Quality Control of Color Printing Software Printing Proofing

In recent years, the printing volume of color papers has been increasing rapidly. Compared with integrated printing companies, due to the characteristics of the management structure and printing requirements, professional color printing companies often involve more factors in color and image control. For example, the source of image manuscripts is complex and the quality is very different; because of the small color gamut formed by the printing suitability of the paper and ink used for printing, the color quality of RGB manuscripts itself differs greatly; Paper and ink are frequently replaced, and the prepress processing time is tight, and color proofing and printing control are not performed. As a result, fluctuations in color and image quality are large.

In the practice of using a color report color and image quality control process, the core is based on the traditional control links and parameters, in the front of the process to join the screen soft proofing process, its role is to meet the color printing On the premise of real-time, all the modifiable colors and images entering the process flow are visually inspected and visually edited through the soft proofing of the screen, so as to achieve the printing effect satisfying a certain or specific style requirement of a certain output condition. Greatly increases the predictability and operability of effects and quality control in the replication process for colors and images that require high real-time performance. With a price of less than 10,000 yuan, the color and image quality of many tens and millions of color newspaper hardware systems can be guaranteed and enhanced.

To form the above-mentioned quality control link based on soft proofing, the key is to establish a color management process based on the printability characteristics of the existing photocopying and coloring press web production line, and the screen can truly reflect the effect of the coloring book. Which need to work in the following areas.

Create printability profile

The printability profile of the entire production line is like the object that the screen soft proofing needs to simulate. Creating a profile is a basic job. Since there are many changes in the entire production line, the dataization of the main control parameters of the production process and the stability control based on accurate detection become the necessary conditions for the generation and use of the entire process profile. The key to accurate data control is to add test strips at the hang-up position or at the end of the newspaper, and use visual and densitometer tests. For the color newspaper of general newsprint grades, the main control parameters include the following.

1. Outlet and print version quality parameters

If the film line, the error occurs less than 2%, 3%, 5% of the outlets to be out of line, 98% of the network does not paste and so on.

2. The main quality monitoring parameters of the printing process

(1) Density range: according to industry standards, the general color newspaper four-color ink density control range: yellow 0.60, magenta 0.80-1.00, blue 0.90-1.10, black 1.00-1.15, many high-level printing factory default scope Both are much smaller than the standard; the best on-site density: yellow 0.75, magenta 0.95, cyan 1.00, black 1.10; ordinary color newspaper printing contrast yellow 0.22, magenta 0.22, cyan 0.22, black 0.27.

(2) Inkwell: 3% to 5% of outlets should be out of order, and 98% of outlets should not be stuck.

(3) other parameters: such as network distortion, gray balance and so on.

After controlling the plate making and printing processes, according to the different paper parameters, different profiles are established for the commonly used paper and ink combinations within the control range of the above-mentioned plate-making printing parameters, and the color management flow is in accordance with the day and the time. The paper and ink combinations use different suitability profiles.

If you want to obtain a more accurate, relatively fine state of the classification of the color newspaper print Profile, the best way is to use a professional color management system to produce. The control interface of the professional color management software ProfileMaker feature file can be seen that the establishment of the characteristic file has two parameters to be determined: one is a basic color conversion parameter based on a color standard such as IT8.7/3, and it is a generation profile. The basic parameters of the method are to generate and print the digital color scale according to various conditions, and use an online colorimeter to complete color scale testing under the control of ProfileMaker, MeasureTooI, and other softwares: the other is for color separation and printing. The process parameters of the conditions are selected, including such parameters as separation parameters, black point gray balance, ink volume control, etc. These parameters determine different ink ratios to achieve the same color effect, and the optimal range requires experience and experimentation. Finally use ProfileMaker to complete the generation of the profile.

Create screen corrections

The corrected screen and its corresponding profile are the basis of the screen-based simulation color management application program discussed in this article. The screen correction refers to the process of establishing the characteristic file through the software and hardware system of the screen Profies generation system based on the optimization and adjustment of the screen display state. In addition, it also refers to the regression adjustment of the existing parameter-type screen profile and the current profile setting status. The screen nano calibration and profile generation can use the following two methods.

1. Screen correction using screen colorimeter

Using a screen colorimeter and its accompanying screen correction software, it is possible to adjust the screen to an ideal state and create a profile of the corresponding state. First, the brightness and contrast of the display are precisely set under the feedback of the screen colorimeter to ensure that the display has the largest tonal range and color gamut. Then the user can set the white point and Gamma value and use this as a basis to form a corrected screen profile by automatically performing a screen test on a group of color blocks.

2. Adobe Gamma Performs Software Screen Correction

This is a method that everyone is familiar with. It is also to set the optimal brightness and contrast of the screen with the help of the software visual bar, and then set the color temperature (ie, the white point, the requirement and the display's hardware set the same color temperature, generally 5000K or 6500K) and Gamma (Window system Generally 2.2), and then through the corresponding visual bar and adjustment function for optimal adjustment of Gamma and color temperature in order to achieve the best, after the formation of a screen profile can be used by the color management process. This parameter-type profile is less effective than a hardware system because of its limited accuracy and direct visual adjustment.

Set up appropriate color management processes and mapping methods

The color management process is used to implement and control the color space conversion of various colors. For the color newspaper professional production line, the most economical and reasonable color management process based on screen soft proofing should be the use of color management functions built into prepress software such as Photoshop and CoreIDRAW.

Photoshop's built-in color management process, where "View -> Proof Color" initiates the soft proofing effect, and "View 1> Proofing Settings" is used to select the simulation object and intent. It uses the RGB-Lab-CMYK-Lab-RGB (for RGB color) or CMYK-Lab-RGB (for CMYK color) processes shown in FIG. 2 to complete screen emulation of the output of RGB files and CMYK files. Its characteristic is that the RGB color space must be converted to the color newspaper CMYK color space to be "constrained", and then converted to the RGB color space again to display the "constrained" color and image effects, thereby providing the screen display with the printing effect for the editing.

In CoreIDRAW, it can directly provide a color management control process based on screen soft proofing. It first converts the Lab color value to the CMYK printing color space, and then converts it to the corrected screen and the RGB color space represented by the Profile and displays it to form a simulation effect. Another important issue is the choice of mapping methods. The mapping method is mainly to solve the two core issues of color space compression and white point transfer. Sataration (saturation) and Perceptual (perceivable) can convert the white point of the source color space into the white point of the target color space, while Absolute (absolute) simulates the white of the source color space in the target color space. point. For screen soft proofing, if it is a screen emulation of the RGB file's color report effect, the white point of the RGB color space needs to be converted into the white point of the CMYK color space by using the Relative, Sataration, and Perceptual methods so as to be on the screen. See the effects of different paper whiteness. If it is a CMYK file, Absolute mode is needed to “pass” the white point of the CMYK color space onto the screen to form the background color emulation effect of newsprint.

Use standard prepress methods and performance standards

After the soft proofing process has been established using the above method, both the pre-screening and pre-press processing of color and image effects should be WYSIWYG. The small print gamut that is unique to newsprint (reflected as lower saturation and brightness) and the more varied paper whiteness (color shift) can be reflected on the screen, and the effect of adjustment is not necessary. Worry about not printing. In terms of quality requirements, following the industry standards for color newspapers, there are the following requirements:

(1) Ink requirements: full and moderate, pure hue, no color cast.

(2) The image quality requirements: clear, moderate contrast; the image is not false, good resolution, can restore the natural tone.

To achieve these requirements can be summarized into the following standardized processes:

(1) Black and white fields and contrast processing: It mainly controls the overall tonal range and image range, and implements the “separation of layers.

(2) Treatment of color shift and saturation: The control of "full and moderate, pure hue, no color cast" is implemented.

The processing method is as follows:

1 Use a pipette tool to neutralise the color cast. The color shift can be determined from the neutral gray region of the image, and the dropper is used to target the region as neutralization of the gray level, that is, the neutral gray region of the color shift is set with a neutral value.

2 Use the Hue/Saturation tool for vivid color control.

(3) Sharpening: Implementing the control that "the image is good and the resolution is good". The quality of newspapers' artwork submissions varies greatly, especially in terms of clarity and resolution. The processing method is to use the SUM sharpening tool.

Keywords: Color Newspaper/Print/Proofing