Section 5.3: Telecentric Lens Size Control
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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 image space, and the exit pupil is the image of the aperture stop
into object space. See Figure 5.11 for illustrations of entrance and exit
pupils for a non-telecentric lens.
Exit Pupil Entrance Pupil
Object Space Image Space
Physical Aperture of System
Figure 5.11: A machine vision lens with entrance and exit pupils located
within the lens, which is typical of most Fixed Focal Length Lenses.
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. Telecentric Lenses can grow quite large and heavy with small
magnifications, as such magnifications require large front optics; 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 5.9 shows two different 4X telecentric lenses of different
f/#s. The lens at the top of Figure 5.9 shows a system that runs at f/6,
and the lens at the bottom runs at a much slower f/16.
If a particularly large field of view is required, Fixed Focal Length
Lenses can be augmented with additional optics in order to produce a
pseudo-telecentric lens for large fields of view. Figure 5.10 shows a 25 mm
Fixed Focal Length Lens that has been augmented with a Fresnel Lens
to make a telecentric lens that has an 8” field of view. This type of augmentation
always has lower image quality than a specifically designed
Telecentric Lens, but can be useful where cost-sensitive, large field of
view applications require telecentricity. This approach allows for minimal
flexibility with regards to working distance, as well as substantial
chromatic aberrations due to the use of a Fresnel Lens.
Figure 5.9: 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 5.10: A fixed focal length lens augmented with a Fresnel lens, making it pseudo-telecentric.
Entrance and Exit Pupils for a Non-Telecentric Lens