Advantages of Telecentric lenses in machine vision applications

Telecentric lenses are essential in eliminating distortion errors caused by standard lenses in machine vision applications.  

Telecentric lenses are defined by lenses only accepting incoming light ray bundles whose principal rays are parallel to the optical axis of the lens as seen in the diagram below

How do the advantages of telecentric lenses help in machine vision applications?  

Constant Magnification

Telecentric lenses can provide constant magnification, and not influenced by varying lens to object distances.  

Example: The image to the right shows 2 screws in which the 2nd screw is 10mm further away from screw 1 when viewed from the Telecentric lens in the setup.












Using a standard lens, screw #1 which is closer will appear to be larger














Using a Telecentric lens, the screws look the same size and not influenced by the lens to object distance.  

This is very helpful in applications to ensure accurate measurements negating any deviations from part placement.

Perspective Error

Standard lenses show significant perspective errors, whereas telecentric lenses are able to eliminate any perspective effects.  In cases where measurements are critical, using a telecentric lens is a must!

Low Distortion

Regular lenses have 1-2% distortion, whereas telecentric lenses have 0.1%

Regular lens will either have “Barrel” (left image) or “Pin cusion” distortion (right image) 






    Barrel Distortion                   Pin Cushion Distortion

In barrel distortion, image magnification decreases with distance from the optical axis. This is common in fish-eye lenses.  

In pincushion distortion, image magnification increases with the distance from the optical axis.  

Using telecentric lenses in machine vision applications will eliminate barrel and pin cusion distortion.  Low distortion is imperative where accuracy is required.  

How can it get any better!  – Use Collimated light!

Telecentric lenses provide a high level of accuracy, but when coupled with a collimated light source, edge definition is increased.

Diffused backlights can generate unwanted border effects and create a “fake” image of the edge of a part.  Collimated backlights allow only parallel rays to pass by objects.  This setup would be setup like the diagram below.  

   
The result is very crisp edge definition as seen in the images below.  The left image is using a standard backlight in which light “wraps” around the cap.  The right images use a collimated back light providing excellent edge definition.