Illumination: Dark field backlight illumination
This technique uses a dark field light behind a transparent object to reveal surface features. It can be used on transparent targets such as glass or plastic that need to be checked for defects such as scratches, or to check the quality of embossed or raised lettering. By using this technique, any surface details are revealed as bright artefacts against a dark background.
This type of backlighting also makes it possible to selectively examine either the upper or lower surface of a transparent object.
The images produced with dark field backlighting benefit from a very high contrast which simplifies any subsequent image processing. The example shows a transparent bowl with heavy scratching on the inside surface.
This image shows how the scratches on the upper surface of the object have been successfully eliminated to reveal the embossed text on the lower surface very clearly. To make the task easier, a lens with a small depth of field was used. Software could now be used to remove the scratches using filter operations, but this would be CPU intensive and involve more processing time. Using this technique however allows features on either the upper or lower surfaces of a clear object to be imaged separately.