positive numbers indicate redder colors, negative numbers indicate greener colors), and b represents the yellow-blue value (positive numbers indicate yellower colors, negative numbers indicate bluer colors).

Using a color difference meter is simple. First, place the white plate correctly, then turn on the power switch. You will see a power-on prompt, and the device will perform calibration. After calibration is complete, you can perform measurements. During measurement, press the test button on the right to accurately read the Lab values.