to the inside or outside of the drum using a variety of complex mechanisms including clamps and vacuum. The size,
weight, and complexity of traditional drums have meant that they are expensive to build and difficult to maintain
(hense older technology).
Exxtra/RIPit developed a unique solution to the problem of attaching the media to the drum: let the media be the drum. ®
The media wraps around two rings, one on each edge of the media, and the drum shape is maintained across the middle
by the media itself and the imager. The result is a drum imagesetter which is lightweight, many less moving parts,
and more reliable because of its simplicity. And it offers image quality, accuracy, and repeatability specs which meet or
surpass traditional heavy drum imagesetters.
The Virtual Drum’s simplicity provides advantages throughout the service life of the imagesetter: no standard ®
maintenance is required other than simple cleaning, and if the machine ever requires service, it is light enough to be
shipped by air carrier to the factory. Time between service calls for Exxtra ImageSetters. is typically measured in years.
What is an Imagesetter? A Platesetter?
An imagesetter is a device which uses a laser to expose photographic film or paper. Platesetters are quite similar,®
exposing plate material for use on a press. Our full range of imaging devices can be used as imagesetters or
platesetters, simply by loading them with the appropriate media. Used as a platesetter, our imagers can be
attached to an online processor, so that they produce cut, processed plates which are ready to hang on a press.
How do Imagesetters and Platesetters differ from Laser Printers?
Laser printers create an image using toner particles which are fused onto the paper, while imagesetters and platesetters®
photographically expose the media, and it is processed chemically. Since toner particles are fairly large in relation to the
size of a halftone dot, that limits the resolution and accuracy of toner-based imaging. Typical laser printers are capable of
600dpi resolution, and line screens up to about 85 lines per inch. Screen values can vary substantially from those specified
in the file on the basis of toner coverage, printer condition, and environmental characteristics, so that producing consistent
screens on a laser printer is difficult. Often, if there is heavy coverage, regions of the page can be “starved” of toner,
rendering them lighter than they should be. Some newer laser printers offer 1200 dpi of addressable resolution, but several
adjacent pixels must be “on” before toner particles will stick to the page, so the effective resolution is considerably lower.
Imagesetters and platesetters use a laser to directly expose media, so they are not affected by the peculiarities of toner or ®
environmental considerations, nor are they affected by the contents of other areas of the page. For that reason, images
produced on an imagesetter are more consistent and repeatable than those produced on a laser printer. Imagesetters
are also capable of much higher resolutions — ours go up to 4000 dpi.
What about toner-based plates?
A number of toner-based plate making systems have become available recently. Used within their limits, they can provide ®
good value — they’re best for single color jobs, or multi-spot color jobs which do not require tight register. The best of them
can produce moderately good photographic reproduction, on images with little highlight or shadow detail. They are not
appropriate for tight register jobs, process color, or screen rulings over 120 lines per inch. Even at low line screen values,
imagesetter-produced screens will look considerably smoother and more consistent.
While the purchase price for a platesetter is higher, the material costs are similar — typically $1 - $2 per plate, depending ®
on a variety of factors. When you factor in the fact that all jobs will look at least somewhat better when printed with
platesetter-produced plates, plus the fact that some jobs must have plates or films made, the difference in price between a
toner-based plate maker and a true platesetter becomes less compelling. Most users — especially those who plan to grow
their businesses — will make more money with a platesetter in the long run.
Why do imagesetters need to offer such high resolutions?
High resolution is critical for creating smooth screens, particularly in gradients. The human eye is able to resolve over 200®
tonal values in the range between black and white, so the goal in generating a halftone screen is to be able to produce at l
east as many tonal values as the human eye can see. Outputting a traditional halftone spot on a digital imagesetter requires
that the spot be “built” from a cluster of adjacent pixels. To be able to produce 256 gray levels with a halftone screen requires
that each halftone spot be composed of between 0 and 256 pixels. Since the space allotted to each halftone spot is usually
square, that works out to a 16 by 16 grid of pixels from which each halftone spot can be “built.” If achieving a full 256 gray
levels is important, then that determines the required resolution. If the desired line screen is 150 lines per inch, and each
halftone spot is given a 16 by 16 grid, 150 lines (halftone spots) per inch times 16 pixels per spot gives you a minimum
resolution of 2400 pixels per inch. Decreasing the line screen or increasing the resolution results in more gray levels being
available, if the RIP supports it.
Modern Rips use a few techniques to increase the apparent number of gray levels, meaning that it is possible to generate ®
higher line screens than the previous paragraph would suggest, while also increasing the apparent number of gray levels
available. Typically this is done by using a “super cell” technique, in which a cluster of four halftone spots are treated as a
single, larger spot, quadrupling the number of available gray levels.
Imagesetters come in two classes: capstan and drum. Capstan imagesetters are analogous to a laser printer –
the media moves continually through the machine while the laser quickly scans across the width of the media producing
successive lines of the image. One of the primary limitations of capstan imagesetters is that the media is moving while it is
being imaged. Any jitter or variation in the speed of media movement results image anomalies which can be quite visible
in screen areas. All capstan imagesetters suffer from some these effects to a certain extent, and the problem typically gets
worse as the imagesetter ages.
Drum imagesetters form the media into a cylindrical shape, and image the media from within the drum (on an internal drum®
imagesetter), or spin the drum and image it from the outside (on an external drum imagesetter). Drums have several
advantages over capstan designs:
1) The media is stationary relative to the drum during imaging. Instead, the imager moves across the media driven by a ®
precision machined screw which is far more accurate than a capstan.
2) Because the laser scans around the circumference of the drum, it is always a consistent distance from the laser to the ®
media, and the beam is always perpendicular to the media, resulting in a sharp, focused laser spot.
3) When imaging a full page, a drum imagesetter is faster than a capstan for a given scanning speed, because the ®
laser scans along the length rather than width of the media.
Spot size is the size of the spot created by the laser as it sweeps ®
across the media. If the spot size is too large, then the quality of the
resulting image is degraded. The ideal spot size is close to —
but not smaller than — the size of the pixel being imaged. If you are
imaging at 2400 dpi, the ideal spot size is close to 1/2400" across.
Spot size is usually measured in microns, and the measurement of
the spot size is taken at the point where the brightness of the spot
drops off by half.
In capstan imagesetters, the typical spot size is in the range of 20 ®
to 30 microns, appropriate for resolutions of 1200 dpi to
1800 dpi. Selecting higher resolution on a machine with a large
spot size yields a smaller improvement in quality than the resolution
value would suggest; you can only write so small with a fat pen.
Exxtra Imagesetters use one or more neutral density filters to ®
match the spot size to the resolution being imaged. At resolutions
above 2000 dpi, Exxtra 300 and 400 series imagesetters use a
10 micron spot, allowing line screens in excess of 250 lines per inch.
The Exxcalibur uses a different spot size for each resolution, down
to a minimum of 7.5 microns.
