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Common lighting techniques

ALLED / 2011-12-26

The effectiveness of a illuminating source in an inspection is determined by the direction at which light strikes an object, and the direction of the reflected light into or away from the camera.  Suitable illumination covers the required field of view, creates a consistently measurable degree of contrast, and does not cause reflected glare. Different light types are required to provide successful illumination for the broad range of products being inspected by automated systems.

Bright Field:
Light is aimed directly at an object, often creating distinct shadows. This type of lighting is effective when used on objects requiring high degrees of contrast, but creates specular reflections when used with shiny or reflective materials.

Dark Field:
Light is projected at an angle to the surface, causing any variations to deflect light up into the camera, creating bright spots on a dark background or field. Nothing is seen by the vision system if there are no aberrations on the surface.

Back Lighting:
An even field of illumination is projected from behind an object, which is seen as a silhouette by the camera. Backlighting is most commonly used for taking measurements or determining part orientation.

Diffuse Light:
Reflected light, providing a non-directional, soft illumination free of harsh shadows that is well suited for highly specular objects. This illumination effect is similar to the type of light found on an overcast day.

Co-Axial Illumination:
A variation of diffuse light in which light is aimed at an angled beam splitter that reflects the light down.  The object is viewed from above through the beam splitter. This light type is particularly helpful on highly reflective objects or in situations where the area of inspection is obscured by shadows from its surroundings.

Each light type has a specific purpose, but is also adaptable for a range of applications not immediately related to its function. For example, a spotlight, which provides bright field illumination, can be placed at an angle to create a dark field effect. For some applications, the best results are achieved by combining multiple light types.

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