Sunday, July 31, 2011

Telecentric Lenses with Collimated Light: Part I, Subtle Defect Detection

[Note to readers: this post from Guest Blogger Spencer Luster of Light Works is the first in a series that will be published every Monday for the next four weeks. Spencer is a lighting and optics guru and we’re very fortunate to have his contributions. Please also note that Spencer retains the copyright to his articles – just in case you were thinking of reposting!]

For several years, telecentric lenses have been standard products for machine vision. A bit less well used are collimated light sources, although many people are familiar with their advantages. These include sharper imaging, very high efficiency, and greatly reduced sensitivity to low-angle reflections from specular (shiny) surfaces. A telecentric lens and collimated back light are often the perfect combination for critical gauging applications.

In the next three articles we'll explore some practical aspects of using this combination, starting with a type of inspection you might not have considered.

Here we look at detecting subtle refractive defects or inhomogeneities in transparent material such as glass or plastic, or detecting small surface variations in highly specular, otherwise flat material. Such Schlieren-like inspection has been known for a long time, but it hasn’t been widely applied to machine vision.

Figure 1 shows a portion of a pair of plastic safety glasses that nominally have no optical power. It's imaged with a conventional lens and a diffuse back light. The only significant feature is a human hair deliberately placed in the field of view.

Figure 2 shows the same object, but this time imaged with a telecentric lens. The light source is still a diffuse back light. Notice that now many features appear, including scratches, dirt, and a long, irregularly curved line to the right of the hair. This is a defect called viscous thread. It’s a thread of plastic left over from previous molding that didn’t quite melt completely. Visually it is extremely faint and in fact this pair of safety glasses is commercially acceptable.

Finally, Figure 3 shows the same object imaged with the telecentric lens and a collimated back light. The viscous thread is much more prominent. We also see a region at the lower left where one surface of the plastic is mildly deformed or "prismed" over a large area. One can also observe a slight graininess over the whole field, the result of subtle surface texture.

The same results can be found when inspecting many other transparent materials. It's a powerful technique for detecting otherwise "invisible" defects or features.

Next post we'll look into aligning and adjusting collimated lights with telecentric lenses.

Spencer Luster

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