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Bandpass Filter Transmission
Section 8.2: Applications with Machine Vision Filtering
www.edmundoptics.eu/imaging 131
T (%)
80
60
40
20
0
490 495 500 505 510 515
(nm)
15°AOI
0° AOI
Thin Film
Layers
45°
Substrate
2x x
Figure 8.3a
Figure 8.3b
Shortpass Filter
Longpass Filter
Transmission
Wavelength
Figure 8.2a
Figure 8.2b
Notch Filter
Bandpass Filter
Transmission
Wavelength
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 diff erent ways fi lters can enhance contrast,
and the fi lter type is dependent on the application. Some common
fi lters used in machine vision are colored glass, interference, neutral
density (ND), and polarization.
Colored glass bandpass fi lters are some of the simplest fi lters available
for drastically improving image quality. These fi lters work incredibly
well at narrowing the waveband that is visible to the vision system,
and are often less expensive than comparable interference fi lters.
Colored glass fi lters work best when used to block out colors on the
opposite side of the color wheel (Figure 8.4).
Figure 8.4: Color wheel demonstrating that warm colors should be
used to fi lter out cool colors on the opposite side of the wheel.
(Continued from page 130)
Shortpass fi lters are the opposite, passing shorter wavelengths and
blocking long. Bandpass fi lters pass a band of wavelengths, while
blocking longer and shorter wavelengths that lie outside the passband.
The inverse of a bandpass fi lter is a notch fi lter, which blocks a
band of wavelengths and passes the longer and shorter. Transmission
curve shapes for these fi lter types are shown in Figure 8,2.
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
fi lter with an optical density (OD) of 4 or greater is more precise than
required and adds unnecessary cost.
Because hard coated fi lters utilize optical interference to achieve
such precise transmission and rejection bands, they introduce diffi
culties when used in machine vision applications. All interference
fi lters are designed for a specifi c angle of incidence (AOI), generally
0° unless specifi cally designated otherwise. When used in machine
vision, these fi lters are generally placed in front of the lens; doing
such causes the fi lter to accept light coming from angles dictated by
the angular fi eld of view (AFOV) of the lens. Especially in the case
of short focal length (large AFOV) lenses, the light that is transmitted
through the fi lter will often display an unwanted eff ect known as
blue shift. For example, a wide-angle 4,5 mm focal length lens will
have a much larger blue shift than a narrow angle 50 mm focal length
lens. As the AOI on an interference fi lter increases, the optical path
length through the fi lter layers increases, which causes the cut-on and
cut-off wavelengths to decrease (Figure 8.3). Therefore, at diff erent
fi eld points in the image, the fi lter will behave diff erently by transmitting
diff erent wavelength ranges: the farther out in the fi eld, the more
pronounced the blue shift. In most cases, interference fi lters can still
provide better fi ltering control over a colored glass fi lter, but be aware
of the potential pitfalls when using an interference fi lter with a wideangle
lens.
Figure 8.3a: Interference fi lters function based on the distance that
light incident upon the fi lter travels. At the correct AOI, the light waves
incident on the fi lter destructively interfere, stopping them from passing
through the fi lter. At a diff erent angle, the destructive interference
is not as eff ective.
Figure 8.3b: An example of blue shift, shown with a bandpass fi lter
used at a 15˚ AOI. 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 fi lter is used at a 0˚ AOI.
Figure 8.2: Transmission curve examples of longpass and shortpass
(a) and bandpass and notch fi lters (b).
Warm Cool
R V
O
Y
B
G
Figure 8.4
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