3-D DIGITIZERS: THIS ARTICLE WAS SCANNED FROM COMPUTER GRAPHICS WORLD 
MAGAZINE JULY ISSUE 1992 Three-dimensional objects of all shapes and sizes 
surround us, yet only a only a small fraction are stored in digital form. 
Threedimensional digitizers enable you to copy these objects into a 
computer. The computer model usually requires editing, bu the work is minor 
compared to creating the object from scratch.    Engineers use 3D digitizers 
for reverse engineering of mechanical parts. Manufacturing professionals 
apply them to the creation of new patterns for molded tooling and investment 
casting. Medical specialists use 3D digitizers to digitize bones andother 
anatomy for the development of prostheses and implants. Video production 
experts use them to capture difficult-to-create shapes for TV commercials 
and a variety of special effects.    The Cyberware 3D digitizer, for 
example, has played an important role in creating the special effects for at 
least three major motion picturesThe Abyss, RoboCop 2, and Termlnator 2. 
Each of the movies featured at least one computergenerated charcter that was 
created by scanning in the heads and facial expressions of real actors. 
   In the medical world, one of the more unique uses of a 3D digitizer 
involves neurosurgeon Richard Bucholz of the St. Louis University Medical 
Center, who uses a special version of the Pixsys Firefly Electro-Optical 3D 
Digitizer as a visualization aid during brain surgery.    The device he uses 
is similar to the general-purpose digitizer, except the LEDs are mounted in 
the handle of his forceps instead of on the tip of a probe. The system also 
requires LEDs located on a ring device that screws to the patient's skull. 
he LEDs on the ring enable the system to track any possible movement of the 
patient's head. The location of the tip of the forceps shows up on a Silicon 
Graphics computer display, and when merged with MRI, PET, and other data, 
the system allows the srgeon to figure out, in real time, the position of 
the forceps within the patient's head to within two millimeters.    In the 
engineering and manufacturing world, 3D digitizers are finding uses in all 
kinds of applications. One hearing aid manufacturer, for example, uses a 
Digibot 3D digitizer as part of an automated system that optimizes the size 
of hearing aids The digitizer is used to scan in the plastic shell housing, 
and then customized Digibotics software fits a mathematical representation 
of the electronic components into the polygonal mesh of the shell. The 
system calculates the smallest possible size for the shell, and then a 
rotary saw mounted on the Digibo system automatically cuts the shell at the 
precise location.Categorizing the Systems    Three-dimensional digitizers 
fall into two broad categories: probe and non-contact. Probe digitizers 
require the user to manually touch the tip of a probe to an object. This 
usually involves several hours of repetitive selections, depending on th 
number of points required to adequately represent the object. Prices for 
probe digitizers range from $3500 to $25,000, but they're less expensive 
than non-contact laser devices.    Laser digitizers, priced from $30,000 to 
more than $265,000, use laser light to obtain x,y,z coordinates without 
physically touching the object. Because they require less mechanical motion 
and do not risk destructive contact, laser digitizers aremore automatic and 
much faster  than probe digitizers.    Laser digitizers can be further 
divided into two smaller subcategories: single point and plane of light. 
Single-point systems illuminate a small spot of light on the object. They 
capture one point at a time, typically by rotating the object. The esult is 
a vertical stack of evenly spaced contours stored as a list of x,y,z 
coordinates. Because of the added mechanical flexibility of moving a single 
beam of light about an object, the single-point technique is an effective 
and accurate solution o laser digitizing. However, this technique is slower 
than the plane-of-light technique.Plane-of-light digitizing systems 
illuminate a line of light on the object. They capture not just one point at 
a time, but rather a string of points that represent the contour illuminated 
by the plane of light. Consequently, they can capture points mre quickly 
than single-point systems. In fact, they usually capture more points than 
are necessary to accurately define the surface of the object. They also 
typically measure points redundantly by producing overlapping grids of data. 
As a result, fils grow very large and often require a reduction of points 
using special point- filtering software.    Laser digitizers work by 
projecting laser light onto the surface of an object. The light is then 
reflected back to one or more sensors. The path of light forms a triangle, 
and the angle formed at the point of contact with the object is called 
theprobing angle. The probing angle decreases as the laser source and 
sensors are positioned closer to one another. The smaller the probing angle, 
the better the digitizer can probe objects with deep concavities, undercuts, 
and openings.    A digitizer with a probing angle of 25 degrees can get into 
areas unreachable by a digitizer with a probing angle of 40 degrees. 
However, accuracy decreases as the size of the probing angle decreases. 
Therefore, developers of laser digitizers mus weigh accuracy against the 
digitizer's ability to probe hard-to-reach areas.The difficulty of reaching 
con-cavities and undercuts extends beyond the probing angle. If a bump on 
the surface of a part obstructs the path of the light (light either coming 
or going), the digitizer must be able to recognize the obstruction. If it 
oesn't, the digitizer will miss this part of the surface.    With the help 
of intelligent software, coupled with adequate degrees of freedom between 
the object and digitizer, certain developers have overcome this problem. In 
these systems, the digitizer automatically probes the object from even angle 
possile until it finds all of the points in the concavity. The only 
restriction to this intelligent scanning technique is the probing angle. If 
the angle is too large, it is impossible for the light to get in and back 
out of the concavity.    Still, even with these systems, it is difficult to 
capture 100 percent of the surface of those objects containing holes and 
deep concavities with small openings. Graphics editors and CAD tools are 
available to patch and fix these missing surfaces Most laser digitizers 
bundle a graphics editor and provide interfaces to CAD systems.    For the 
last several years, the major players in the 3D digitizer market were 
Cyberware, Laser Design, Polhemus, and Science Accessories. Today, they 
continue to dominate the market, but they are now sharing the field with 
several newcomers, inclding Digibotics, Faro Technologies, Perceptron, and 
Pixsys. All have shipped or plan to ship products this year.New Players 
Enter the Market    What these newcomers bring to the market is the ability 
to mix clever probing techniques with proven technologies. That, combined 
with aggressive pricing, may provide the appeal that's necessary to greatly 
expand the market for 3D digitizers.Digibotics' Digibot system, for example, 
combines laser technology and personal computing with speed and accuracy. 
The use of a basic '386-based PC and Microsoft Windows, coupled with its 
RS-232 serial interface and small size, makes it a true deskto peripheral. 
   Faro Technologies is targeting the lucrative AutoCAD market with its 
Metrocom product. It is the only 3D digitizer company to offer a powerful 
ADS interface to AutoCAD. The low price, reason able accuracy, and ability 
to han dle a large work volume makes i an attractive choice for reverse en 
gineering of large objects, such a~ automobile body parts.    Perceptron 
offers a produc called Lasar that blends a uniqu~ mix of laser and radar 
technolog~ with a small size, portability, and a 126-feet ranging length. 
The re sult is a digitizer that may open new markets in the theme park and 
construction ndustries. The technology also has potential for machine vision 
applications, such as robotic guidance and part inspection. The technology 
may even make the product attractive to surgeons in the operating room. 
   While these new technologies are exciting and show impressive potential, 
no company is yet poised to become the AutoCAD of the 3D digitizer industry. 
The industry is still small, with less than 1000 general-purpose units 
installed worldwide. Thatmeans the race to be the first company to produce a 
machine that copies 3D objects as easily as a copy machine copies paper 
documents is still wideopen.          CGWHow Accurate is Accurate?Comparing 
the accuracy of 3D digitizers can be confus-ing. The terms "accuracy, 
precision," resolution," andrepeatability" are often misused, 
misunderstood mislead-ing, or just plain omitted. A company may claim its 
3Ddigitizer is accurate to +/- 0.010 inches. Yet it is usuallyimpossible to 
achieve this accuracy all of the time, unlessyou are digitizing a very 
simple shape, such as a sphere. Acomplex shape with holes and concavities, 
such as an en-gine block, is a much better test of accuracy.Several factors 
can affect the accuracy of laser digitiz-ers. If a part contains detail that 
is smaller than thewidth of the laser beam, accuracy drops 
significantly,since the beam hits and spreads over more than one sur-face. 
Consequently, the detector is unable to accuratelylocate the center of the 
beam. Corners and surfaces thatform sharp angles can cause a similar spread 
of light.Some experts claim that the major source of error is thediameter of 
the laser beam.Laser digitizers are also sensitive to shiny and 
darksurfaces. Ideally, the object should be light in color, yetdull or flat. 
If the surface is shiny, reflection can cause ashift in the location of the 
point. If the surface is darkthe digitizer sensors cannot adequately see the 
point. Us-ers of laser digitizers, therefore, often coat the surface ofthe 
object with a white, water-soluble substance, such astempera paint, which 
usually rinses clean from mostparts.Buyer confusion can also occur when 
digitizer suppli-ers publish numbers related to the specific parts (for 
ex-ample, linear rails and stepper motors) that make up thesystem. This can 
be misleading. The precision of the sys-tem's mechanics does not reflect the 
accuracy of the proc-ess. In fact, the precision of the mechanics is 
usuallyalways better than the accuracy of the process.As for the term 
"resolution," that refers to the densityof the points. If a digitizer has a 
maximum resolution of0.010 inch, this means the points are never spaced 
closerthan 0.010 inches apart. It would be impossible for amachine to offer 
O.OO9-inch accuracy if the maximumresolution is 0.010 inches. Therefore, a 
well designed sys-tem will always have a resolution that is greater than 
itsaccuracy.
