Before explaining what a Virtual Drum is, let's review why an internal drum is absolutely
necessary for a great color separation imagesetter.
The two qualities a great imagesetter will possess are:
For high quality images, you must have a small laser writing spot ...really small.
Well no ... really, really small!!!
Actually, the spot has to be sized proportionately to the writing resolution. Just as you ®
couldn't expose the imagesetter film using a flashlight beam, you don't do skywriting or
paint a billboard with a ten micron laser diode beam.
High quality halftone photographic images are exposed at 2400 dots per inch or more,®
so if the writing spot is as big as one-thousandths (1/1000th) of an inch (that's 25 microns)
in diameter, there will be too much overlap. The proper size spot is about 10 microns, or
four ten-thousands of an inch - for this you need a drum type imagesetter rather than a
capstan. Small spots happen as the spinner in the imagesetter gets closer to the film,
and a drum structure lets you get at least twice as close.
and only with a drum structure, can you have both stationary film and small spots.
Forming the film
So if the flexible film has to be formed into the shape of a drum -®
diameter drum, then spin this drum, and then write
rasters on this film as it goes around.
That's an external drum structure.®
of a precision machined drum, and then transport a spinner down the axis of the drum. That's an internal drum structure.
needs to shaped - the massive drum structures are only there to help this happen! (and by the way, the drum structures are really
massive - some manufacturers brag that their internal drum structures weigh over 250 pounds, and they're aluminum!).
Suppose that there could another way to bring the film into a drum shape without having such massive structures - you'd have®
accomplished the essence of what you need to do, without using brute force, and you'd have a truly elegant system.
wrapping the film around a glass or Plexiglas drum,
and transporting a spinner down the axis and
exposing the film through the glass. That would
be an internal external drum system.
Probably pretty good, - but it's very expensive to®
make a precision glass drum - and maintenance
would be a chore to keep the drum surfaces clean
and dust-free so that particles don't get between
the laser and the film.
supports the wrapped film, but you could cut away the middle,
you'd be left with two rings that would keep the film in its drum
shape. Voila! - q.e.d. - abracadabra. A two ring ceremony!
That's a Virtual Drum! The film is the drum, the only time you®
have a drum is when the film is wrapped around the rings.
The integrity of the film itself produces a strong, smooth, uniform
film plane surface that is more perfect than any you can get by
forcing the film to conform to a machined mechanical drum surface.
A drum that is better than the usual internal or external drum.
A two pound solution - neat, nifty, and office friendly.®
and in December 1996 they awarde us Patent No. 5,589,900.
Of course, although you can't make a great imagesetter without great ideas, it takes more than just one idea to produce a great®
imagesetter. With a big head start towards keeping the build costs low and offering great value to the end-user, we carefully
designed the surrounding structure and electronics using state-of-the-art techniques so that the end product would be
low-cost, light weight, reliable and user-friendly.
