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Harnessing the Power of Electronic media for the Print media

Difference between
Capstan Imagesetters & Drum Imagesetters

    CAPSTAN IMAGESETTERS 

The most inexpensive imagesetters are capstan, or flatbed, recorders, which take their name from the capstans, or rollers that feed the imaging media through the device. Capstan recorders are capable of continuous imaging of a roll of media, for extremely high-productivity. They are used for a wide variety of applications, including B & W and low-end colour work. Unfortunately, most capstan recorders do not support high-resolution imaging, making high-end colour work requiring high screen rulings difficult.

Recorders in general, image output pages by exposing consecutive lines into the media using a laser beam. In capstan recorders specifically, imaging proceeds from the line to line as the media moves across the optical assembly. Since the media in capstan recorders is transported lengthwise by rollers as the laser beam is projected across the width of the medium, stretching and shifting may occur, resulting in registration problems. To prevent these kinds of difficulties, precision transport systems that maintain consistent tension throughout the entire length of the media roll are a necessity.

Another feature of capstan devices is that the laser itself does not move and that the beam must be projected across the scan line. It is important that the laser spot be the same size and shape, no matter where it is on the scan line, since variations in dots may result in moire and other artefacts. Capstan optical systems are also highly susceptible to wobble, typically caused by the force and speed of the spinning elements. The optical system must be carefully designed both to move smoothly and to create laser spots of exactly the same size, shape and intensity across the entire width of the media.

Drum Imagesetters come in two types

  External drum recorders:  

 

External drum recorders are famed for their superior imaging quality. Unlike internal drum recorders, where the head is inside the drum, in external drum recorders the imaging head is placed outside the drum. The difference in positioning has a direct effect on imaging quality.

In internal drum imagesetters the imaging head is positioned in the center of the drum. This design creates a variety of challenges. First of all, placing the head reliably in the precise center of the drum requires the finest in engineering and construction. Secondly, since the head itself spins at extremely high speed, controlling vibration is a crucial issue. Thirdly, since the imaging head is in the center of the drum and the center becomes further from the drum surface as drum circumstance increases, the distance from the imaging head to the media increases in drums that support larger format media.

Unfortunately, as the distance between the imaging head and media increases, the light becomes less focused and more sensitive to vibration. This results in consistencies in spot size, shape, sharpness, and position, otherwise known as addressability. Poor addressability is expressed in the creation of soft dots, resulting in a noticeably wider fringe in the final halftone screen. This makes it difficult to maintain good registration between colour separations.

In external drum imagesetters, on the other hand, the imaging head is placed outside the drum, so its exact position and distance from the media are not predetermined by inherent design factors. In fact, the imaging head can be positioned more or less whenever desired, and it is generally placed extremely close to the media. The imaging head is then moved parallel to the central axis of the drum with the aid of a precise tracking mechanism during imaging head can always place spots from a position perpendicular to and at the same distance from the media, providing consistent spot addressability at all times, regardless of drum size.

The external drum design also allows external drum recorders to employ multiple beam imaging heads. Such multiple beam heads can expose several lines at one time, providing highly increased imaging speed. Specialized control systems are employed to ensure that such multiple beam heads track smoothly across the media so that there is no loss of imaging precision

 

  Internal drum recorders: 

 

As with external drum recorders, the main components of internal drum devices are the drum, the optical system, and the media handling system. Unlike in external drum devices, however, the drum in an internal drum recorder does not rotate. Instead, the imaging head itself rotates at high-speed to image the entire sheet of media (see figure).

Internal drum recorders are famed for their superior media handling capabilities. Since the media is loaded into, instead of onto, the drum, media position is easy to control and maintain. This makes internal drum imagesetters the perfect choice for difficult to handle media such as the aluminum-based PS plates used in CTP work. Furthermore, internal drum recorders are easy to fit with punch systems; since the drum does not spin, punches can be mounted right onto the drum to provide superior repeatability and reduce operator labor.

The imaging heads for internal drum recorders travel through the center of the drum and contain mirrors or prisms which project the laser light onto the film. The movement of the optical assembly must be perfectly smooth to prevent artifacts such as banding from appearing in the film. Furthermore, lens systems which condense laser beam and correct for variations in laser beam shape and size are required to compensate for the increased distance between the media and imaging head.

 

Courtesy
-Screen News Box Vol 4-

 

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