Jul 6 2010
As production lines become increasingly fast, quality-control procedures must also keep pace. The surface of objects, such as flat panels and glass sheets, can now be monitored for defects-without contact-using an automated technique developed by Xiaoming Yin and co-workers at A*STAR's Singapore Institute of Manufacturing Technology1.
The new system exploits the way a beam of light is reflected from the surface of an object. Similar to waves in the ocean, light travels in a series of wavefronts defined by the light's phase. When a beam of light is reflected by an object, information about the surface shape is carried back by the reflected wavefront. The measurement of wavefronts is therefore a non-contact-and reliable-method for characterizing surfaces. “Other optical systems, such as interferometers, are not suitable for mass-production lines due to their sensitivity to vibrations,” explains Yin.
Wavefronts are measured using a device called a Shack–Hartmann wavefront sensor (SHWS), which comprises an array of tiny lenses. Each lens takes a sample of the wavefront and focuses it into a spot on an electronic detector similar to those found in digital cameras. The shape of the wavefront is then reconstructed by comparing the resultant image with one generated using a known surface. The resolution of the system is largely determined by how well the center of the focused spots can be established. A number of different computer algorithms have been tested to accomplish this, but with varying degrees of success.
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