So how does AFI work?
First, a laser illuminates the subject with a pattern of stripes, or fringes.
The pattern comes from splitting the laser beam into two point sources of
near-infrared light, each bearing a wavelength of 780 nanometers (780
billionths of a meter). By making fine adjustments to the two sources of light,
operators can manipulate the thickness and placement of the fringes - hence the
term Accordion Fringe Interferometry. (Interferometry is the measuring of
objects using interference patterns as a tool.) To take accurate readings of
complex shapes, such as a nose, operators must be able to adjust the fringes.
Initially, they lay only a few thick fringes over the subject, allowing a
reading of the general outline of the head. When they thin the fringes to get a
more accurate reading, that understanding of the general shape becomes crucial
in order to fill in the details - seeing the forest despite the trees, you
Imagine stripes of light streaming through a venetian blind. Hold a ball up to
the light, and the stripes curve around it. If you were to observe the ball
from the sun's point of view, however, the lines would appear straight. Exactly
how these lines bend depends on the shape and size of the ball. The same
principle applies to AFI: An operator takes a picture of a subject covered in
stripes of light (fringes) from a known angle. Knowing exactly where these
fringes are projected from—the sun, in the venetian blind example—together with how they appear to bend as viewed from that angle, is enough to
triangulate the height of each individual point.
In the near future, detectives may be able to
compare 3-D photos of shoe soles (above) with corresponding 3-D images of shoeprints shot at crime scenes.
The system is still under development, but its potential applications are
limited only by the imagination. (As Derr says, "If you can think of it, we can
design it!") For crime-fighting, besides the possible uses already mentioned,
Shirley and his colleagues have investigated using AFI to take images of
suspects' ears, which in three dimensions serve as well as fingerprints in
identifying individuals. Capturing a shoeprint in a patch of dirt would take
less than a second; later, specialists could test whether 3-D measurements of a
suspect's shoe matched the print.
Potential uses go far beyond detective work. Shirley and company fully realize
how invaluable knowing the exact dimensions of a car door, satellite dish, or
airplane wing, precise to a few hundred nanometers, would be to manufacturers.
(Scalability is another of the system's strengths; the same principles that
work on a square inch work just as well on an object measuring hundreds of feet
square.) Imagine enhanced "virtual reality" games with photo-realistic, 3-D
characters and places, or faxes and copying machines that offer printouts in
three dimensions. With 3-D printers already on the market, such machines are
anything but pie-in-the-sky.
Rob Meyer is Production Assistant of NOVA Online.
The technology reported here was developed at Lincoln Laboratory and was sponsored by the Department of the Air Force under Air Force
Contract #F19628-95-C-0002. Opinions, interpretations, conclusions and
recommendations are those of the author and are not necessarily endorsed by
the United States Air Force.