What is Telecentricity? | Telecentric vs. Conventional Lenses | Application Examples
The increased popularity of imaging technology over the last decade has spurred the demand for a wide variety of lenses that can provide optical designers with images suitable for all types of analysis. One such example is the telecentric lens, which is frequently used in the machine vision industry for measurement and alignment applications. In order to understand what makes telecentric lenses ideal for machine vision, it is important to look at what it means to be telecentric, compare telecentric lenses to conventional lenses, and examine the most common applications involving telecentricity.
WHAT IS TELECENTRICITY?
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Telecentricity is a unique property of certain multi-element lens designs in which the chief rays are collimated and parallel to the optical axis in image and/or object space. A key characteristic of telecentricity, then, is constant magnification regardless of image and/or object location. There are three classifications of telecentricity depending upon the optical space(s) in which the chief rays exhibit this behavior.
Classification 1: Object-Space Telecentricity
It occurs when the system stop is placed at the front focal plane of the lens, resulting in an entrance pupil location at infinity. A shift in the object plane does not affect image magnification.
Figure 1: 0.5X Object-Space Telecentric Lens (Note the Parallel Chief Rays in Object Space) [View Larger Image]
Classification 2: Image-Space Telecentricity
It occurs when the system stop is placed at the rear focal plane of the lens, resulting in an exit pupil location at infinity. A shift in the image plane does not affect image magnification.
Figure 2: 0.5X Image-Space Telecentric Lens (Note the Parallel Chief Rays in Image Space) [View Larger Image]
Classification 3: Double Telecentricity
Also known as bilateral telecentricity, it occurs when the system stop is placed at the common focal plane, resulting in both the entrance and exit pupils being located at infinity. Shifting either the image or object planes does not affect magnification given that double telecentric systems are afocal.
Figure 3: 0.9X Double Telecentric Lens (Note the Parallel Chief Rays in Both Image and Object Spaces) [View Larger Image]
