Spotting defects in curved reflective surfaces

Researchers from the Toyohashi University of Technology (Toyohashi, Japan) have developed a vision-based inspection system that is capable of detecting defects on highly specularly reflective curved surfaces.

Specular reflection is the reflection of light from a surface, in which light from a single incoming direction is reflected into a single outgoing direction. Specular reflection differs from diffuse reflection, where incoming light is reflected in a broad range of directions.

Inspecting objects with highly specular curved surfaces, such as those that are chrome plated, can prove a challenge for developers of vision systems. As such, the researchers claim, few vision systems have been built for the task.

To address the problem, the Japanese researchers developed an imaging system that illuminates a part from the side by four separate banks of white light LEDs. As a part is illuminated in turn by the four banks of LEDs, a CCD camera captures images of the part which sits directly beneath it.

Next, processing is performed on the images to remove the specular reflections and enhance any defects on the parts. After this, the defects are collated in a synthetic image that has been reconstructed from the other images. Noise is then removed from the synthetic image and the images of the defects are extracted from them using a combination of template matching and morphological techniques.

The researchers have shown that the system is capable of reliably detecting defects on many specularly reflective curved surfaces. In the future, they would like to increase the performance of the system as well as implement it on a production line.

A technical article describing the design of the system was recently published in the International Journal of Innovative Computing, Information and Control. Entitled "Developing a new automatic vision defect inspection system for curved surfaces with highly specular reflection," it can be found here.

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-- Dave Wilson, Senior Editor, Vision Systems Design