STEMMER IMAGING

Illumination: Dark field illumination

Illumination mainly used to highlight surface defects, scratches or engraving.

This technique is mainly used to highlight surface defects, scratches or engraving. Dark field illumination usually uses a low angle ring light that is mounted very close to the object. Dark field is the opposite of bright field because most of the light reflected from the surface of the target will fall outside the FOV of the camera with the camera only "seeing" scattered light that is reflected by a defect on the surface.

 

Dark field illumination are normally flat ring lights that must be mounted very close to the test object. Unlike bright field lights, most of the light is reflected away from the camera.

 

The image of the coin shows the type of the effect that can be obtained when using dark field lighting.

 

The sequence of images shows the principle of dark field illumination when detecting surface scratches or defects on flat objects.

 

On this flat, unmarked surface there is a low angle of incidence, and virtually all of the light is reflected out of the field of view of the camera. The resultant image would be very dark with no distinctive features implying a defective object.

 

Here one can see that when surface features are present, the light hits them and is scattered in all directions. Some of this scattered light will enter the camera to reveal the scratch or feature.

 

As this detail shows, light hitting a sharp edge will disperse widely

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Learn more:

• Illumination
• Suitable illumination technologies and Important factors affecting lighting