How to realize the standardization process operation of color management (I)

The input device of the manuscript can be a scanner (planar or roller type), PhotoCD or digital camera, and the color file can be output by a laser photosetting machine, a digital proofer, a printer, and the like. This involves the accuracy of input and output devices and the quality standardization of printed materials such as colorants and their mapping. This article will discuss several elements of color management.

In the era of electrical extensions, color separations from the manuscript to the screen are managed within the machine. In the previous “copying of the manuscript”, as long as the ratio of the manuscript color to the dot output of the output color separation is correctly set, the extension can output Color separation film that reproduces the original color. Now, prepress processing systems mostly go from originals to RGB files, from RGB files to CMYK files, from RGB files to monitors, and from CMYK files to different devices for color separations, plates, and color proofers (or Media) and the conversion process between different color spaces. Therefore, the prepress processing system has divided this internal process into multiple mutually independent and interconnected processes.

Determination of the color rendering properties of the device

To standardize the color management system, we must first determine the color rendering performance of the device. The determination of the color rendering performance is the calibration of the coloring of the device. Before the color data is converted between different media, the calibration of the device must be performed. Therefore, equipment calibration is the basis for its feature description.

The calibration process is to ensure the color matching between devices (such as scanners, monitors, printers, etc.). Calibration shall determine the deviation of the color rendering performance of the same device at different moments, so that the color information has consistency in time and consistency in the process of acquisition and transmission, to ensure that all devices in their own way (such as RGB brightness, CMYK Density) forms the correct color.

1. Scanner calibration

Scanner calibration refers to the calibration of a specific light intensity measured from a manuscript. The calibration items include brightness, contrast (Y value), white field (balance of R, G, and B colors). Because the deviation of the device itself, the ambient temperature and humidity and other factors will affect the accuracy of the calibration of the whole machine, the scanner must have a special calibration subroutine in addition to its own calibration.

2. Display calibration

    The display calibration is to make the display card display the color according to a specific numerical value in the image file and have consistency. The display itself also has brightness, contrast (Y value), white field. The color that people see is not only influenced by the color filter or the color of the color, but also by the visual influence brought by the ambient light source; the color temperature of the display also affects the accuracy of the coloration, and the color temperature should be consistent with the input and output (originals and proofs) Lighting conditions match. The color temperature of general lighting sources is set at 5000K.

The contrast (Y value) describes the relationship between the input value of the display and the output brightness, which affects the distribution of various shades between highlights and shadows. In the absence of a color management system, we can set the display gamma value to a condition that compares the contrast between the printed image produced by the same digital file and the image on the screen. After using the color management system, the system mainly specifies the gamma value of the display, such as "EFI scaling RGB" specifies a gamma value of 2.20.

3. Color proofer calibration

In order to effectively perform color correction, the device variables must be compensated so that the device is output according to the color rendering rules specified at the factory. Some devices have an organic correction function, while others need to be corrected by software provided by the manufacturer or a third party. Currently, some display manufacturers have developed a set of standardized calibration methods. For example, Apple has developed a 24-bit color display to control gray balance, white point and black point, color temperature, and gamma curve.

4. Offline proofer calibration

The SWOP "Web offset printing publication specification" is generally used as a description of the offset printing process. In the SWOP, parameters such as ink and paper color, on-site density, and dot gain range are specified. When using a film and toner color proofing system, the substrate, pigment, and calendering layer should also be selected to simulate the printing conditions of the printer as closely as possible.

Characterization of the color rendering performance of the device

The feature description is to digitally describe the color rendering performance of scanners, display screens, color printers, inks, and other color media. That is, the appropriate color system is used to describe the color space of each media. A method for the color gamut of different input and output devices can define its color gamut characteristics to determine the manner in which the input and output devices output colors after scaling, and how the known devices are displayed under unknown conditions. The data describing the color rendering range of the device established after the feature description is called the device color rendering file. The color rendering file is a color conversion model that is stored in the system and can be combined by different color rendering models to achieve color reproduction between different input and output devices.

The realization of the characterization of the device's color rendering requires the close cooperation of the color management system development, management personnel and equipment manufacturers, and media manufacturers (such as inks and phosphors).

For example, Adobe's Postscript Level 1 page description language allows manufacturers to add color rendering libraries (CRD) device color rendering languages ​​to RIPs. With CRD, you can automatically convert files described in RGB or La*b* color spaces to files described in CMYK color space before output, and output four-color color separations. Photoshop also supports CRD functionality.

The color display also has a corresponding device description file, but it is limited to work under the factory-specified parameters. If the brightness and contrast of the display change, the data will not be accurate. Therefore, the color management system also often provides software, users can develop their own device color description files as needed.

(1) Scanner characterization. The printing institutes such as GATF, RIT, UGRA, and FORGRA, as well as companies such as Heidelberg, have designed test images for the characterization control of printing presses. EFI, KEPS and Agfa have used their own drawings to characterize the printing process. The output pattern is measured with a spectrophotometer and describes the color gamut of the output device in the CIE La*b* and CIE xyz color spaces.

(2) Display feature description. The display color rendering file provided by the monitor manufacturer only works in the manufacturer's specified operating environment. After the operator adjusts the brightness and contrast, the feature description is not allowed. Some software, such as Kodak color management system and Agfa, FotoFlow software, as well as specialized software, such as RasterOps color correction software and Super Mac, Super Match color correction software, can also perform display characterization.