Both of these technologies are very exciting and have the potential of
making a 3D fax machine when used in combination.

Laser Digitization is a process of scanning a 3D object (somewhat
redundant) and saving the dimensional data on a computer.  Basically the
object is placed on a turntable and a laser beam is bounced off the
object onto sensor.  The computer determines the distance to the object
and places the data in memory.  Once the turntable has completed one
rotation the laser move down (or up) a fraction of an inch and scans
again and continue thus for the entire height of the object.  Just as
with a desktop (2D) scanner the resolution depends on the amount of
movement between data readings.  The smaller the amount of movement the
higher the resolution and thus finer detail.  There are several firms
that provide this service (one in Monterey, CA that I know of).  They
scan objects and even live people for use in special effects in movies.

On the other end there is the "output".  In Hollywood they put the data
into special modeling programs which can represent the image from the
data on a computer screen.  These images can be manipulated and combined
and place on film for use in a movie.  It is also possible to make 3D
representations of the data using Rapid Prototyping processes.

There are several different techniques for making Rapid Prototypes (some
in plastic and some in metal).  As the name implies these techniques
were developed to make prototypes of new products .  Instead of scanning
(a non-existent object) the data would come from a computer drafting
drawing which was "modeled" and then sent to the rapid prototyping
machine.

Now these machine are fantastic!  However, not without limitations, as
one would imagine.  Again the magic of lasers comes into play.  A
computer controlled laser scanning mechanism is place above a vat of
photosensitive polymer.  Immersed in the vat is a movable platform that
is placed just beneath the surface.  The laser starts by tracing the
bottom edge of the object to be reproduced on the surface of the
polymer.  This cures (solidifies) the polymer which now rests on the
movable platform. The platform then moves down a fraction of an inch
(e.g. 0.001") and the laser traces the next layer (based on the data)
and so forth.

If this is hard to understand, then think in terms of a coil pot made of
clay.  To make a cylindrical vase you start with a flat coil of clay
forming a disk. For the next layer you make a ring of clay and place it
along the circumference of the disk and so forth forming a cylinder.
The thinner the clay "rings" are the smoother the surface will be.

The fidelity of object depends on the resolution of the machines on
either end.  In the case of a actual prototype, the resolution of the
prototyping machine is the limitation.  I don't know what the highest
resolution of the laser scanning device is but I would guess it is less
than the Rapid Prototyping Machines (about 0.001).  The parts that come
from the RP machines still need clean up work after the process.
Normally the RP part would be cleaned-up and used to make a mold for
multiple part.

So what are the pros and cons of using these techniques for reproducing
museum objects? If you had an priceless object that could be harmed
during the process of making a silicone mold directly, laser scanning
might be an alternative.  However, if the object is "photosensitive" the
laser could do some damage.

The main disadvantage as I see it are the limitations (particularly the
resolution) of the current RP machine.  These machine are great for
producing 3D objects from drawings of non-existent objects.  But if you
want to reproduce fine detail its hard to beat a properly made silicone
mold.  Basically with RP you end up with a fine coil pot!

There's certainly a lot more to it than this, but I've already gone on
too long.  I hope this helps.

-Mark-

Ok- One more thing, what was that stuff about a 3D fax machine.  A 2d
fax machine is basically a scanner connected to a modem (to send digital
information across standard telephone line) connected to a printer on
the other end.  Replace the 2D scanner with the laser scanner, connect
it to a modem, and dial up a RP machine connected to a modem on the
other end and you have it!  If you decide to try this, just remember
this is going to be a very long fax!

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