iJUNCO->Machine Vision Classic Case — Surface Scratch Detection Case

Surface inspection has always been a difficult point in the machine vision industry. For the detection of scratches and dirt on the surface of polished materials, common lighting methods include coaxial light, high angle, low angle, and backlight. (As shown below)

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The above methods generally have obvious effects for hard scratches, but the surface requirements of some workpieces are relatively high, and the effects for some soft scratches are often not very obvious. According to our daily work experience, most of the renderings in machine vision are presented through low or high angles. For scratches, we have proposed two solutions based on low and high angle methods.

Industry applications:

Glass, metal, LCD panel, mobile phone screen, plastic and other surface inspection.

Low angle mode:

The machine vision industry usually uses a low-angle method for scratch detection, as shown below:

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Before we detect a defect, we first need to understand how the defect is formed, and how it reminds and helps our work. The method of forming a scratch is basically because two objects are in direct contact with each other, and a relative displacement is caused by an external force, thereby causing a scratch. The above information shows that scratches are directional.

When considering the effect of low-angle lighting, if we use a group of rays parallel to the scratch to illuminate it (as shown in the left below), the scratch will be blurred by the light, and the effect is not obvious in the image; A beam of light perpendicular to the scratch shines on it, and the scratch will be highlighted by the light, and the effect is very obvious in the image (as shown in the bottom right).

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From the above analysis, we used eight strip-shaped light sources to time-expose the workpiece and designed the following lighting scheme (as shown below):

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A ring-shaped light source is divided into eight channels for control, time-sharing exposure, and eight consecutive acquisitions. Finally, a software algorithm is used to superimpose all defects, and finally detect the surface of the workpiece with high standards for scratches. (As shown below)

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High-angle method:

According to experience, under the effect of coaxial light with soft scratches, the higher the working distance of the light source, the more obvious the effect. However, the same light source, the higher the working distance of the light source, the smaller the light emitting surface of the light source, the light source brightness will be weakened accordingly, the effect and the actual situation can not have both.

Many times, our production line employees use fluorescent lighting when observing product surface information (see the figure below). People who observe carefully will find that the naked eye detection method is often to make the fluorescent lamp project into the product, and then swing the product to make the shadow of the fluorescent lamp move inside the product. In this way, the light spot illuminates a part of the product, thereby reflecting the information on the surface of the product, so that the human eye can clearly determine whether the product is defective, and weak defects can be easily seen.

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According to the above description, for the workpiece with specular reflection, according to the law of light reflection (as shown in the lower left figure), the lighting method in the lower right figure is used to form the specular reflection effect, which can well solve this problem.

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As one of the companies in the machine vision industry, iJUNCO provides various machine vision inspection solutions. This article mainly exemplifies the treatment method of surface scratch detection from the perspective of the lighting solution, but the scratch detection is actually only in visual inspection. It's just a small testing category. If you have any questions or needs, please feel free to contact us.