Laser digitization and rapid prototyping

A recent offline inquiry regarding "laser digitization" and "rapid
prototyping" prompted a little research on my part.  Since the
inquiry, I've spoken to vendors and researchers and done some WWW
surfing.  Though my understanding of these processes is by no means
complete, I've come across some information that might be useful:

Laser digitization is a technique used to scan 3-dimensional
objects.  The technique is conceptually simple: A laser is projected
onto an object at various points over its surface.  The reflected
beam is analyzed and recorded. This process is repeated rapidly
until the object's entire topography has been mapped.

During scanning, the object is usually mounted securely to a
rotating platform. The initial digital file is a collection of
points known as a "point file." This file is usually a pretty
manageable 2MB for a medium-sized object (i.e. a vase).  The point
file is most often converted to create a "mesh" of connected dots.
These connected dots form an image of the object rendered through
multiple triangular facets.  The resulting ".stl" files are
compatible with a range of computer aided design (CAD) applications.
A 2 MB "point" file might turn out to be a 10 MB .stl file.

Many laser digitizers use a very low power, class 3A, helium neon
lasers (emitting the conventional and familiar red laser light).
Laser power tends to be less than 5 milliwatts (in range with a
laser pointer).  Laser light exposure time for each point on the
scanned object is measured in subseconds (0.25 seconds is one
reported value).  Scanning a medium sized object (like a vase) might
take 30 minutes or more depending on the number of measured points
required. The distance between individual measured points or
"resolution" can be as good as 0.5mm.

>From a conservation point of view my first impression (gathered from
secondary sources only) is that exposure to the laser light would
not be a problem (for almost any object) based on very the brief
exposure time and the low intensity of the light.  Of greater
concern is that fact that industrial practitioners routinely coat
objects prior to scanning to increase their reflectance properties.
Presumably cultural material would not be coated, perhaps resulting
in poor scans.  This type of problem can be overcome by increasing
the laser power or through other operator manipulations.  Another
conservation/preservation issue might be mounting since the scanned
object (most often but not always) needs to be placed onto a
rotating platform.  Since there is really no track record for
applying these techniques to a wide range of cultural material,
there are likely a fair number of other conservation issues that
could arise that are beyond my current understanding of the process.

"Rapid prototyping" is a term used to describe the automated,
robotic fabrication of an object using a computer-based model of the
object (a CAD file).  I've been told about a recent example of rapid
prototyping might be somewhat more illuminating: An artist made a
small-scale, clay sculpture.  This clay model was then laser
digitized. The resulting CAD file was used to control fabrication
equipment (specialized bandsaws) to make a large-scale version of
sculpture in Styrofoam.  The Styrofoam prototype was used as the
basis for traditional bronze casting techniques.  Based on my grasp
of rapid prototyping, the term really boils down to a range of
minimally interventive, automated techniques used to make high
quality duplicates of 3-D objects.

Used in a cultural context, the possibilities of laser digitization
are intriguing for scholarship and documentation.  Conceivably an
anthropologist with access to laser digitization tools could quickly
make a scan of an intriguing arrowhead, bone etc.. and "show it" to
scholars anywhere through a 3-D computer model (a CAD file) sent via
email. A low bandwidth illustration of this idea is Cyberware's
"Models Page" http://www.cyberware.com/models/index.html.  In
addition, multiple similar objects can be scanned to create a large
comparative catalog. An example of cataloging can be found at the
University of Texas' "Guide to Human Osteology"
<URL:http://www.dla.utexas.edu/depts/anthro/kappelman/osteolog.html/>.
Another potential application is the production of high quality
facsimiles of fine art objects for display (or for sale).

For a good overview of the field see Terry Wohlers' site at
<URL:http://lamar.colostate.edu/~wohlers/>?

Lots of additional information can be found at these vendor sites:

    <URL:http://www.3dscanners.com/>
    <URL:http://www.cyberware.com/>
    <URL:http://www.digibotics.com/>
    <URL:http://www.gks3d.com/>

I hope this information helps and would really appreciate hearing
from anyone who has used these techniques with cultural material or
has detected errors/oversimplifications in this quick write-up.

Paul Messier
Conservator
Boston Art Conservation

                                  ***
                  Conservation DistList Instance 11:50
                 Distributed: Monday, December 8, 1997
                       Message Id: cdl-11-50-011
                                  ***