TECHSPEC® Telecentric Lenses can be found on pages 78-101
or www.edmundoptics.co.uk/telecentrics
Section 4.1: The Advantages of Telecentricity
Figure 4.1
(center) and a Telecentric Lens (right). Note that in the image taken
with a Telecentric Lens, it is impossible to tell which object is in front
of the other. With the Fixed Focal Length Lens, it is quite obvious that
the object that appears smaller is positioned farther from the lens.
While Figure 4.2 is drastic in terms of a working distance shift, it
illustrates the importance of minimizing parallax error. Many automated
inspection tasks are imaging objects that move through the field of
view of an imaging system, and the position of parts is rarely perfectly
repeatable. If the working distance is not identical for each object that
the lens is imaging, the measurement of each object will vary due to
the magnification shift (see Section 2.3 on magnification and how it is
defined, page 12). A machine vision system that outputs different results
based on a magnification calibration error (which is unavoidable
with a Fixed Focal Length Lens) is a non-reliable solution and cannot
be used when high precision is necessary. Telecentric Lenses eliminate
the concern about measurement errors that would otherwise occur
due to factors such as a vibrating conveyor or inexact part locations.
Conventional Lens
Telecentric Lens
Figure 4.2 Setup Fixed Focal Length Lens Telecentric Lens
www.edmundoptics.co.uk/imaging 29
introduction fundamentals lens specifications real world performance telecentricity lens mechanics lens selection guide
Section 4: Telecentricity and Perspective Error
The ability to quickly perform repeatable, high-accuracy measurements
is critical to maximize the performance of many machine vision systems.
For such systems, a telecentric lens allows the highest
possible accuracy to be obtained. This section discusses the
unique performance characteristics of Telecentric Lenses
and how telecentricity can impact system performance.
Zero Angular Field of View: Parallax Error Elimination
Conventional lenses have angular fields of view such that as the distance
between the lens and object increases, the magnification decreases. This is
how human vision behaves, and contributes to our depth perception. This
angular field of view results in parallax, also known as perspective error,
which decreases accuracy, as the observed measurement of the vision
system will change if the object is moved (even when remaining within
the depth of field) due to the magnification change. Telecentric Lenses
eliminate the parallax error characteristic of conventional lenses by having
a constant, non-angular field of view; at any distance from the lens, a
Telecentric Lens will always have the same field of view. See Figure 4.1 for
the difference between a non-telecentric and a telecentric field of view.
A Telecentric Lens’s constant field of view has both benefits and
constraints for gauging applications. The primary advantage of a Telecentric
Lens is that its magnification does not change with respect to
depth. Figure 4.2 shows two different objects at different working distances,
imaged both by a Fixed Focal Length (non-telecentric) Lens
Figure 4.1: Field of view comparison of a conventional and Telecentric Lens. Note the conventional lens’s angular field of view and the
Telecentric Lens’s zero angle field of view.
Figure 4.2: The angular field of view of the Fixed Focal Length Lens translates to parallax error in the image and causes the two cubes to
appear to be different sizes.
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