This series of lectures covers many aspects of the prepress field, such as web, page design and page description languages, data formats and digital proofing systems, color and color management, and more. This article will introduce you to film imaging, various screening technologies, and various dot shapes.
First, the graphic recorder and its working principle The graphic recorder is a kind of precision device. It generally uses a laser as a light source. It can use a subtle laser beam (sometimes split the laser beam into several segments first) on the film or the plate. The surface of the material is focused and the dots forming the image are formed thereon. If the graphic recorder uses film as a photosensitive material, we generally call it a "laser image-setter"; if it is directly imaged on the plate, we can call this graphic recorder a "printing plate-making machine". . The arrangement of imaging materials on the Internet depends on the electronic data of the prepress system, and satisfactory image quality is achieved through the arrangement of the dots. Similarly, just like photographs taken with a camera, the image formed on the film must be subjected to a development process before it can be seen with the naked eye.
Why do lasers are used as light sources in graphic recorders? This is because the intensity of the laser beam after focusing is very high, even if it is a very small dot, it can be guaranteed to be intact and not lost. This is crucial for the quality of the image, and it is also decisive. When the resolution and RIP are introduced later, We will also cover this aspect.
The finer the laser beam of the graphic recorder, the smaller the generated dots and the more dots per unit area, the smoother and smoother the dots or lines that are composed, and therefore, the reproduced image details The better the level of reproducibility, the sharper and sharper edges are not blurred. Therefore, the resolution of the laser imagesetter is generally between 1200 and 5000 dpi. The resolution of a laser imagesetter is a measure of the number of laser points it can create within one inch, that is, any vertical or horizontal line is composed of many individual small dots, each inch (equivalent to 25.4 Mm) There are about 1200 to 5000 small ideas. Once the straight line is tilted and it is no longer in the vertical or horizontal direction, it can effectively prevent the edge of the line from appearing jagged.
Second, the dot-oriented imaging approach From this, we can draw the following conclusion: Modern laser imagesetters are based on the operation of the network point, each independent small dot in the image area must define and accept the laser beam The "scanning", as to whether or not the laser beam should be exposed or not, is only determined by the image itself. Therefore, the entire image area is "divided" into many independent dots. We will also talk about resolution issues later. The working principle of the laser image-setter is a bit similar to that of a stylus printer. In order to enable everyone to understand the working principle of the laser image-setter, we will use the stylus printer as an example to explain. We know that the print head of the printer moves along the surface of the paper. When it is necessary to record, the print needle on the print head will receive the corresponding pulse signal and write a black dot here. However, the laser imagesetter also differs from the stylus printer in that the dots printed by the printer can be seen immediately, and the dots formed by the exposure of the laser imagesetter are invisible at the time, and they have only undergone development and fixing. Only then can it be seen by the naked eye.
According to different operating methods and working principles, the laser photo-setters can be divided into three major categories: outer drum photosetters, inner drum photosetters, and winch photosetters. The main difference between these three technologies is film ( Or other imaging materials) and their imaging methods of positioning.
Outside drum phototypesetting machine. The outer drum phototypesetter absorbs and fixes the material (such as film) to be exposed and fixed on the outer surface of the drum by means of vacuum suction. During the exposure and imaging process, the drum rotates at a high speed, and the laser head is parallel to the axis of the drum. The direction is moved so that the laser beam can be scanned spirally over the entire drum surface. The operation and maintenance of this laser imagesetter is relatively simple.
Drum phototypesetter. If an internal phototypeetter is used, film and other imaging materials are to be mounted on and fixed to the inner surface of the drum body. During the exposure and imaging process, the drum will always remain stationary. The only moving/moving part is a small tilt. Mirrors, small mirrors reflect the laser beam impinging on it (the direction of the incoming laser beam is parallel to the axis of the drum and is reflected on the inner surface of the drum). The laser beam can also be scanned in a spiral manner by the rotation of the small mirror. The full surface of the imaging material. However, the maintenance of the inner drum laser imagesetter is difficult, which is one of its shortcomings.
Winch type photosetter. The winch type imagesetter uses a mechanical imaging device that moves an imaging material, such as a pull film, and gradually passes the laser imagesetter so that the laser beam moves along the surface of the imaging material and moves in only one direction, in the direction Movement in the vertical direction is affected by the imaging material itself. However, some people think that this mechanical pull will affect the repeatability of the offset.
Virtual internal drum photosetter (virtual internal). In order to save the production costs of the photo-composing machine, reduce the cost and increase the production efficiency, some people have conceived such a virtual inner drum photosetter. In such an image-setter, the photographic film is in the form of a tube, and the optical exposure imaging apparatus is projected into the tubular film, which is somewhat similar to the design of the internal photographic imagesetter. However, the issue that people are most concerned about is the stability and sharpness of the film size. This issue requires further in-depth research and discussion.
Platesetter. The content of the laser image-setter is briefly introduced here. Let's take a look at the CTP plate-making machine. Since Drupa'95, there have been many manufacturers have successively introduced various types of CTP direct platemaking machines. One of the most outstanding advantages of this type of direct platemaking machine is that it eliminates the need for film and film, fixer, and eliminates imposition and printing. Other processes save production time, reduce environmental pollution, and greatly improve image quality. The working principle of the CTP platemaking machine and the laser imagesetter is very close, but this is just for the offset CTP platemaking machine. So far, flexographic platesetters are still the only CTP platesetter that uses an external drum structure. The UV (digital-ray) platesetter currently uses a platform-type structure that can be repeated. It is believed that this type of CTP platemaking machine will surely gain wider application and popularity in the near future.
Each of the above-mentioned graphic recorders has its own specific advantages and disadvantages. Usually, when selecting a suitable graphic and recording device, in addition to taking into account the requirements for use, the quality, price, and performance of the device are also decisive. the elements of
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