TECHSPEC® Telecentric Lenses can be found on pages 88-111
or www.edmundoptics.eu/telecentrics
Section 4.1: The Advantages of Telecentricity
Figure 4.1
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 WD shift, it illustrates the
importance of minimizing parallax error. Many automated inspection
tasks are imaging objects that move through the FOV of an imaging
system, and the position of parts is rarely perfectly repeatable. If the
WD 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.eu/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 (AFOVs) such that the
distance between the lens and object increases and the magnification decreases.
This is how human vision behaves, and contributes to our depth
perception. This AFOV 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
DOF) due to the magnification change. Telecentric lenses eliminate the
parallax error characteristic of conventional lenses by having a constant,
non-angular field of view (FOV); at any distance from the lens, a telecentric
lens will always have the same FOV. See Figure 4.1 for the difference between
a non-telecentric and a telecentric FOV.
A telecentric lens’s constant FOV 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 (WDs),
imaged both by a fixed focal length (non-telecentric) lens (center) and
Figure 4.1: FOV comparison of a conventional and telecentric lens. Note the conventional lens’s AFOV and the telecentric lens’s zero-angle AFOV.
Figure 4.2: The AFOV 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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