Display correction and establishment of its characteristic file

With the rapid development of open DTP desktop publishing systems and video technologies, the role of displays has become increasingly important and has become one of the important devices in the prepress process. The professional application of digital photography is becoming more and more common. The user's requirements for the display are also relatively high. They hope that the images will get the same display effect on different displays, and the colors displayed on the display are exactly the same or very close to the finished printed products. Can greatly save the time to modify the photos and proofs proofs, more importantly, to enable users to get satisfied with the high-quality, device-independent color reproduction, and truly achieve "What you see is what you get."

To realize the "what you see is what you get" color, it means that the color consistency among a large number of devices, including the consistency of the color between the scanner, display and printing equipment, the current major manufacturers generally use device-independent colors Management technology. This technology introduces the device-independent color space, establishes the device's color characteristics description, and performs color space conversion, color gamut matching, and error correction according to the user's requirements to compensate for the lack of color reproduction capabilities of existing devices. Therefore, the user should first establish a set of color management system in the actual operation process, so that the entire workflow in different devices, such as digital cameras, scanners, monitors, printers, etc. based on a color standard. In simple terms, users need to create a color profile for each device. For a display, it generally includes two basic processes: display calibration and creation of a device profile of the display, ie establishing a Profile of the display. Discuss the two processes separately below.

At present, the monitor adjustment system can be divided into two categories: an external adjustment system and a built-in adjustment system. The biggest difference between the two is that the external adjustment system cannot adjust the color of the monitor. Only the Look Up Table (referred to as LUT) in the display card in the computer can be used to generate the desired color. Because most displays have gamma values ​​(the relationship between RGB input values ​​and output luminance) fixed, some displays do not provide the ability to adjust color temperature and RGB independent intensity. Therefore, the external adjustment system will cause the brightness of the display to decrease and the color gamut to narrow. For example, the color temperature of a typical monitor is 9300K (bluish), and the standard for the printing industry is 5000K (reddish). To adjust the monitor's color temperature to 5000K, the method is to reduce the blue and green output intensity of the “white point” (when the RGB input value is 255) in the display LUT to achieve a reddish effect. After tuning, when the blue input value is 255, the true output may only reach 220, while the true green output may only reach 240, the brightness decreases, the most serious is the output color can be reduced (Before the adjustment, it was 255 × 255 × 255, only 255 × 240 × 220 after adjustment). As a result of the gamut reduction, it is easy to have a banding situation.

Plus adjustment system according to the different methods of adjustment, it is divided into pure software and software with hardware two. The use of pure software to adjust the monitor needs to be observed with the naked eye, characterized by low cost and ease of use, but due to the difference in color perception of each person, plus the same observer at different times and different mental states will also produce different Judging, there is no objective criteria for a calibrated display to determine whether it is "accurate." Based on the above restrictions, professional users generally adopt software and hardware adjustment methods. The so-called "hardware" refers to a colorimeter or spectrophotometer.

A monitor with a built-in calibration system can avoid the above situation, because when the calibration software adjusts the monitor color temperature, it can control the energy of the electron gun in the monitor, and does not need to forcibly reduce the output brightness of a certain primary color, so it can achieve an ideal Brightness and widest color gamut. However, this type of display has only a few choices. Previously only some CRT displays worth tens of thousands of yuan have been effective but they have not been popularized by general users due to the high purchase and maintenance costs. Therefore, the widely used system is an external adjustment system.

In our testing process, we used the software and hardware adjustment methods. The hardware device used was a spectrophotometer and the software was its companion software GretagMacbeth's ProfileMaker.

The specific adjustment process is as follows:

1. During the test preparation stage, the graphics card driver is upgraded to the latest version, and the refresh rate of the monitor is adjusted to be lower than the maximum value. In our experiment, 85 Hz is used, and the color setting is 32-bit true color.

2. After the monitor is turned on, it should have a warm-up time of half an hour, so that the monitor is in a stable state to obtain a stable effect;

3. In the display calibration process, a relatively dark working environment is required. Therefore, the indoor light source needs to be turned off in order to prevent the light from being projected onto the glass of the picture tube and affect the reading of the spectrophotometer.

4. Turn off all desktop graphics and set the background color of the monitor to neutral gray so that it does not affect the vision during the calibration and helps to adjust the gray balance. Right-click on the table, click "Properties" to enter the display properties, then click on the desktop tab, select "None" in the background, then click on the "pattern" in the lower right corner, select "50% grayscale", and then click "OK ". The background of this display is grayed out;

5. After the above preparations are completed, open the Measure Tool of GretagMacbeth, select Monitor, enter the Monitor Calibration dialog box, and correct the display. In general, the standard color temperature for the prepress process is 5000K, so the monitor should also be calibrated to 5000K. Since the gamma of the picture tube is mostly around 2.2, the Gamma value used in the test is 2.2. Set the contrast to 100%. Click "Next" and follow the wizard to calibrate the display. By adjusting the intensity of R, G, B three channels, the actual display color temperature reaches 5000K, the display brightness reaches 72.1cd/m2;

6. After correcting the display, save the settings and apply the settings to the current monitor.

After the calibration of the display is completed, the sample target color patches of the display need to be measured to generate the device profile of the display. Note that the external environment should be kept constant during the measurement process to ensure that the characteristics of the monitor are not changed and that the best state after calibration is maintained. The specific test procedure for generating a profile is as follows:

1. Open the Measure Tool of GretagMacbeth and select "Tools→Instrument Configuration". In this dialog box, select "Emission" to perform white point correction on the device. Close the dialog box after correction.

2. After selecting “Char”, select the original data file of the monitor target to be tested, and perform measurement. To ensure the accuracy of the measurement data, during the measurement process, the displayed color should be displayed in the center of the display in a quarter area, and Keep the measurement conditions consistent with the conditions during calibration;

3. Save the test data file, open GretagMacbeth's ProfileMaker tool, select the type of monitor profile to be generated, the general user can select "Default", select "Large" for professional users, generate a monitor profile, save the profile Go to the system folder color and apply the display to the display system.

At this point, the calibration of the display and the establishment of the device profile Profile are completed. Manufacturers of general input/output devices will provide standard ICC profiles for their products. However, due to the slight differences in the specifications of each device at the time of shipment and the different rate of change in color performance during use, these standard Profiles cannot meet actual requirements, so professional users will regularly adjust and update Profiles for their devices to ensure Each time the device is used, it works in the best condition and reproduces the color better.

Source: Coinprint