机器视觉中的滤波
Filtering in Machine Vision
There are many different types of filters in machine vision that can
be utilized to improve or change the image of the object under
inspection. It is important to understand the different technologies
behind the various types of filters in order to understand their
advantages and limitations. Although there is a wide variety of filters,
almost all can be divided into two primary categories: colored glass
filters and coated filters.
COLORED GLASS FILTERS
Colored glass filters are incredibly common in machine vision, and
are created by doping glass materials with elements that lectively change their absorption and transmission spectra. The dopants vary bad on which wavelengths are considered for transmission, and the manufacturing process is then nearly identical to standard optical glass manufacturing. Colored glass filters are advantageous for a couple different reasons: they are of relatively low cost when compared to interference filters and, more importantly, they do not demonstrate any shift in wavelength transmission when ud with wide angle lens or at an angle. However, colored glass filters also typically feature wide cut-on wavebands, do not have curves that are as sharp or accurate as coated interference filters, and do not have transmission throughput levels (percentages) as high as interference filters. Figure 1 shows the transmission curves for veral common colored glass filters. Note that the filters feature wide cut-on wavebands and have relatively shallow slopes describing their transmission functions.
Figure 1: Transmission curves for veral different colored glass filters
Infrared (IR) cutoff filters can be either colored glass filters or a type of coated filter that is uful for
both monochrome and color cameras in machine vision applications. Since the silicon nsors in most machine vision cameras are responsive to wavel engths up to approximately 1µm, any IR light incident on the nsor that may have been caud by overhead fluorescent lights or other unwanted sources can create inaccuracies on the nsor. On a color camera, IR light will create a fal color on the nsor that can
degrade overall color reproduction. For this reason, many color imaging cameras come standard with
IR-cut filter over the nsor. With monochrome cameras, the prence of IR light will degrade the contrast of the overall image.
There are a multitude of other types of colored glass filters. For instance, daylight blue filters can be ud for color balancing when polychromatic light sources and color nsors are ud.
COATED INTERFERENCE FILTERS
Coated filters typically offer sharper cut on and cut off transitions, higher transmissions, and better blocking then colored glass filters. In addition to colored glass filters, there are a range of coated filte
rs, they range from hard coated fluorescent filters to dichroic filters to polarization filters. Each coated filter undergoes a unique manufacturing process to ensure the proper performance. Wavelength-lective optical filters are manufactured by depositing dielectric layers on a specific substrate of alternating high and low indices of refraction. The surface quality and uniformity of the substrate establishes the baline optical quality for the filter, along with tting wavelength limits where the transmission of the substrate material falls off. The dielectric layers produce the detailed spectral structure of a filter by creating constructive and destructive interference across a range of wavelengths, as well as providing much sharper cut-off and cut-on bands when compared to colored glass filters.快速变白的方法
Many types of hard coated filters exist, such as bandpass, longpass, shortpass, and notch filters, each with a specified blocking range and transmission range. Longpass filters are designed to block short wavelengths and pass long
wavelengths. Shortpass filters are the opposite, passing shorter wavelengths and blocking longer. Bandpass filters pass a band of wavelengths while blocking longer and shorter wavelengths. The inver of a bandpass filter is a notch filter, which blocks a band of wavelengths and pass the longer and shorter. Transmission curve shapes for the filter types are shown in Figure 2.
friday怎么读>党组会议制度堵车路上Figure 2: Transmission curve examples of longpass and shortpass (a) and bandpass and notch filters (b).
Filters designed for deep blocking (high Optical Density) and steep slopes (sharp transition from blocking to transmission) are ud in applications where preci light control is critical. Most machine vision applications do not require this level of precision; typically, any filter with an Optical Density (OD) of 4 or greater is more preci than required and adds unnecessary cost.
Becau hard coated filters utilize optical interference to achieve such preci transmission and rejection bands, they introduce some difficulties when ud in machine vision applications. All interference filters are designed for a specific Angle of Incidence (AOI), generally 0° unless specifically defined otherwi. When ud in machine vision, the filters are generally placed in front of the lens; doing such caus the filter to accept light coming from angles dictated by the angular field of view of the lens. Especially in the ca of short focal length (large angular field of view) lens, the light that is transmitted through the filter
will often display an unwanted effect known as blue shift. For example, a 4.5mm focal length lens (wi
de angle) will have a much larger blue shift than a 50mm focal length lens (narrow angle). As the AOI on an interference filter increas, the optical path length through the filter layers increas, which caus the cut-on and cut-off wavelengths to decrea (Figure 3). Therefore, at different field points in the image, the filter will behave differently by transmitting different wavelength ranges: the farther out in the field, the more pronounced the blue shift. In most cas, interference filters can still provide better filtering control over a colored glass filter, but be aware of the potential pitfalls when using an interference filter with a wide-angle lens.
Figure 3a: Interference filters function bad on the distance that light incident upon the filter travels. At the correct angle of incidence, the light waves incident on the filter destructively interfere, disallowing them from making it through the filter. At a different angle, the destructive interference is not as effective, effectively changing the type of filter.
Figure 3b: An example of blue shift, shown with a bandpass filter ud at a 15° angle of incidence. Note not only the shift towards a lower center wavelength, but the shallowing of the slope as well. Th
e dashed curve is ideal, when the filter is ud at a 0° angle of incidence.
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, e our application note. Filtering provides a simple way to enhance the contrast of the image while blocking out unwanted illumination. There are many different ways filters can enhance contrast, and the filter type is dependent on the application. Some common filters ud in machine vision are colored glass, interference, Neutral Density (ND), and polarization.
Colored glass bandpass filters are some of the simplest filters available for drastically improving image quality. The filters work incredibly well at narrowing the waveband that is visible by the vision system, and are often less expensive than comparable interference filters. Colored glass filters work best when ud to block out colors on the opposite side of the color wheel (Figure 4).
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Figure 4: Color wheel demonstrating that warm colors should be ud to filter out cool colors on the opposite side of the wheel.
Color 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 parated 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%.
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%.
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In this particular example, minor fluctuations in ambient light, such as individuals walking past the system, can decrea 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 enclo the inspection system, the entire lighting scheme of the sy班主任励志寄语
stem 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 increa of nearly a factor of 10.
Figure 7: Capsules being viewed with a monochromatic camera and green colored glass filter yielding a contrast of 86.5%. Neutral Density Filters
Neutral density filters are ud 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 nsor. 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 caud by a harsh reflectio
n from an object, but the optical density decreas with radial distance away from the center of the filter.
为什么而学习Polarizing Filters
Polarization filters are another common type of filter ud in machine vision applications as they allow better imaging of specular objects. In order to properly u polarizing filters, it is important that both the light source and the lens have polarization filters on them. The 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 tup with a polarizer on the light source and an analyzer on the lens.
Figure 8: Images taken with no filter (a) showing high glare and with polarization filters (b) which reduce glare.
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, otherwi, 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 increa 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 ud for specific applications.
