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Section 4.3: Telecentric Lens Size Control
f/6
f/16
Figure 4.9
Section 4.4:
Telecentric Design Topics – ADVANCED
In previous sections, the lenses discussed have been divided into two
different categories: Telecentric Lenses and non-telecentric lenses.
However, there are two types of telecentricity: object space and image
space telecentricity (which refer to the entrance pupil and exit
pupil locations, respectively). The pupils of an optical system are images
of the aperture stop; typically, machine vision lenses have embedded
aperture stops, with individual lens components surrounding
it on either side. The entrance pupil is the image of the aperture stop
into object space, and the exit pupil is the image of the aperture stop
into image space. See Figure 4.11 for illustrations of entrance and exit
pupils for a non-telecentric lens.
32 +44(0) 1904 788600 | Edmund Optics® targets Entrance and Exit Pupils for a Non-Telecentric Lens
Exit Pupil Entrance Pupil
Figure 4.11
Object Space Image Space
Physical Aperture of System
Figure 4.11: A machine vision lens with entrance and exit pupils located
within the lens, which is typical of most Fixed Focal Length Lenses.
Object Space Telecentricity
When a machine vision lens is described as simply being telecentric, it
is most likely object space telecentric. A lens is said to be object space
telecentric when the entrance pupil is located at infinity in image space
(everything behind the objective), as shown in Figure 4.12.
In an optical design, object space telecentricity is accomplished by
placing the aperture stop of the system at the focal point of the front
group. The entrance pupil at infinity explains why the field of view is
constant and non-angular. Since the chief ray, which defines the field
of view of a system, crosses the optical axis at the aperture stop and
also crosses at the entrance pupil, the ray will be parallel to the optical
axis when the pupil is at infinity, and the angular field of view is zero
(Figure 4.13 on page 33). By contrast, any entrance pupil located a finite
distance away from the lens will result in a non-parallel chief ray, as
shown traced in Figure 4.14 for a non-telecentric lens.
Lenses that are only object space telecentric have the ability to be
focusable since the back half of the lens can be designed with moving
elements, which is advantageous for systems that need the variability
of working distance. They can also be designed to be variable magnification
lenses, as the rear elements can be used to control magnification,
or the sensor plane can be moved for small magnification shifts.
Object space-only Telecentric Lenses are more easily capable of
hitting larger image sensor formats, even in more compact form factors,
as the rays in image space are allowed to diverge; the back of the
lens therefore does not need to exceed the sensor dimensions.
Object Space Telecentricity
Object Space Image Space
Entrance Pupil Located at
Infinity in Image Space
Exit Pupil
Embedded in System
Focal Length of Front System Half
Figure 4.12
Entrance Pupil Location
Entrance Pupil
Figure 4.14
Figure 4.14: The entrance pupil’s location is where the object space
chief ray crosses the optical axis, as shown with a non-telecentric lens.
The field of view of any Telecentric Lens is limited by the front optic
diameter; the larger the required field of view, the larger the front
optic diameter. Additionally, Telecentric lenses can grow quite large
and heavy with small magnifications, as such magnifications require
large front optics for light collection; this can be prohibitive in some
setups where weight or size is a concern. The f/# of a lens also plays
into the size of the lens, particularly at large magnifications where the
front optics also grow with the speed of the lens. Figure 4.9 shows two
different 4X telecentric lenses of different f/#s. The lens at the top of
Figure 4.9 shows a system that runs at f/6, and the lens at the bottom
runs at a much slower f/16.
Figure 4.9: Two 4X telecentric lenses, operating at f/6 (top) and f/16 (bottom), showing how f/# can partially dictate element size for telecentric lenses.
Figure 4.12: A lens which is object space telecentric, with the entrance
pupil projected to image-side infinity.