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introduction fundamentals lens specifications real world performance telecentricity lens mechanics lens selection guide
more than doubles. This type of spacer also has advantages for spaceconstrained
systems as the total track length (length from the image
plane to the object plane) is reduced.
It is also important to consider the performance impact spacers
have on the optical system. The WD range over which the lens physically
operates before adding the spacers is generally where the best
performance is, based on the optical design, so performance will
typically suffer as these distances are altered with spacers. Figures
5.3 and 5.4 show the MTF curves for the lens from the example
above at f/4 and on a 2/3” sensor, at the minimum WD and with the
11mm spacer, respectively. As a rule of thumb, a spacer should not
be used if it is more than half of the focal length. If used correctly,
spacers can be an excellent way to adapt a lens to a specific applica-
WD
f f
d
Figure 5.2
Object Image
Figure 5.2: An illustration of the relationship between image
distance, WD, and focal length.
No Spacer 11 mm spacer
Focal Length 35 mm 35 mm
Lens Length 41 mm 52 mm
Image Distance 42,9 mm 53,9 mm
Working Distance 165 mm 74,1 mm
Total Track 223,5 mm 143,6 mm
Magnification 0,22X 0,54X
Field of View (1/2") 28,5 mm 11,88 mm
Table 5.1: Comparison of specifications of the same 35mm focal
length lens (focused at minimum WD) with and without a spacer.
35mm MTF With No Spacer
TS 5.50mm
TS 4.00mm
TS 0.00mm
Diff. Limit
0 50 100
Spatial Frequency in Cycles per mm
Contrast (%)
100
80
60
40
20
0
Figure 5.3: 35 mm focal length lens at the minimum designed WD.
35mm MTF With 11mm Spacer
TS 5.50mm
TS 4.00mm
TS 0.00mm
Diff. Limit
0
0 50 100
Spatial Frequency in Cycles per mm
Contrast (%)
100
80
60
40
20
Figure 5.4: 35 mm focal length lens with 11 mm spacer.
tion, as long as the limitations and performance impacts are fully
considered. Making illumination monochromatic will help mitigate
these performance issues. Using a lens within its design range is
the best option for optimal performance. Longer focal length lenses
tend to respond better to spacers, as they are often simpler designs
compared to shorter focal length lenses.
Shims
Shims follow the same basic concept as spacers but are used for
fixed magnification lenses such as telecentric lenses. Shims are thin
(0,025 - 1,0 mm) stainless-steel spacers used to control WD with fine
precision to guarantee the best image quality. Flange distance can
vary slightly from the nominal design because of tolerancing in the
housing design or the sensor placement; as such, shims may be used
as thin spacers between the lens and camera to customize and correct
for this deviation.
As image distance varies, so can image quality. If image distance is
shifted too far from the ideal design, a noticeable blur or degradation
in MTF can occur. This may occur when switching a lens between
different cameras—even if using the same camera and lens model as
there may be small variations from one component to the next. Minor
adjustments can be made using shims to bring the MTF and focus
back to optimum levels. During the setup of each new system or line,
slight adjustments to the image distance may be required. Therefore,
shims are often included with many telecentric lenses. With critical
optical parameters set for each system, software thresholds and calibration
procedures will be repeatable from one system to the next.
Telecentric lenses are often chosen for applications requiring demanding
manufacturing measurements. Oftentimes, these applications
include other challenges, such as limited WD ranges due to
swinging robotic arms, contaminations nearby, or the existing mechanical
layout of a machine into which the new measurement system
must be fit. Like spacers, adding or removing shims at the rear of
a telecentric lens takes advantage of the relationship between image
distance, WD, and focal length (Figure 5.2) and adjusts the WD slightly
into a usable range. In monochromatic applications, shims can also be
used to compensate or refocus for chromatic focal shift (see Section
3.5). Shims give users precise control of image distance to achieve the
best possible solution for their application.
Focal Length Extenders/Multipliers
Another way to increase the magnification of a machine vision system
is by using a focal length extender. A focal length extender is like a
spacer in that they are both placed in between the back of the lens
and the camera. However, a focal length extender will not change the
WD range. Focal length extenders contain a negative set of elements
that change the focal length of a lens by a multiplicative factor. A
25 mm focal length lens with a focal length extender of 2X will have
an effective focal length of 50 mm and will therefore halve the FOV at
the same WD range.
Focal length extenders can also be stacked upon one another and will
have a multiplicative effect on the focal length of a lens. A lens with a
25 mm focal length used with two focal length extenders of 1,5X and
2X will have a new focal length of 75 mm.
In the same way spacers do not come without compromise, potential
image quality reduction should be considered when using focal length
extenders. Because the individual lens elements in an objective have
all been collectively designed to balance optical performance, adding
an additional negative element into the optical train will reduce performance
by introducing additional optical aberrations. Focal length
extenders also reduce the amount of light throughput in a lens by
changing the f/#. A focal length extender of 2X will decrease light
throughput by a factor of four. Effects on image quality must be considered
before implementing a focal length extender.
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