Shims follow the same basic concepts as spacers, but are used for
fixed magnification lenses such as telecentric lenses. Shims are very
thin (0,025 - 1,0 mm) stainless steel spacers used to control working
distance with fine precision in order to guarantee the best image quality
resource possible for a given setup.
imaging lenses imaging filters microscopy cameras illumination targets 44 +44 (0)1904 788600 • EDMUND OPTICS® Flange distance is defined as the ideal distance from the shoulder
of the camera to the sensor plane and can vary slightly from the nominal
design distance due to tolerancing. This departure from nominal is
due to variations in housing design and sensor placement inside of a
camera. An exact or custom distance may be required to control one
of the following: working distance, image quality, or system to system
repeatability. Shims act as thin spacers that can be placed between
the lens and camera to customize this distance.
As image distance varies, image quality can also vary. If the image
distance is shifted too far from the ideal design, a noticeable blur or
degradation in MTF can occur. For example, this may occur when
switching a lens between different cameras - even if the same model
of camera and lens are being used, there may still be small variations
from one camera to the next that may cause working distance or image
quality changes as each new system is assembled. In this case, 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, which is why many
telecentric lenses come with shims included. With critical optical parameters
dialed in 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, or the ability to get a certain
view of an object under test. Often times these applications include
other challenges such as limited working distance range due to swinging
robotic arms, contaminations nearby, or limited space due to an
existing mechanical layout of a machine into which the new measurement
system must be fit. Similar to using lens spacers, adding or
removing shims at the rear of a telecentric lens takes advantage of
the relationship between image distance, object distance, and focal
length (Figure 7.1) and adjusts the working distance slightly into a usable
range. In monochromatic applications, shims can also be used to
compensate or refocus for chromatic focal shift (see Section 6). Shims
give users precision 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 similar to a
lens spacer in that they are both placed in between the back of the lens
and the camera. A focal length extender, however, will not change the
working distance range where the lens will work; focal length extenders
contain a negative set of elements that change the focal length of the
machine vision lens by a multiplicative factor. For example, 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 have half of the field of view
it originally had at the same working distance ranges.
Another useful advantage of focal length extenders is that they can
be stacked upon one another, and have a multiplicative effect on the
focal length of a lens. For example, a 25 mm focal length lens used
with two focal length extenders of 1,5X and 2X will have a new focal
length of 75 mm, as 75 is the product of 25, 1,5, and 2.
Much in the same way that the use of spacers does not come without
a compromise, potential degradation of image quality should be
considered when using focal length extenders. Because the individual
lens elements in an objective have all been specifically designed and
engineered to balance each other out in terms of optical performance,
adding an additional negative element into the optical train will reduce
that performance by introducing additional optical aberrations that the
lens was not designed to balance out. Focal length extenders also reduce
the amount of light throughput in a lens by changing the f/#.
For instance, a focal length extender of 2X will decrease light throughput
by a factor of four. The potential negative effects on image quality
should be considered before implementing a focal length extender.
of Imaging Lenses
Imaging lenses used in many industrial machine vision applications
have special requirements beyond those of standard imagining lenses.
The lenses used in factory automation, robotics, and industrial inspection
have to be able to work in specific and demanding environments,
which could involve vibrations, shocks, temperature changes, and contaminants.
Because of these environmental requirements, new classes
of ruggedized lenses are being designed specifically to work in a multitude
of different scenarios and therefore create different types of ruggedization.
There are three distinct types of ruggedization available:
1. Industrial Ruggedization
2. Ingress Protection Ruggedization
3. Stability Ruggedization
Industrial Ruggedized lenses are designed to survive vibration and
shock without damage to the lens or change in focus and f/#. To
achieve this, flexibility is sacrificed by eliminating moving parts and
making them easier to lock down.
A standard fixed focal length lens utilizes a complicated iris and
focus mechanism. A typical iris is comprised of thin leaves and ball
detents for adjusting f/#, which can spring out of place during shock
and vibration. For Industrial Ruggedization, the iris is removed and
replaced with a fixed aperture stop. Additionally, the typical focusing
mechanism of a threaded barrel within another threaded barrel is
replaced by a single thread and rigid locking mechanism.
Double Threaded Focus Ball Detents
Multiple Leaf Iris
Single Threaded Focus
Figure 7.4: Standard Lens with complex mechanics and an adjustable
iris vs. Industrial Ruggedized Lens with simplified mechanics.
Industrial Ruggedization is ideal for applications where the system will
be set up once and not changed afterwards. An added cost advantage is
also present in this type of lens due to the removal of the complex movements
and adjustments, which results in a significant part reduction and
cost savings. There are many applications for Industrial Ruggedization,
such as high vibration factory environments, situations where the camera
is rapidly accelerated and decelerated, inspection systems where many
similar camera setups are repeated, and robotic vision.
Ingress Protection Ruggedization
Ingress Protection Ruggedization ensures that a lens assembly is sealed
to prevent moisture and foreign debris from entering the lens through
the use of O-rings and RTV silicone. This protection is typically added
to an Industrial Ruggedized lens, since an adjustable focus and iris
would be problematic for sealing. These lenses are used in environments
of high humidity/moisture, sputter, dust, or small particles, and
also where space is not available to fully enclose the lens and camera.
Figure 7.5 O-ring Seal
Figure 7.5: Ingress Protection Ruggedized Lens with an O-ring to
seal out contaminants.