The composition and characteristics of the laser imagesetter
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The laser imagesetter is a new device developed in the 1970s. It uses the electronic computer to proofread the input text symbols, edit it, and then exposes it to the photosensitive material through laser scanning technology to make the scanning beam have high brightness. To a very thin beam, the scanning resolution can reach 40 lines/mm. Therefore, the photo-aligning machine can better reproduce the character glyph outline and the pen front. The quality of the photo text is high and the typesetting speed is fast, which has become the main force of text typesetting. After more than 20 years of continuous development, the technology is increasingly perfect, and there are many kinds, from the use of laser helium (he-ne) laser, argon ion laser and light-emitting diode (led), etc., from the scanning method is different, There are several main methods such as prism scanning, mechanical roller scanning (outer roller scanning) and inner cylinder scanning, and galvanometer scanning, but the quality of the galvanometer is high and used less.
1. Prism scanning laser imagesetter. The prism scanning (scanning mirror) laser phototypesetting machine is a type of phototypesetting machine which is currently used more. It is composed of an input part, an information processing part and a laser scanning recording part.
(1) Input part: input mode, can be directly input by the input machine (usually using PC), or can be input by the user's own floppy disk, and the image is input by scanner.
(2) Information processing part: by the operation control panel, the electronic computer and the hard magnetic drive, which according to the input information and the operation panel, issue instructions to complete the control, edit the proofreading, set the layout, and control the exposure and other four main programs, so that The imagesetter automatically ejects the desired text layout. The computers used in the modern typesetting system are all microcomputers, such as ibm386, 486, Macintosh, etc., and the layout (page) description language has gradually adopted the postscript language.
(3) Laser scanning and recording part: This part is the key to ensure the image quality of text exposure. Now the scanning part of lasercopm laser imagesetter is taken as an example to illustrate its working process. Its host computer uses laser plane line scanning (ie tv scanning). The method records the dot matrix shape information output by the computer processing. The scanning method has a simple and reliable structure, uses less optical components, and has less light energy loss.
The laser beam (=632. 8 nm) output from the 氦-氖 laser enters the acousto-optic modulator, and the first-order light carrying the text information output by the acousto-optic modulator is used as the recording beam. The beam is adjusted by a neutral filter to the energy of various photosensitive materials, and then the beam is quasi-valued by a beam expander, and then projected onto a conical mirror scanner having eight reflecting surfaces ( In recent years, the model has been changed to five reflecting surfaces. The light beam reflected by each surface forms a spot on the photosensitive material through a wide-angle focusing lens, and the x-direction scanning line with a diameter of 0. 025 mm, while the conveying mechanism drives the y-direction of the photosensitive material. A displacement of 0. 025 mm, a plurality of x-direction scan lines and a displacement of the photosensitive material in the y direction are combined into a text image.
2. Mechanical outer drum scanning laser imagesetter. The mechanical outer cylindrical scanning laser imagesetter basically consists of an input part, an information processing part and a laser scanning recording part. The composition principle and function of the first two parts are basically the same as those of the prism scanning type laser imagesetter, and the main difference is in the laser scanning recording part. The scanning roller is a hollow cylinder made of light metal aluminum to reduce the moment of inertia to ensure smooth rotation and rotate at a constant speed during operation. The optical fiber and the lens are placed on the carriage, and the stepping motor and the ball screw are used to move back and forth lightly according to the instruction. In order to increase the scanning speed, the laser is divided into four equivalent scanning light speeds through the optical system. Simultaneous imaging on the scanning cylinder. At the time of photographing, under the control of the typesetting command, the modulator transmits the laser beam to be exposed as required to the surface of the photosensitive material on the photosensitive drum through the optical system for imagewise exposure, and when the photosensitive material is exposed to one line (roller rotation) After one turn, the carriage immediately drives the fiber to move the distance of the four scan lines, and then performs the next scan exposure, and then loops until the entire layout is scanned. This kind of scanning system has simple optical system, easy manufacturing of optical components, simple mechanical structure and convenient manufacturing. However, due to the influence of the inertia of the drum, the rotation speed can not be too high, which limits the typesetting speed and is subject to mechanical vibration. The quality of the image is also affected by the influence of other factors.