Soft proofing with LCD monitor
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A liquid crystal monitor that replaces the original CRT monitor (liquid crystal Monitor) has been developed and is already being used in some companies. There is a strong trend in the publishing and printing industry to switch from CRT monitors to LCD monitors. With the digitization of the workflow, it is expected to complete the color reproduction of the printing on the liquid crystal monitor. This article describes the need for soft proofing on a liquid crystal monitor and the possibility of practicing soft proofing.
Advantages and disadvantages of LCD monitors
The advantages of LCD monitors are summarized below compared to CRT monitors.
1) Saves land due to short depth and consumes less power (no heat).
2) It is a perfect plane and does not exhibit geometric distortion.
3) There is no deterioration of focus and color shift of RGB.
4) Long life and less change over time.
5) The setting is arbitrarily high.
On the other hand, from the perspective of the use of graphics in a broad sense, including printing, it is not as good as the CRT monitor:
1) The color space is narrow.
2) Contrast, chromaticity, and gradation characteristics vary greatly within the field of view.
3) Hierarchical characteristics are not straightforward.
4) The characteristics of the monitor are uneven (untrustworthy).
To overcome these shortcomings, a liquid crystal monitor that could replace the CRT monitor was developed.
Principle of liquid crystal monitor
Briefly explain the working principle of the liquid crystal monitor, and the pixels constituting the base are divided into 3 (sub-pixels) for use in the three primary colors of RGB, and RGB color filters are placed in front of them. By applying a moderate voltage to each of the sub-pixels, the liquid crystal element then changes the angle to control the amount of light transmitted by the back. This amount of transmitted light then passes through the filter swatch, mixing the necessary amount of RGB to represent a particular color. Figure 1 shows the most general TN mode liquid crystal operation.
Color space
The color space is determined by the RGB color filter and the back light color, so the color filter characteristics have a great influence. In the past, the characteristics of the monitor were biased towards giving priority to brightness, and the color space was generally narrower than that of the CRT monitor.
At present, the industry standard for monitor characteristics is sRGB, and so far includes notebook PC-like monitors. Most LCD monitors have a narrower color space than sRGB. The majority of CRT monitors follow EBU (European Broadcasting). Federation) standard color space. The so-called sRGB, the green coordinates are slightly different, but can be said to be roughly the same. sRGB is an area where the RGB operation area of the Adobe application software is maintained, and the color space of the digital camera is also generally supported. Therefore, the color space of the sRGB (EBU) is also advocated in the LCD monitor.
Field of view and chromaticity
Hierarchical property locator
Among the characteristics of liquid crystal monitors, the most important feature of users related to printing technology is the "angle of view". Moreover, the definition of the field of view in the liquid crystal monitor means that the contrast between white and black represented on the screen can maintain a certain value (usually 10:1), and the displacement of the chromaticity and the displacement of the gradation characteristics are not considered at all. That is to say, even in the product sample values, even if the liquid crystal monitor having the same angle of view characteristics is given a little angle, the change in chromaticity and the change in the gradation characteristics (the phenomenon of whitish skin, etc.) hardly exist. .
It seems that there is no problem when viewed from the front, but for a 21-inch class monitor, when viewed from a distance of 50 cm, the two ends become viewed with an angle of view of 20 degrees or more, which is inevitably affected. If you confirm the picture of the product with the customer, the effect will be more obvious. This feature depends on how the liquid crystal works.
3 ways of LCD
TN mode
At present, the most commonly used TN (Twisted Nematic) type liquid crystal panel. Most of the notebook PCs, most of the 15-inch LCD monitors, use this type of display panel. The 17-inch grade of cheap goods is also the display panel in this way. As shown in Figure 2-1, a voltage is applied to the pixels, indicating that the liquid crystals that are originally parallel on the plane turn to an angle, thereby changing the chromaticity and gradation characteristics. With the use of the viewing angle compensation sheet, the left and right viewing angles are relatively wide, and the appearance of the screen is obviously different, so it is not suitable for the printing process.
VN mode
17-inch grade wide-angle angle products, 19-inch grade products use VA (Vertical Alignment) display panel, when the voltage is not applied to the pixel (dark) state, the liquid crystal pair indicates that the plane remains close to vertical Angle, so the dependence of the angle of view is large. According to the fact that the tilt direction of the liquid crystal is multiplied (multi-domain), the effect is lost, and the field of view is enlarged, the displacement of the chromaticity is suppressed, and the displacement of the gradation characteristic is also present, so that the recommendation is not achieved. The level of use for the printing process (see Figure 2-2).
IPS mode
The IPS (In Plane Switching) method is that the liquid crystal changes the angle on the representation plane, and rarely changes the chromaticity and gradation characteristics due to the angle of view (see Figure 2-3). Further, as shown in FIGS. 2-4, the sub-pixel is divided into two to change the angle (dual-domain), so that the influence is further reduced.
Because of its good color reproduction characteristics, the display panel of this type is mostly used for large display panels of 18 inches or more, and is generally used in the field of medical images, and can be said to be most suitable for the printing process.
Monitor calibration
The characteristics required for the monitor have been described above, and calibration is required periodically in order to maintain the accuracy of its characteristics. In most cases, the monitor is color-coded, and based on the results, the video output of the personal computer is adjusted, which is called software calibration. This situation leads to a change in the color temperature of white, which is equivalent to reducing the brightness of one or two of RGB, which is equivalent to reducing the 256-order tone to use. The case of changing the γ value from the original value is also equivalent to selecting and subtracting several of the 256-order tones to become γ of the approximate target, resulting in loss of the tone. When representing a smooth level, there are steps or lines, or the case of seeing colors, that is, these are the root causes.
For printing process applications that require the correct level of characteristics, it is desirable to have a hardware calibration that meets the required values (eg, white temperature 500K/γ1.8). However, on the LCD monitor, the adjustment of the brightness, because of the RGB common backlight adjustment, can not be adjusted individually for the highest brightness of RGB like the CRT monitor.
In order to solve this problem, multi-bit processing of 10 bits or more is performed inside the monitor, and it is most effective to redistribute RGB individually by the 8-bit 256-order modulation function table.
Printing applications
Since the color reproduction characteristic of the monitor is reliable and reliable, it can be practically utilized in the field of printing for the following applications.
1) Image processing of data captured by a digital camera (photographer)
2) hotographic processing of the image processing department of the printing company
3) Image display monitor in the scanner input department
4) DTP operations in the publishing department
5) Image processing for newspaper image reporting agency
6) For the production work of the advertising production company
It is not limited to the above-mentioned fields, and applications can be considered in areas related to the need for correct color reproducibility, its business and post-process, and quality improvement.
Soft proofing practice
It is well known that digitalization and printing standards for workflows are becoming popular in the print publishing industry. All of these are aimed at shortening the delivery time, reducing the unnecessary cost, and stabilizing the quality. In the change of these jobs, the requirement for the monitor is that the color of the printed information content can be correctly reproduced on the monitor screen, and finally all the corrections are implemented on the monitor, that is, "soft proofing"
As the benefit of the user, with the digitization of the printing workflow, the DDCP of the color proofing is further evolved, and all the corrections can be completed by the monitor display, thereby saving the cost of proofing, transmitting the cost and time, while making The stage can also reduce the output for confirmation, and can also reduce the time spent on correcting the correction after proofing.
The key to this is the use of CMS based on a color profile. In fact, experiments have been carried out on the monitor to determine whether the color reproduction is correct or not. For the experiment, at least the print data is required, the sample actually printed by the data, and the ICC setting file of the printer that prints the sample. In order to satisfy these requirements, a standard component of "JAPAN COLOR Color Reproduction Printing 2001" is used, and this standard component includes all of the above.
Job configuration for color settings in Photoshop: For CMYK, select the Type 1 setting file for JAPAN COLOR 2001.
Job Configuration: For RGB, select sRGB. The setting of the monitor is selected from the white temperature 5000K/γ1.8, and the setting file generated after the calibration of the brightness of 80cd/m2 is set in the system.
Under this condition, each color code of the print data is displayed on Photoshop, and the Yxy value is obtained using the Minolta CS-1000, and converted to the Lab value and compared with the original data. Regardless of the luminance component "L", the comparison is made in the "ab" plane, but with respect to the color in the sRGB color gamut, a good result that ⊿E(ab) is within 2 is obtained.
Depending on the variety of prints, this accuracy requirement may be differently reflected, but in general, soft proofing on liquid crystal monitors has reached an ideal level.