uu #11: Understand and define the fundamental
parameters of the imaging systems.
By narrowing down the specific parameters required for the imaging
system, the wide range of available lenses and sensors can be
reduced to a manageable selection. Fundamental parameters of an
imaging systems are a great place to start and are detailed in the
next section.
www.edmundoptics.eu/imaging 5
introduction fundamentals lens specifications real world performance telecentricity lens mechanics lens selection guide
u #4: Illuminate for the occasion.
For a lens and sensor to effectively work together, strong contrast
must be reproduced by properly lighting the object. The characteristics
of the object under inspection and the nature of any defects
must be understood so that the proper illumination geometry is used.
Remember that these lights can be very large (see Best Practice #1).
Learn more about illumination geometries on pages 155-160.
uu #5: Color matters.
The wavelength (color) selected for the illumination can have enormous
impact on system performance. In an application using highquality
optics and a top-of-the-line sensor, switching from broadband
to monochromatic illumination, or between specific wavelengths,
can significantly improve performance. As with Best Practice #4, the
proper choice of wavelength can make the difference between high
contrast and no contrast and can be the difference between system
success or failure. Learn how proper filtering techniques can impact system
performance on pages 124-126.
uu #6: High resolution and large depths of field
struggle to co-exist.
As shown in Section 3.4, maximizing resolution and depth of field
(DOF) requires the same variable—the f/#—to move in opposite directions.
It is impossible to have very high resolution over a large
DOF. To achieve this may require more elaborate solutions such as
using a liquid lens or multiple imaging systems.
uu #7: There is no single lens that can do everything.
As resolution requirements increase, the ability to decrease aberrations
(attributes of optical design that adversely affect performance)
becomes increasingly difficult over a wide range of operating parameters.
Even without budget constraints, there are physical limitations.
For this reason, a wide range of lens solutions for similar applications
are required. More details on lens performance are found in Section 3.
uu #8: Thoroughly understand the object to be
inspected.
The ability to produce the highest level of contrast for an inspected
object is the foundation for imaging. An understanding of the object’s
properties, such as its materials or finishes, is critical for success. Additionally,
it is not enough to just know what features are considered
good or bad. Rather, to guarantee high levels of reliability and repeatability,
the range of details to be inspected and the margins for good
and bad must be understood.
uu #9: Be a control freak.
The ability to control the environment into which the imaging system
is deployed can significantly affect the reliability and consistency of
results. Control also reduces the likelihood of unintended problems.
Whether using filters to increase contrast, baffles to eliminate unwanted
light from entering the system, or measurement devices to monitor
light sources for spectral stability, controlling the environment will
reduce unforeseen difficulties in the future. Some of these techniques
are extremely low-cost ways to protect and increase the performance
of an expensive imaging system.
uu #10: Speak up.
Do not be afraid to ask why something will or will not work. Suppliers
should be able to explain why components are or are not capable
of achieving the desired result. The answer will not always be
the same (e.g. the laws of physics or design deficiencies). However,
designers and manufacturers must be able to explain the capabilities
of their products.
Image Plane
Blue
Focal
Plane
Green
Focal
Plane
Red
Focal
Plane
#5: Depending on the wavelength(s) of light, there exists multiple
points of best focus. This image shows three points for three different
wavelengths.
#7: Comparision of two different lens sizes with significantly different
applications.
Telecentric Lens
Fixed Focal Length Lenses
/imaging