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Laser scanner sees wrinkles in plastic

20 Nov 2007

A customized laser scanning system produces an accurate 3D analysis of wrinkles in laminated plastics.

Wrinkles can be a serious problem for manufacturers of laminated plastics and textiles. Different schemes for measuring and characterizing surface wrinkles in these materials have been developed, but don't provide true 3D data on the wrinkle's size and shape.

Now, however, a team at the University of Texas, Austin, has developed a laser-based system that enables 3D analysis of wrinkles in laminated plastics. According to researcher Bugao Xu the team has already designed a unit to apply the technique in industrial applications (Measurement Science and Technology 18 3724).

"The current version of our system can acquire 3D surface data on a 10 µm scale in x-y direction, and to a depth of 4 µm," Xu explained to optics.org. "It can assess wrinkle density, sharpness, amplitude and position. The process can work on almost any surface, except those that are extremely reflective."

The system exploits a laser sensor equipped with a 780 nm diode source. The beam size measured 0.3 mm at a sensing range of 30 mm, and the beam was slightly tilted to minimize the effect of specular reflection. A resolution of 12 pixels per mm and a 35 x 35 mm2 scanning region was found to give the best results, generating a 420 x 420 pixel grid.

The data from the laser was turned into an X-Y profile of the reflected light and a 3D surface map of the sheet was formed by collecting multiple profiles, traversing the sample in a step-wise pattern.

Applying customized image processing algorithms to the results of the scan produced an intensity bitmap corresponding to the surface variations. Two key wrinkle characteristics could then be measured: the maximum amplitude is calculated from the difference between the maximum and minimum grayscale values of the profile, while the wrinkle sharpness is assessed from the gradient of the image.

"Developing the wrinkle detection algorithms was the major challenge we had to overcome," Xu commented. "Existing standards for surface roughness and texture attempt to produce statistical summaries of the profiles of surface features, but are based on a set of parallel 2D line profiles, which cannot be thoroughly effective for 3D surface wrinkles."

The team's set-up was a simple laboratory process, but a larger scale production unit is on the drawing board. "The process is ready for commercialization," said Xu. "Our next step is to apply it to more varieties of industrial materials, such as fabrics and tissues."

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