Research on the evaluation of printing effect

In recent years, display technology has developed rapidly, and liquid crystal displays have become mainstream. Liquid crystal displays have been widely loved for their advantages of low energy consumption, low radiation, and good display uniformity, and have become the standard configuration of most computer systems. But it also has obvious shortcomings such as small viewing angle and color change when viewed in different directions, so it is not popular in the printing industry. With the emergence of some high-end LCD monitors such as EIZO, Apple Cinema, etc., the performance of LCD monitors has been greatly improved. To a large extent, the above shortcomings have been overcome, and the color gamut that is very close to printing can be displayed.

At present, the printing industry generally adopts color management technology to enable various devices to present more consistent colors, which provides a prerequisite for the application of soft proofing. Some soft proofing products have appeared on the market, such as VirtualProof from Kodak and RemoteDirector from ICS.

In summary, screen soft proofing is a very new technology, which is constrained by many factors in equipment, technology and use, so it is very necessary to conduct relevant experiments and research. This project designed a soft proofing experiment, conducted a preliminary evaluation of the soft proofing effect, and obtained some meaningful results.

In this experiment, the display and printer are calibrated and characterized by color management, and the printed hard copy proofs are compared with the display effect. The observer evaluates the display results against the hard copy proofs and checks the correlation between the subjective evaluation results and the measured color difference. The result of this test soft proofing.

The display used in the experiment is an EIZOColorEdgeCG19 liquid crystal display, the color management software is MonacoOPTIXPro 2.0, the measuring instrument is GretagMacbethEye-one, and the white field color temperature is 6500K.

The printer for generating hard copy proofs is EpsonStylusPro 7600, the color management software is MonacoPROFILER4.7.2, and the measuring instrument is X-RiteDTP70. The observation of the proofs was carried out in GretagMacbethJudgeII standard indoor lighting, and the light source was simulated D65. First, color management the monitor and printer to make the color difference as small as possible. The color-corrected display is tested with IT87 / 3 color scale and gray series samples of a total of 963 color measurement data (CIELAB), and the average color difference between the screen color measurement value and the standard data is 3.02, the maximum color difference is 30.81, and the standard deviation Is 5.07. Among them, samples with a color difference of less than 1 accounted for 53.6%, samples with a color difference of less than 2 accounted for 72.1%, and samples with a large color difference were all colors that exceeded the display color gamut. The color-corrected printer is tested with the same 963 color data as above, and the average color difference is 1.35, the maximum color difference is 6.15, and the standard variance is 0.86. The samples with color difference less than 1 accounted for 41.8%, and the samples with color difference less than 2 accounted for 82.0%.

The color measurement value of the hard copy sample is directly compared with the measurement value displayed on the display, the color difference is increased, the average color difference is increased to 3.54, the maximum color difference is 30.90, and the standard deviation is 4.95. Among them, the samples with color difference less than 1 accounted for 28.1%, the samples with color difference less than 2 accounted for 58.3%, and the samples with color difference less than 3 accounted for 70.5%. The color difference distribution of the display, printer and display relative to the printer. Since the color outside the color gamut has the greatest influence on the display color difference, the number of samples in the large color difference area mainly depends on the color difference of the display, and the sample in the small color difference area depends on the interaction of the two, so from the perspective of objective evaluation of color difference The color difference of soft proofing can be accepted by the printing industry, indicating that the monitor and printer after color correction can reach a relatively good state, and the colors presented are basically accurate.

In order to evaluate the effect of soft proofing, the actual visual evaluation must be carried out. Use visual evaluation to check whether the color difference of soft proofing is acceptable. To this end, the experiment designed subjective evaluation.

The subjectively evaluated sample is the S8.Tif file in ISOSCID, which contains 104 color patches, which are relatively evenly distributed in the color space. Convert the image file to LAB mode in advance, print the LAB value on the monitor display and printer that have undergone color management, and measure the color difference between the display and the print according to the previous method. There are 4 levels of "Slight Color Difference", "There is Large Color Difference" and "There is Significant Color Difference". Assign a value of 1 to 4 to each level. The larger the value, the greater the color difference. The color of the color scale is displayed in Photoshop. Profile is set correctly in Photoshop.

When observing, use black paper to dig a hole of a color block, and compare only one of the 104 color blocks at a time to avoid the influence of adjacent colors on the observation result. Between undetectable and weak color difference, it means that most color blocks can only detect very weak color difference. The effect of soft proofing is better. The color difference of subjective evaluation is only 1.9. The corresponding relationship between the color difference level and the CIELAB measured color difference The reason for the low correlation coefficient between the two is that the data points are scattered. This is more in line with the law of subjective feelings. The correlation coefficient r obtained after logarithmic fitting is 0.4446, which is greatly improved than the linear correlation coefficient.

In this paper, we objectively test and subjectively evaluate the effect of soft proofing on the screen through experiments. The objective test results show that under the condition of strict color management of the display and the output device, the average color difference produced by the screen soft proofing is 3.54, and the main reason for the large color difference is that the display and the printing color gamut are not completely consistent. The color of the excellent gamut can produce a color difference of up to E ab = 30, and has a good display effect for the colors in most gamuts, so it can be applied to printing proofing. This experiment is only a preliminary experiment of soft proofing. The color difference tolerance of soft proofing has not been investigated. The unevenness of CIELAB color space and the impact of complex images on color evaluation have not been considered. Therefore, other color difference formulas are needed in the next step. Inspection and the influence on the color of complex images, such as using S-CIELAB square and CIECAM02 color appearance model to check.

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