Digitization of Image Manuscripts--Scanning (1)

Most of the image manuscripts involved in the usual prepress production are analog manuscripts. These types of images contain continuous brightness change information. However, since the prepress computer can only process digital information, it is necessary to make such images available on digital computers. For digital processing, the analog signal must be digitized. The scanner is a common computer system information acquisition and input device. It adopts photoelectric conversion principle to convert continuous tone images into digital images for computer processing, and realizes digital input of image information. It is the basis for realizing digital production operations in the prepress field. The processing of the image by the scanner is based on the idea that the image is a collection of pixels. That is, the picture is first decomposed into pixels, then the pixels are read, and finally the pixels are recombined to recombine the images. Here we come to know about the scanner.

1 The structure of the scanner

The typical scanner is mainly composed of four parts: an optical system, a photoelectric conversion component, an electronic system, and a mechanical system, as shown in FIG. 1 . The optical system of the scanner: It is mainly composed of a light source and some optical components. When the scanner scans an image, the light is emitted from the light source, and is reflected on the original to form reflected light or transmitted light, and then is focused on the photoelectric conversion component by the optical focusing function of the lens. Photoelectric conversion component: It converts the optical signal transmitted from the optical system into an electrical signal that can be recognized by the electronic system. At present, commonly used photoelectric converters include photomultiplier tubes (PMTs) and photocoupler devices (CCDs). Electronic system: It converts the electrical signal converted by the photoelectric converter into digital signals that can be recognized by the computer and can be processed by analog/digital converters and then transmitted to the computer. Analog-to-digital converters are the most important part of an electronic system. Its number of bits directly affects the scanner's bit depth. Mechanical system: A system that drives the relative movement of the scan head and the original.

2 Classification of Scanners

1) Flatbed scanner

Platform scanners are the most common type of scanner available today. Its scanning area is a transparent glass plate, scanning the range from A4 ~ A3. The photosensitive element of the platform scanner is a line array CCD, as shown in Figure 2, which is a few centimeters long array of several thousand photosensitive elements (photodiodes) arranged on a few millimeters wide silicon substrate. Its scanning of images is performed on a line-by-line basis, that is, taking in thousands of pixels at a time and converting them together into electrical signals. Since the sensitivity of each light-sensing element on the line array is not necessarily the same, it is unavoidable that the platform scanner scans with local colors, brightness, and other deviations. With the maturing of CCD technology, these hardware defects can be completely resolved, coupled with the advantages of platform scanner price, leading to the CCD scanner has a tendency to replace the PMT scanner.

2) Drum scanner

Drum scanner is a set of photoelectric system as the core, through the rotation of the drum to drive the scanning of the original movement to complete the scan. Its photosensitive element is a photomultiplier tube (PMT). As shown in Fig. 3, PMT is a vacuum tube composed of positive and negative electrodes. There are several dynodes inside the vacuum tube, and the principle of using the electrodes in the tube to emit light can be achieved by receiving light. Photoelectric conversion. The scanning of the image by the roller scanner is performed on a point-by-point basis. Therefore, its scanning accuracy is high, the speed is high, and the processing width is also relatively large. However, due to its large area and relatively high cost, it is difficult to widely promote it.

3) Film scanner

When an object such as a slide is scanned, the light source is required to reflect the light source through the object rather than the object, and the size of the object is generally small and requires a high resolution for scanning, so a professional film scanner comes into being. The structure of the film scanner is compact, and the reflective lens, lens, CCD array, and light source are mounted on the fixed frame and cannot be moved. The transmissive original can be moved. During scanning, the CCD array and the light source are separated from the two films. The film manuscript fixed on the holder is driven by a stepper motor and moves slowly. The light emitted by the light source passes through the film manuscript and is reflected on the reflective lens, and then refracted. , Focus, is received by the CCD, converted by the CCD element into an electrical signal, after decoding, generate image data. Film scanners generally have a large-capacity memory, and the scanned image can be stored therein. When necessary, the computer can be connected to a computer and transfer the image data to the host computer. Since the color of the negative film is exactly complementary to the normal color, the film scanner generally has a color correction device to make the color of the scanned image normal. At present, there are many products in the market that specifically target 35mm film scanning. This type of scanner is used in professional fields such as hospitals, high-grade studios, research institutes, and so on. The general resolution is high, the scanning area is small, and the processing function for film characteristics is provided. Most products also have matching output devices, which can achieve photo-quality output.

(to be continued)