Collimated and telecentric backlight in machine vision applications

 

The diffuse backlight lighting utilises the principle of the  Lambertian scatterer  to generate a constant radiance over the spatially extended lighting field. The light has no preferred direction of propagation and scatters evenly and homogenously in all directions. Although required for many applications, this can pose a problem during precise measurement work, for example. For spatially highly extended objects and round outer edges, non-directional lighting generates a partial shadow. Instead of a binary transition from white to black in the image, the result is a grey gradient over several pixels. This makes determination of the exact edge location more difficult.

To solve this problem, we can work with directed, homogeneous, backlight lighting. With this type of lighting, the beam angle of the light is reduced by using special optical films. This very nearly approximates collimation. The effect achieved with backlighting is similar to that obtained by telecentric lighting. The light rays from the lighting are significantly more directed and achieve a more precise image of object edges. Edge probing is consequently more exact than with diffuse backlight.

Video about collimated backlight

For deep objects and objects with convex edges in particular, this can optimise the determination of the exact edge location and improve the accuracy of the Machine Vision system. Collimated backlight lighting also has advantages when working with transparent and semi-transparent test objects. The directed rays of light are refracted more directly by edges and irregularities. These therefore appear as pronounced areas of darkness against a bright background. This simplifies the detection of scratches, inclusions, embossing and engravings in glass or plastics.

This effect can be enhanced by the use of telecentric backlight lighting. Collimation is achieved by using an optical system in front of a divergent source of light radiation. The beams of light from this telecentric lighting run parallel to one another.

Use of telecentric lighting also significantly enhances the depth of field and the telecentric region of a telecentric lens.

Video about telecentric backlight

By using a telecentric lens, exactly this parallel light – which runs perpendicular to the plane of the image sensor – is recorded and passed on to the camera. The result is a bright image. If an object is located between the lighting and the lens, this object appears in the image as a perfectly dark body. Edge transitions are clear and extend over only a few pixels from bright to dark. This makes edge probing both straightforward and highly accurate.

As with directed homogeneous backlight lighting, the outlines of transparent objects can be captured as sharp images by using telecentric lighting. Here, the bundle of parallel rays crosses only the perpendicular planes of a body. These appear bright in the image while all other areas are dark. The object’s outline is therefore dark in the image. A similar result is achieved with irregularities such as scratches or inclusions in transparent test objects.


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