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.
Ray diagram of a dark field backlight illumination: 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.
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.