5.4.2 Capstan devices
In devices of this type (Figure 5.10) slow scan is carried out by pulling a film or photo paper from the roll feed cassette through the exposure area to the developing machine. The line scan is provided by the laser beam move by a mirror. The advantage of this type of devices is in their low cost and high degree of automation of loading and extraction of exposed material. At the same time, they are characterized by relatively low resolution, format, accuracy and repeatability of the output pages. Despite the complex optical system, the exposure aperture was limited to 25 microns, and the output image width rarely exceeded 400 mm.
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Figure 5.10 Output roll fed device with the recording beam deflection by a multi-faceted mirror Resolution of the order of 1000 dpi (40 lines/mm) and other parameters of such scanning devices had fully met the requirements of the electronic phototypesetting and facsimile transmission of newspapers, however, were insufficient to output the integrated, in particular color layouts in computer publishing systems. To this end, technical solutions were found that allowed to increase the accuracy of the film transportation, reduce the deviation of the page sizes to 0.015 mm and increase the accuracy of the exposure beam positioning. The principal improvement was achieved with the laser displacement by LEDs line array. Similarly the use of multiple ink nozzle arrays allowed for creating large format plotters. To provide an acceptably imperceptible, sufficiently high screen frequency, the discussed above solutions were used.
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5.4.3 On-drum and in-drum recording
The output section of the on-drum type is traditionally inherent in the color scanners. The maximum format of output reached 750 x 1100 mm with the accuracy of the separations size repeatability of ±0,005 mm. The use of argon-ion lasers with the relatively short-wave radiation and short – focus lenses allowed for obtaining a recording spot of 5~10 microns and, respectively, the resolution up to 5000 dpi (200 lines/mm).
Their relative disadvantage was in the complexity of film loading and removal. These operations were carried out manually or semi-automatically in non-actinic ambient light. Scanner manufacturers have tried different ways to cope with this kind of inconvenience. So, in Chromograph 296 the drum with a fixed over it film was in a removable light-proof cassette, while in Chromograph 300/350 there was used a flat cassette from which a sheet of film format 400 x 500 mm automatically transferred to the cylinder and fed back to the cassette after recording. All this did not eliminate the additional manual operations of charging and removing the film from the cassette in the dark room. The use of cassettes proved to be unpromising in terms of increasing the scanner basic parameter— the recording format. Later, the output sections of the on-drum scanners became remotely controlled modules placed in a dark room.
With the format growth, not only the drum dimensions but also its mass increase. At the same time, maintaining the performance at the same resolution, i.e. the number of recorded lines per unit drum length requires increasing the speed of rotation. It, in turn, had increased the dynamic loads, the drive power for fast acceleration and created the problems with holding the film by a vacuum suction.
Therefore, the required performance at a technically acceptable speed of rotation of about 1000 -1200 rpm is provided by simultaneous recording of several (6 -10) independently controlled exposing spots. To this end, the laser beam is split [5.24] in the beam-splitting system (Figure 5.11) or in an acousto-optic deflector [5.25]. Even more lines are exposed in parallel, when instead of a laser the line of LEDs is used, installed in the recording head parallel to a certain segment of the drum or along the entire its length.
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Figure 5.11 The splitting of the laser beam by semi-transparent mirrors and parallel recording of multiple lines allow for reduce the speed of the drum rotation Compared to the trivial line-by-line scan, parallel exposure systems require some extraordinary output data organization in the RIP. In in-drum systems, a line scan is provided inside the cylindrical chamber (1) by rotating the mirror (2) tilted to its axis at an angle of 450 (Figure 5.12). To reduce the influence of rotation eccentricity on the beam path, the mirror was subsequently replaced by a prism. The camera has a longitudinal recess required for film (3) loading and moving the mirror bracket (4). Therefore, the length of the recorded line is not equal to the entire camera circumference and is limited only by its part — arc. This, apparently, explains the name “arc” scanning system, adopted in the Russian literature of the 70s when on this principle the equipment Gazeta-2 of newspapers photo facsimile transfer for the remote, decentralized printing was created [5.26]. In this equipment, the photographic material was not only automatically cut off from the roll, inserted into the camera and clung to its inner surface, but also output after recording directly into the developing machine. Slow scan was provided by moving the camera in the longitudinal direction. In the later imagesetters and CtP devices of this type, camera is usually stationary, and the entire mechanism of the slow scan moves inside it (Figure 5.12). The bracket (4) to this mechanism is rigidly connected via the longitudinal recess of the chamber to the external feed drive, lead screw (5) and guides (6). |
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Figure 5.12 Scan image inside a cylindrical camera by rotating mirror or prism Here, unlike on-drum systems, there is no need to turn off the line scan to remove or install the recorded material. This was especially effective for analog photo-facsimile newspapers transmission, because it had eliminated the loss of time on phasing the scan lines of the receiver and transmitter prior to the next page transmission. The relatively small mass of the mirror or prism allows for increase the line scan frequency to tens of thousands of rpm and thus provide high performance in the usual (line-by-line) organization of data controlling the exposing beam.
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