3D VISION
Rethinking 3D scans of reflective surfaces
Greg Blackman talks to Isak du Preez, who’s new approach to 3D deflectometry has attracted interest from BMW
I
sak du Preez first had the idea for a 3D scanner for reflective surfaces in South
Africa after talking to a friend who had spent the summer counting hail dents on cars. Insurance companies pay by the dent to repair hail damage; they want to know the number of dents, their size and their location. Du Preez thought it might be possible to scan a car’s surface in 3D to find the dents. In 2016 he built a miniature
prototype of the scanner. A rudimentary draw machine moved a patterned light arch over a toy car. He used his cell phone camera to capture reflections from the shiny car body and developed software to calculate the surface profile in 3D. Since then six large scanning
machines have been built, mostly for scanning cars, and du Preez is working on a seventh
for BMW. Counting hail dents is no longer the goal; now it’s end- of-line inspection of cars, which is usually a manual task or uses a robot arm to scan the surface from different angles. Scanning car windscreens is another potential application, or large panes of curved architectural glass. Du Preez has also had interest from aerospace firms for inspecting some of their components. Any reflective surface – typically a large smooth surface – could benefit from this technology. Te technology is a type
of deflectometry, although when du Preez started out he’d never heard of the term. Te strength of du Preez’s approach is to bring the precision of 3D deflectometry to large curved surfaces – previously not addressable by deflectometry – in a practical and economical way. Objects several metres
One of the early scanners using security cameras, and the 3D data generated by the software
wide can now be scanned with sufficient precision to detect micrometre-level local shape aberrations. Deflectometry works,
traditionally, with a camera and
a flat panel display, with each pixel of the camera mapped to a scene point on the display, normally the display pixels. When a reflective surface is introduced, what each pixel of the camera sees can be traced back, via specular reflection from the surface, to points in the surrounding scene. Te mapping is based entirely on geometric information; it depends only on the surface shape and the positions of the camera and display. Te technique relies on
A scanned hood of a car with dents mapped 6 IMAGING AND MACHINE VISION EUROPE DECEMBER 2021/JANUARY 2022
specular reflection, so the surface does have to be shiny or at least glossy, but any reflection is extremely sensitive to shape – hence, hail dents only micrometres deep can be
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Isak du Preez/Axiscan
Isak du Preez/Axiscan
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