targets illumination cameras microscopy imaging lenses imaging resource guide 130 +44 (0)1904 788600 • EDMUND OPTICS® Filters
Consider the example shown in Figure 5, where gel capsules are being
inspected. As shown, two red capsules are on the outer sides of
a pair of green capsules and under a white light backlight. This is a
sorting application where the pills need to be separated by color to
reach their respective locations. Imaging the capsules with a monochrome
camera (Figure 6) provides a contrast between the green and
red capsules of only 8,7%, which is below the minimum advisable
contrast of 20%.
In this particular example, minor fluctuations in ambient light, such
as individuals walking past the system, can decrease the already low
contrast value of 8,7% enough so that the system is no longer capable
of operating properly. Several solutions to this problem exist: a bulky
and costly light baffling system can be built to completely enclose the
inspection system, the entire lighting scheme of the system can be
reworked, or a filter can be added to enhance the contrast between
the green and red pills. In this instance, the simplest and most cost
effective solution is to utilize a green colored glass filter in order to
improve the contrast between the two different colored capsules. As
shown in Figure 7, the contrast improves from 8,7% to 86,5%: an increase
of nearly a factor of 10.
Neutral Density Filters
Neutral density filters are used in certain applications where it is advantageous
to have additional control over the brightness of an image
without changing the exposure time or adjusting the f/#. Although
there are two primary types of neutral density filters (absorbing and
reflecting), their overall responsibility is the same: uniformly lower the
light that is transmitted through the lens and onto the sensor. For applications
like welding where the imager can be overloaded regardless
of the exposure time, neutral density filters can provide the necessary
drop in throughput without needing to change the f/# (which can impact
the resolution of the system). Specialty neutral density filters, like
apodizing filters, exist to help with hotspots in the center of an image
caused by a harsh reflection from an object, but the optical density decreases
with radial distance away from the center of the filter.
Polarization filters are another common type of filter used in machine
vision applications as they allow better imaging of specular objects.
In order to properly use polarizing filters, it is important that both
the light source and the lens have polarization filters on them. These
filters are called the polarizer and the analyzer, respectively. Figure 8
shows an example of how polarization filters can make a difference
when viewing specular objects. In the Figure 8a, a CCD imager is being
inspected with brightfield illumination and Figure 8b shows the
same illumination setup with a polarizer on the light source and an
analyzer on the lens.
As shown in Figure 8b, augmenting the system with polarizers provides
superior performance as the harsh reflections are absorbed by
the filter on the lens. To ensure the maximum extinction of unwanted
glare, the polarizer on the light source must be aligned with its polarization
axis 90° from the polarization axis of the polarizer on the lens,
otherwise, the lens will still transmit some of the harshly reflected
light into the system, causing glare.
It is critical to understand that filters exist to manipulate the contrast
of an image in order to help increase the accuracy of the imaging
system. Whether it is simple color filtering or polarization filtering,
each filter exists to solve a unique problem; it is important to understand
what filters should be used for specific applications.
Figure 5: Four liquid capsules under inspection with the same vision
system, shown here in color.
Figure 6: Capsules being viewed with a monochromatic camera,
yielding a contrast of 8,7%.
Figure 7: Capsules being viewed with a monochromatic camera and
green colored glass filter yielding a contrast of 86,5%.
Figure 8a Figure 8b
Figure 8: Images taken with no filter (a) showing high glare and with
polarization filters (b) which reduce glare.
More information on filters, including
technical content and transmission curves,
can be found on each TECHSPEC® filter’s product page