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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, a machine vision lens has an embedded
aperture stop with individual lens components on both sides.
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 nontelecentric
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 FOV is constant
and non-angular. Since the chief ray, which defines the FOV 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 AFOV 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 WD. 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 FOV of any telecentric lens is limited by the front optic diameter;
the larger the required FOV, 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.