Telecentric Lenses

Telecentric lenses are designed to have a constant magnification regardless of the object's distance or location in the field of view. This attribute is ideal for many machine vision measurement applications, as measurements of an object's dimension will be independent of where it is located.

Types of Telecentric Lenses
In order to achieve a telecentric lens design, all of the chief rays (rays from an off-axis point that pass through the center of the aperture stop) have to be parallel to the optical axis in either image space or object space, or both. For an object space telecentric lens, the chief rays will be parallel to the optic axis on the object's side of the lens (object space). This is accomplished by setting the aperture stop at the front focal plane of the lens, which results in an entrance pupil at infinity. Since chief rays are directed towards the center of the entrance pupil, the chief rays on the object side of the lens will be parallel to the optical axis. An example of an object space telecentric lens is shown in Figure 1.

For an image space telecentric lens, the chief rays will be parallel to the optical axis on the image's side of the lens (image space). This is accomplished by setting the aperture stop at the back focal plane of the lens, which results in an exit pupil at infinity. Since the chief rays must pass through the center of the exit pupil, the chief rays on the image side of the lens must be parallel to the optical axis. For a double telecentric, or bi-telecentric, lens, the front and back focal planes are made to overlap so that the aperture stop is located where both the entrance and exit pupils are at infinity. In a bi-telecentric lens, neither the image nor object location will affect the magnification. Figure 2 shows an example ray trace through a bi-telecentric lens and illustrates how the chief rays pass through the system.

Conventional Lenses
In conventional lenses, the entrance and exit pupils are not located at infinity, so the chief rays will not be parallel to the optical axis. In this case, the magnification of the object depends on its distance from the lens and its position in the field of view. Figure 3 shows an example ray trace through a conventional camera lens; notice how the chief rays are angled with respect to the optical axis in both image and object space. Note that both Figures 2 and 3 feature the same lens design; only the aperture stop location was varied to shift the telecentric system to a non-telecentric one.

Example Images
Figures 4 and 5 show photographs taken with a telecentric lens and a standard camera lens, respectively. With the telecentric imaging system (Fig. 4), the height of the two screws appears to be the same, even though the object planes are separated by approximately 45 mm along the optical axis. With the conventional imaging system (Fig. 5), the two screws appear to be different heights, and therefore a machine vision system based on this lens will lead to incorrect dimensional measurements.

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Figure 4: An image of two identical 8-32 cap screws imaged with our previous generation MVTC23013 0.128X Telecentric Lens. Although the screws appear to be the same size and in the same object plane, they are actually separated by a distance of approximately 45 mm along the optical axis.

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Figure 5: The same two screws photographed with our MVL7000 conventional zoom lens. In this image, the perspective error due to the separation of the screws would lead to incorrect height measurements.