The laser imagesetter is a new device developed in the 1970s. It uses an electronic computer to proofread, edit and process the input text symbols, and then exposes and images the photosensitive material through the laser scanning technology to make the scanning beam high in brightness. To very fine beams, the scanning resolution can reach 40 lines / MM, so this phototypesetting machine can better reproduce the outline and stroke of the text glyph. The high-quality text typesetting and fast typesetting speed have become the main force of text typesetting. After more than 20 years of continuous development, the technology is becoming more and more perfect, and there are many types. From the use of laser light He-Ne laser (HE-NE) laser, argon ion laser and light emitting diode (LED), etc., from the point of view of different scanning methods, There are several main methods such as prism scanning, mechanical drum scanning (outer drum scanning) and inner cylinder scanning. Galvanometer scanning is also used, but the quality of the galvanometer is high and less used.
1. Prism scanning laser imagesetter. Prism scanning (referred to as rotating mirror scanning) laser phototypesetter is a type of phototypesetter currently in use. It consists of an input part, an information processing part, and a laser scanning recording part.
(1) Input part: The input method can be directly input by a recorder (usually a PC), or by the user's own floppy disk, and the image is input by a scanner.
(2) Information processing part: it consists of four main programs: operation control panel, electronic computer and hard magnetic drive, which sends out instructions according to input information and operation panel to complete control, edit proofreading, group layout, and control exposure. The imagesetter automatically ejects the required text layout. The computers used in modern typesetting systems are all microcomputers, such as IBM386, 486, Macintosh microcomputers, etc. The layout (page) description language has gradually used the unified POSTSCRIPT language.
(3) Laser scanning and recording part: This part is the key to guarantee the quality of text exposure imaging. The scanning and recording part of the LASERCOPM laser imagesetter is used as an example to illustrate its working process. Its host uses laser plane line scanning (ie TV scanning) It records the dot-matrix information output after computer processing. This scanning method has a simple and reliable structure, uses less optical components, and has less light energy loss.
The laser beam (= 632.8NM) output by the helium-neon laser enters the acousto-optic modulator, and uses the first-order light with text information output from the acousto-optic modulator as the recording beam. The light beam is adjusted to the energy suitable for various photosensitive materials through the neutral filter, and then the beam is quasi-valued by the beam expander, and then projected onto the cone rotating mirror scanner. The cone scanner has eight reflecting surfaces ( In recent years, the model has been changed to five reflective surfaces). The light beam reflected by each surface forms a spot on the photosensitive material through a wide-angle focusing lens. The X-direction scanning line with a diameter of 0.025MM, and the conveying mechanism drives the Y-direction of the photosensitive material 0.025MM displacement, multiple X-direction scanning lines and the displacement of the photosensitive material in the Y-direction are combined into a text image.
2. Mechanical cylindrical roller scanning laser imagesetter. The mechanical cylindrical roller scanning laser imagesetter is basically composed of the input part, the information processing part and the laser scanning recording part, etc. The composition principle and function of the first two parts are basically the same as the prism scanning laser phototypesetter, the main difference is the laser scanning recording part The scanning drum is a hollow cylinder made of light metal aluminum to reduce the moment of inertia and ensure a stable rotation. When working, it rotates at a constant speed. The optical fiber and lens are placed on the carriage, and driven by the stepper motor and the ball screw, it moves back and forth briskly according to the instructions. In order to increase the scanning speed, the laser is divided into four equivalent scanning light speeds through the optical system. Simultaneously imaged on the scanning roller. During phototypesetting, under the control of typesetting instructions, the modulator transmits the laser beam to be exposed as required through the optical system to the surface of the photosensitive material on the scanning roller for imaging and exposure. After one turn), the carriage immediately drives the optical fiber to move the distance of four scanning lines, and then performs the next scanning exposure, and then cycles until the entire layout is exposed. For this type of scanning system, the optical system is simple, the optical components are easy to manufacture, the mechanical structure is also simple, and the manufacturing is convenient. However, due to the influence of the rotational inertia of the drum, the rotation speed cannot be too high, which limits the layout speed and the mechanical vibration. Influenced by other factors, the imaging quality is also affected.
3. Inner cylinder scanning laser phototypesetter. The main feature of the scanning and recording part of the internal cylinder scanning laser phototypesetting machine is that the photosensitive material is adsorbed on the inner wall of the drum. During the phototypesetting, the drum does not rotate, but rotates on the side of the scanning head while moving to expose the photosensitive material For imaging, generally only 270 degrees out of 360 degrees of the drum is used as the scanning area of ​​the photosensitive film to prevent blur caused by reflection. The optical character system of this type of equipment is relatively simple. After the laser has been turned twice, it is adjusted by a collimating mirror to ensure that the laser beam is concentric with the inner cylinder and is accurately focused on the inner surface of the drum. The rotating mirror generally uses air (air cushion Type) bearing support, there is no mechanical contact, so there will be no friction and errors, so as to ensure that the rotating mirror rotates evenly and error-free for a long time, and under the drive of the ball screw, moves along the axis of the inner cylinder at a uniform speed to make the laser scanning line uniform It is distributed on the photosensitive material on the inner surface of the drum to ensure the scanning quality of the layout. When the photosensitive material is automatically fed into the surface of the inner drum, an air cushion is generated under the action of several air holes to ensure that the film is not scratched, and through vacuum suction, the film is firmly adsorbed on the inner surface of the drum and does not move to ensure scanning Accuracy, no distortion in output, there are many manufacturers of internal cylinder scanning laser imagesetter, and the product models are different, but the basic structure and principle are the same, but they are unique in the degree of automation and specific composition. A product of Haier Company is taken as an example to illustrate their general working process. Figure 5-30 is the internal structure diagram of the QUASAR laser typesetting machine of Linotype-Hell Company. Its working process is Driven by the conduction roller, the film is fed from the cassette to the inner cylinder. After vacuum suction and fixation, laser scanning exposure is performed. After the plate making is completed, the punching device punches (to ensure the convenience of post-process registration). Finally, the scanned and exposed film enters the loading box under the transmission of the driving roller. After a set of plates is prepared, the slicing knife cuts off the film, and the exposed film can be sent for development, fixing and other post-processing. The above processes are all carried out automatically to reduce the influence of human factors and ensure stable quality.
Compared with the scanning system of the prismatic scanning laser imagesetter, the inner cylinder scanning laser imagesetter has the following characteristics:
1. The optical structure of the scanning system is relatively simple, the optical components are relatively small, the weight of the machine is relatively light, the optical path efficiency is high, the working reliability and the quality of the exposure record are good. For example, when the resolution is 1280 lines / CM, the size can be 0. 0078125MM micro pixels can expose 1638400 pixels per square centimeter, and can generate and expose 1021870000 pixels on the DIN-A4 layout. Therefore, it can print high-quality prints.
2. A single mirror on the rotating mirror that rotates the recorded image in the drum, supported on the trolley in the exposure drum, driven by a synchronous motor, because it is located in the center of the drum, keeps the same distance as the photosensitive material and rotates at a constant speed and accurately before and after It moves (the exposure roller and the photosensitive material are fixed), so the reflected laser beam can be evenly projected onto the photosensitive material at equal distances and perpendicularly, and the recorded geometry is always the same.
3. Since the laser beam illuminates the photosensitive material at the same distance, the same optical path and the same angle, the intensity of light is the same, so the degree of blackening of the photosensitive material is the same, that is, the density value is the same.
4. Because the exposure intensity and the image are clear and uniform, and the dot reproduction is perfect, the dot structure is extremely hard and the gradation is steep.
5. The scanning area is large, and the maximum can reach 757 * 550MM.
Due to the above-mentioned characteristics, this type of product has been valued by users, and has gradually become the main force of laser phototypesetting.
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