
Illumination
Dark field illumination
Low light incidence angle
At first glance, images taken with darkfield illumination appear unfamiliar. The light is incident at a low angle. According to the principle of angle of incidence = angle of reflection, all the light is deflected away from the observer (camera), so the field remains dark.
Bevelled edges, scratches, embossing, depressions and elevations disturb the path of the light. At these anomalies, the light is reflected towards the camera, or usually just scattered. These defects then appear bright in the camera image.

Slightly oblique illumination
The scheme shown here shows the smooth transition from bright field to dark field illumination. The surface is still brightened by scattered light effects on the slightly structured surface to such an extent that the component is easily recognisable. Bevelled edges and notches, however, are already brighter than the other image areas, as is typical for dark field.
Oblique light coming from the side
Surface slightly lightened by scattered light, deep edges of the notch lettering and slanted contours lightened.

Working principle
Edges facing away from the light appear dark, edges facing the light appear light.

Very low incident light
The scheme shown here illustrates the effect of a very strong dark field. The light falls extremely flat on the surface. Even small scratches, imperfections, dust grains and even fingerprints on polished surfaces can be clearly seen, while the surface appears very dark.
Low angle dark field
Surface completely dark, brightened edges of engraving and metal edges. Dust and lint particles visible.

Working principle dark field
Edges facing away from the light appear dark, edges facing the light appear light.

Highly effective dark field illumination must be positioned very close to the surface. The maximum working distance is 0.5 to 2 cm. At greater distances the surface is no longer illuminated.

Hard disk with dark field
Very low incident light produces a strong emphasis on the edges. Fingerprints and dust visible.

Dark field circuit board

Connector with pins

Slightly low angle dark field

Extreme dark field
Important for Machine Vision
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In general, a small working distance is required for dark field illumination.
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The larger the diameter of the illumination, the more directional the light will appear, as the amount of stray light decreases with distance. For an object with a diameter of 50 mm, a illumination with a diameter of 150 mm can be used.
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Stray light can be further filtered by artificial shading using a pinhole aperture/plate with a cut-out directly above the object.
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Extremely directional dark field illumination can be generated using line lasers, too, which incide on the object from one, two, or several sides. For this purpose, the laser is defocused in order to form line a few millimetres wide which is directed on the object at a shallow angle.
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