The goal:
- Only when the desired inspection characteristics or defects can be visualised with sufficient contrast can they be processed by the image processing software.
- Usually the object is illuminated by an external light source. This principle sounds very trivial, but in practice it turns out to be one of the main difficulties in industrial machine vision to make the defect in the object visible to the camera at all.
Direct frontlight illumination
Surface partly very bright but overexposed, partly dark due to total reflection.
Diffuse dom light illumination
Surface smoothly illuminated due to homogeneous incident light.
Dark field illumination
Lateral edges are very bright. Scratches and dust are visible.
Backlight illumination
Part appears as black silhouette only. No surface information, contours ideal for metrology applications.
Some examples to illustrate the difficulties
- magine a transparent glass bottle, the embossed text on the bottom of which has to be read. The test object and the features are made of the same material and, what is more, they are transparent! The same applies to many embossed fonts on flat material surfaces.
- Similarly, a scratch on a metallic surface often does not result in any further changes, but in a notch that needs to be detected. Again, a test situation where a defect must be detected despite the identical material. Embossing and moulding in materials are similar.
Diffuse incident light
Diffuse light causes matte surfaces with low contrast of stamped font
Dark field
Low angle illumination causes emphasis on the edges and instamped font
Electromagnetic radiation interacts with objects
The key role is played by light and its interaction in a chain of effects between lighting, test object and camera.
Since the physical properties of the test object cannot be influenced, or can only be influenced to a limited extent, the object determines the lighting and the camera! The camera in turn defines the optics with its working distance and connection thread, and its data format and data rate also determine the image capture map.
Conclusion:
It is worth taking a closer look at light and how it interacts with the material and the camera!
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