imaging resource guide imaging lenses filters cameras illumination targets
Bandpass Filter Transmission
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Figure 3a: Interference lters function
based on the distance that light
incident upon the lter travels. At the
correct angle of incidence, the light
waves incident on the lter destructively
interfere, disallowing them
from making it through the lter. At a
di erent angle, the destructive interference
is not as e ective, e ectively
changing the type of lter.
Applications with Machine Vision Filtering
When designing a machine vision system, it is important to enhance
the contrast of the inspected object’s features of interest. For an introduction
to contrast, see Section 2.3, Filtering provides a simple way to
enhance the contrast of the image while blocking out unwanted illumination.
There are many di erent ways lters can enhance contrast, and
the lter type is dependent on the application. Some common lters
used in machine vision are colored glass, interference, Neutral Density
(ND), and polarization.
Colored glass bandpass lters are some of the simplest lters available
for drastically improving image quality. These lters work incredibly
well at narrowing the waveband that is visible by the vision system,
and are often less expensive than comparable interference lters.
Colored glass lters work best when used to block out colors on the
opposite side of the color wheel (Figure 4).
Figure 4: Color wheel demonstrating that warm colors should be
used to lter out cool colors on the opposite side of the wheel.
(Continued from page 128)
Many types of hard coated lters exist, such as bandpass, longpass,
shortpass, and notch lters, each with a speci ed blocking range
and transmission range. Longpass lters are designed to block short
wavelengths and pass long wavelengths. Shortpass lters are the opposite,
passing shorter wavelengths and blocking longer. Bandpass
lters pass a band of wavelengths, while blocking longer and shorter
wavelengths. The inverse of a bandpass lter is a notch lter, which
blocks a band of wavelengths and passes the longer and shorter.
Transmission curve shapes for these lter types are shown in Figure 2.
Filters designed for deep blocking (high Optical Density) and steep
slopes (sharp transition from blocking to transmission) are used in applications
where precise light control is critical. Most machine vision
applications do not require this level of precision; typically, any lter
with an Optical Density (OD) of 4 or greater is more precise than
required and adds unnecessary cost.
Because hard coated lters utilize optical interference to achieve such
precise transmission and rejection bands, they introduce some di culties
when used in machine vision applications. All interference lters are designed
for a speci c Angle of Incidence (AOI), generally 0° unless speci
cally de ned otherwise. When used in machine vision, these lters are
generally placed in front of the lens; doing such causes the lter to accept
light coming from angles dictated by the angular eld of view of the lens.
Especially in the case of short focal length (large angular eld of view)
lenses, the light that is transmitted through the lter will often display an unwanted
e ect known as blue shift. For example, a 4,5 mm focal length lens
(wide angle) will have a much larger blue shift than a 50 mm focal length
lens (narrow angle). As the AOI on an interference lter increases, the optical
path length through the lter layers increases, which causes the cut-on
and cut-o wavelengths to decrease (Figure 3). Therefore, at di erent eld
points in the image, the lter will behave di erently by transmitting di erent
wavelength ranges: the farther out in the eld, the more pronounced
the blue shift. In most cases, interference lters can still provide better ltering
control over a colored glass lter, but be aware of the potential pitfalls
when using an interference lter with a wide-angle lens.
Figure 3b: An example of blue shift,
shown with a bandpass lter used at
a 15° angle of incidence. Note not
only the shift towards a lower center
wavelength, but the shallowing of
the slope as well. The dashed curve
is ideal, when the lter is used at a
0° angle of incidence.
Figure 2: Transmission curve examples of longpass
and shortpass (a) and bandpass and notch lters (b).