Prepress Technology 30 Questions (19-24)

19, what kind of scanner? What's the difference?

A: There are two types of scanners: one is a roller scanner; the other is a flatbed scanner that can be divided into high-end professional scanners and middle- and low-range flatbed scanners. The roller scanner is developed from the electronic color separation machine. Its sensing technology is a photomultiplier tube. Figure 5-1 shows the working principle of the roller scanner. The flatbed scanner is a CCD device to complete the scan. Different operating principles determine the difference in performance of the two scanners:

(1) The highest density range is different: The highest density of the roller scanner is up to 4.0, while the average middle and low-range scanner is only about 3.0. Thus the drum scanner can sweep out more details in the darker areas and increase the contrast of the image.

(2) Different image sharpness: The roller scanner has four photomultiplier tubes: three for color separation (red, green, and blue) and one for a false light mask. It can make unclear objects clearer and improve the clarity of the image, while the CCD does not have the function of this main surface.

(3) The degree of detail in the image is different: after printing with a photomultiplier, the details of the image are clearly printed, the dots are fine, and the texture is small. However, the quality of the image scanned by the flatbed scanner is relatively poor in the fineness of the image. some.

20. What is Pantone? Why use Pantone to set colors?

A: Pantone is a famous American ink brand and has become a standard for printing color. It has made all the inks produced by it into the chromatogram and color standard. Pantone's color standard has become a recognized language for color communication. Users need a certain color and they can calibrate according to the color standard. Due to the wide use of the Pantone color standard, computer design software has a Pantone color library and uses it for color definition. When using the Pantone library to set the colors, the selection method is Pantone. However, it should be noted that most Pantone colors are spot colors. If you want to use four colors, you should set the color type to primary color. Figure 1-13 is an interface that defines the Pantone color.

21. What is color separation? Is separation important? How to color separation in Photoshop?

Answer: Color separation is a printing professional term, referring to the decomposition of various colors on the original into yellow, magenta, cyan, and black. The color separation works in computer printing design or graphic design image software. That is, the color mode of the scanned image or other source image is converted to the CMYK mode.

In general, the scanned image is in RGB mode, and the image taken with a digital camera is also in RGB mode. Most of the images downloaded from the Internet are also in the RGB color mode. If it is to be printed, it must be separated into yellow, magenta, cyan and black colors. This is a requirement for printing. If the image color mode is RGB or Lab, there may be only dots on the K plate when outputting, that is, the RIP interpretation only interprets the color information of the image as gray.

In Photoshop, the color separation operation is very simple: just convert the image color mode from RGB mode or Lab mode to CMYK mode. The specific operation is to perform Image/Mode/CMYK. The color of the image is thus represented by the colorant (ink) with 4 color channels. When the image is exported, the image will be generated from the color channel data and divided into yellow, magenta, cyan, and black four-color film films.

22. In the color separation, when the image is transferred from RGB to CMYK, the naked eye can see some color changes on the screen. What is the reason? What is the effect on the image color?

A: When the image is converted from the RGB color mode to the CMYK color mode, some vivid colors on some images will produce obvious changes. This change is sometimes observable, and it will generally change from bright colors to dull ones. Some colors. This is because the color gamut of RGB is larger than the color gamut of CMYK, which means that some colors that can be represented in the RGB color mode are beyond CMYK-expressible color ranges after they are converted to CMYK. Replace with similar colors. Therefore, these colors produce more obvious changes. Actually, if there is a color exceeding the color gamut in the image of the RGB color mode, it can be previewed with the Gamut Warning.

23, what is the color space? What is the significance of clarifying the gamut space?

We edit images in RGB, CMYK, and Lab, and the essential difference is working in different color gamut spaces. The color gamut refers to the range of colors that can be expressed by a certain color model, and also refers to the range of colors that can be represented by specific media such as screen display, printer output, and print copy. The color of the visible spectrum of the natural world forms the largest gamut space, which contains all the colors that the human eye can see. In the color mode, the Lab color space is the largest, and it contains all the colors in RGB and CMYK.

24. What is the best mode for designing the color mode of an image?

A: In Photoshop, the color modes of an image are RGB mode, CMYK mode, GrayScale mode, and other color modes. The use of any mode for designing image patterns depends on the end use of the design image. If the designed image is to be printed or printed on printing paper, it is best to use the CMYK color mode so that the color seen on the screen is close to the output printing color or the printed color. If the design is for electronic media displays (such as web pages, computer projections, video recordings, etc.), the color mode of the image is best to use the RGB mode, because the RGB mode is more vivid and richer in color and the picture is better. And there are only 3 channels in the image, the data volume is smaller, and the disk space is less. If the image is gray, GrayScale mode is better, because even if the image is expressed in RGB or CMYK color mode, it still looks neutral gray color, but its disk space is much larger. In addition, if the gray image is to be printed, if it is expressed in the CMYK mode, there are four editions of the film and the printing, the cost will not be great, and it may cause color cast problems when the gray balance is not well controlled during printing. When the ink volume is too large, color shifts occur in gray images.