Understanding the growing list of polarization applications requires a discussion of the latest polarization cameras and sensors and an understanding of the benefits and limitations associated with the use of polarization technology.

Dichroic, thin-film, and wire-grid polarizers represent the most common components used in machine vision to linearly polarize light. Wire-grid polarizers specifically can withstand the power levels of lasers, which can be useful in factory and scientific environments.

Focusing a Gaussian Beam to a Spot. In many applications, such as laser materials processing or surgery, it is highly important to focus a laser beam down to ...

The focus limit switch tells the camera where to focus within certain ranges in distance. For example, if you set the switch to 6.5m-∞ then the camera knows not to worry about focussing on anything closer than 6.5 meters. Knowing this information, your camera will shoot faster.

Dechow is a regular speaker at conferences and seminars worldwide, and has had numerous articles on machine vision technology and integration published in trade journals and magazines. He has been a key educator in the industry and has participated in the training of hundreds of machine vision engineers as an instructor with the AIA Certified Vision Professional program.

Widelens

Polarization filters function by selecting or restricting light waves traveling in a single plane. The orientation of the filter determines the orientation of the light waves that can pass through the filter, thereby selecting one set of light waves to advance and preventing others from continuing (Figure 2).

Take, for example, parts on a conveyor belt rapidly passing underneath a camera for inspection. It’s impossible to achieve perfect, replicable results during parts inspection, even with polarized light and filter working in concert, because there will likely be variances in the position of objects relative to the camera, which affects the angles at which polarized light reflects off the surface of the object. However, a new technology and new components becoming readily available overcome the limitation.

Polarization, a filtering technique used for decades by photographers for image enhancement, also sees widespread use in commercial applications, including machine vision image acquisition. With polarizing cameras and imaging components increasingly entering the vision application mainstream since 2018, new applications and uses for polarization in machine vision continue to emerge.

The polarizers used by photographers rotate, to allow the photographer to attune the polarizer to the precise angle at which the light is being polarized by the surface of the subject in the scene. The photographer tunes the angle of polarization as needed for different scenes, not a practical technique for automated imaging in machine vision.

However you do need to make sure your shutter speed is longer than the focal length of the lens. For example, when you zoom your lens out to 200mm, make sure the shutter speed is at minimum 1/200th of a second or faster. If you zoom it to 300mm focal length, then the shutter speed needs to be at least 1/300th of a second and so forth. This will give you the best chance of getting a sharp image while hand holding your camera.

New technologies and components A polarizing image sensor makes a polarization filter part of the image sensor, versus a separate component placed over the sensor. The Photron Crysta by Photron (San Diego, CA, USA; www.photron.com) and PolarCam from 4D Technology (Tucson, AZ, USA; www.4dtechnology.com), to cite two examples, have been in the marketplace for some time and make use of on-sensor polarization. The IMX250MZR/MYR (monochrome/color) Polarsens sensor from Sony (Tokyo, Japan; www.sony.com) represents a recent implementation of on-sensor polarization technology. The polarization elements are fabricated right into the silicon, under the microlens (Figure 4), which makes the sensor unique. In the color sensor, color filters are located directly underneath the polarizers. The polarizers in Sony’s sensor, a microscopic implementation of a wire-grid polarizer over every lens on the sensor, have 0°, 45°, 90°, and 135° polarization angles in four-pixel groups. The groups are interpolated, reducing the sensor’s overall functional resolution by 4x. In other words, each four-pixel block equates to one pixel of output.By interpolating the images from each four-pixel block, the sensor gathers considerable information about the linear polarization occurring on the surface of the object being viewed. These interpolated pixels allow a very accurate definition of the exact direction and intensity of the polarization within each four-pixel group.

Polarization - Definition of Concepts, Techniques, and TechnologiesPolarization, a filtering technique used for decades by photographers for image enhancement, also sees widespread use in commercial applications, including machine vision image acquisition. With polarizing cameras and imaging components increasingly entering the vision application mainstream since 2018, new applications and uses for polarization in machine vision continue to emerge. Understanding the growing list of polarization applications requires a discussion of the latest polarization cameras and sensors and an understanding of the benefits and limitations associated with the use of polarization technology.Polarization concepts Light is a transverse wave, i.e. its electromagnetic and magnetic fields are disturbed at right angles relative to the direction of travel of the light waves. Or, it can be said that light waves oscillate perpendicular to their direction of travel. Natural light, and virtually all artificial light (LEDs, incandescent lights, fluorescent lights, etc.) is unpolarized or weakly polarized. Natural light travels in any radial direction from the source of the light (Figure 1). Imagine a beam of light. Light waves oscillate 360° from every point along that beam. (An oversimplification, to illustrate the concept.) Polarized light, on the other hand, is light in which the waves travel in only one, specific direction. Light can be polarized in nature by absorption, refraction, reflection, scattering, and birefringence (double refraction). For example, when light strikes water, it can be reflected linearly perpendicular, i.e. polarized in that specific direction, to the surface of water, which we experience as glare. For another example, as the sun moves across the sky the angle of the sun’s light striking a window will change. At some point the light will reflect off the window, or be polarized, at an angle perceived as glare. The sky is blue because sunlight strikes the molecular structure of the atmosphere and scatters, which polarizes the light in a specific direction. As the angle of the sun relative to the atmosphere changes the polarization angle of the light also changes, and the human eye perceives color changes from dawn to midday to dusk.

Primelens

Zoom lens are used for instances where you want to fill the frame with a subject that is far in distance and you wish it to appear much closer. Good examples would be sports or animal photography where you cannot be physically close to the subject. Or maybe it is possible but you’d rather not disturb the subject, choosing instead to photograph it naturally. This is particularly useful for street photography where you don’t want the person to know they are being photographed until after the shot is taken.

Applications and implementation Common polarization techniques in machine vision include inspection of glass and highly reflective materials. Raw and machined metal have grain at a microscopic level that can lead to linear polarization, making the use of a polarization filter useful. Plastic and glass, automotive, packaged materials, semiconductor, and LCD inspection applications commonly make use of polarization filters. The proper way to employ polarization in an inspection environment typically involves polarizing both the light source and the reflected light (Figure 3) with a polarizer placed at the source of the light, at an angle that benefits the application, and a second polarizer placed at the same or a complementary angle, onto the lens of the camera conducting the inspection. Even if the light projected onto an object is polarized in order to limit the potential number of reflective polarization angles, the reflective properties of the object may still create unwanted polarization when the light reflects off the object. Employing two filters allows for finer control of the light angles that enter the camera, ensuring the desired crispness of the image. Placing a polarizer only over the camera lens, and thus only polarizing the light reflecting off the object to be inspected, may sometimes be an effective technique but far more often not. To demonstrate the point, consider the example of photography. The polarizers used by photographers rotate, to allow the photographer to attune the polarizer to the precise angle at which the light is being polarized by the surface of the subject in the scene. The photographer tunes the angle of polarization as needed for different scenes, not a practical technique for automated imaging in machine vision. Take, for example, parts on a conveyor belt rapidly passing underneath a camera for inspection. It’s impossible to achieve perfect, replicable results during parts inspection, even with polarized light and filter working in concert, because there will likely be variances in the position of objects relative to the camera, which affects the angles at which polarized light reflects off the surface of the object. However, a new technology and new components becoming readily available overcome the limitation.

Dollyzoom

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Using a telephoto zoom lens will totally change the overall feel of the image. Especially when photographing subjects that are further in distance from you. Not only does it bring the main subject closer, but it also brings all the background elements closer as well.

Returning to the example of the sun reflecting off the surface of water and producing glare, a photographer may employ a polarization filter to remove that glare, improving the image by filtering light at the specific angle causing the glare.

Look on the side of your lens and check if it has a focus limit switch. You will recognize it because it will have a distance metering associated with it. For example my Canon lens has a switch on the side that says 1.8m -∞, as well as 6.5m-∞. If your lens doesn’t have this switch then ignore this section.

The IMX250MZR/MYR (monochrome/color) Polarsens sensor from Sony (Tokyo, Japan; www.sony.com) represents a recent implementation of on-sensor polarization technology. The polarization elements are fabricated right into the silicon, under the microlens (Figure 4), which makes the sensor unique. In the color sensor, color filters are located directly underneath the polarizers.

Telephotolens

Similarly, in machine vision, artificial polarization techniques help developers select or restrict the direction of the light waves that enter a camera lens and strike an image sensor. The three basic ways to artificially polarize light are linear polarization, circular polarization, and elliptical polarization, with the latter two methods serving as extensions of linear polarization and not widely used in machine vision.

If you are photographing long exposures or landscapes with a telephoto lens then it is best to use a tripod. If using a tripod, look on the side of your lens for letters VR (vibration reduction for Nikon) or IS (image stabilization for Canon) and turn them off.

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Use of multiple filters, each of which prevent the passage of light waves at different orientations, allows finer attenuation of light. A filter only allowing vertical light to pass, followed by a filter only allowing horizontal light to pass, would act together to block most of the light waves passing through the filter pair.

When photographers hear the term telephoto they usually imagine a long lens between 70 and 500mm focal length or longer. Personally I’ve used 3 different brands and the technique for using them is basically the same.

On-chip polarization sensors, now available to virtually any camera manufacturer, drive advances in software and solutions that developers and manufacturers provide for machine vision systems. An increasing number of viable polarization applications can now be expected.

For example, I live in a coastal town where I photograph boats anchored out at sea with islands in the background. If I use a wide angle landscape lens, the islands in the background seem much smaller and insignificant than when I use my telephoto zoom lens. For these types of shots I prefer to see layers. Hence, I’ll use my telephoto zoom lens.

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For another example, as the sun moves across the sky the angle of the sun’s light striking a window will change. At some point the light will reflect off the window, or be polarized, at an angle perceived as glare.

Light is a transverse wave, i.e. its electromagnetic and magnetic fields are disturbed at right angles relative to the direction of travel of the light waves. Or, it can be said that light waves oscillate perpendicular to their direction of travel.

DOLP, or degree of linear polarization (the intensity and direction of linear polarization as it’s interpreted from the four angles of each four-pixel group), and AOLP, or angle of linear polarization, (the angle of the light as it reaches the sensor plane) can be adjusted. Adjusting these parameters allows fine control of polarization similar to how a photographer rotates their polarization filter to achieve the desired crispness of image (Figure 5).

Polarized light, on the other hand, is light in which the waves travel in only one, specific direction. Light can be polarized in nature by absorption, refraction, reflection, scattering, and birefringence (double refraction). For example, when light strikes water, it can be reflected linearly perpendicular, i.e. polarized in that specific direction, to the surface of water, which we experience as glare.

With more than 35 years of experience, David Dechow is the founder and owner of Machine Vision Source (Salisbury, NC, USA), a machine vision integration firm. He has been the founder and owner of two successful machine vision integration companies. He is the 2007 recipient of the AIA Automated Imaging Achievement Award honoring industry leaders for outstanding career contributions in industrial and/or scientific imaging.

The proper way to employ polarization in an inspection environment typically involves polarizing both the light source and the reflected light (Figure 3) with a polarizer placed at the source of the light, at an angle that benefits the application, and a second polarizer placed at the same or a complementary angle, onto the lens of the camera conducting the inspection.

The sky is blue because sunlight strikes the molecular structure of the atmosphere and scatters, which polarizes the light in a specific direction. As the angle of the sun relative to the atmosphere changes the polarization angle of the light also changes, and the human eye perceives color changes from dawn to midday to dusk.

Each line in a time delay integration (TDI) setup is successively struck with each of the four polarization angles (Figure 6). The four images then combine into a composite image, providing a full-resolution polarization image, versus the four-pixel interpolated image created by an area sensor like the IMX250MZR/MYR.

Digitalzoom

The dynamic process allows for the correction of glare that may originate from different angles on individual items to be inspected, making it easier to extract features, scratches, or even to see material stress in plastic.

Software can then dynamically handle the selection of either individual wave angles, or the computational manipulation of the variety of wave angles, to analyze the image in ways that only were possible previously by providing sets of filters, multiple images, or even multiple sensors, to acquire multiple polarized analyses of the individual image, itself.

Just because it’s a long lens doesn’t mean you need to use a tripod. In fact I rarely use a tripod when using my telephoto lens. I find them too restrictive when photographing birds and wildlife.

All lenses require different techniques for photographers to grasp. Just because you know how to use one lens doesn’t mean you can use the same technique on the next. As strange as it sounds, telephoto and macro lenses have a lot in common. Personally I treat them much the same way.

Opticallens

Blurring backgrounds is easier with a telephoto lens than it is with a wide angle lens. Set your camera aperture on f/8 or lower, zoom the lens out to its longest focal length, and get as close to the subject as possible while still being able to focus. You also need to ensure as much distance as possible between the subject and any background elements. You won’t see any blur if the subject is against a brick wall.

No technology provides a global cure for imaging challenges, however. Polarization is another tool in the toolbox of a vision system designer or integrator.

Placing a polarizer only over the camera lens, and thus only polarizing the light reflecting off the object to be inspected, may sometimes be an effective technique but far more often not. To demonstrate the point, consider the example of photography.

Common polarization techniques in machine vision include inspection of glass and highly reflective materials. Raw and machined metal have grain at a microscopic level that can lead to linear polarization, making the use of a polarization filter useful. Plastic and glass, automotive, packaged materials, semiconductor, and LCD inspection applications commonly make use of polarization filters.

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A telephoto zoom lens is my favorite piece of glass. Over the years I’ve photographed with Canon, Nikon and Sigma telephoto zoom lens. Before we let you in on the secrets of using these long lenses, we’ll first discuss a couple of basics.

Fresnellens

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Why is this important to know? There has been many instances where I’ll be trying to photograph a subject that is only a few meters from me and the lens keeps swimming in and out, not able to focus. I’ve fixed this problem every time by setting the correct metering. ie I set 1.8m -∞ for subjects closer than 6.5 meters away, and set 6.5m-∞ for everything 6.5 meters or further in distance.

A polarizing image sensor makes a polarization filter part of the image sensor, versus a separate component placed over the sensor. The Photron Crysta by Photron (San Diego, CA, USA; www.photron.com) and PolarCam from 4D Technology (Tucson, AZ, USA; www.4dtechnology.com), to cite two examples, have been in the marketplace for some time and make use of on-sensor polarization.

The first time I used my Canon EF 70-200mm f/4L USM lens on a tripod, was to photograph beautiful night shots of the Opera House in Sydney. Looking at the images on my computer I saw they were all out of focus and appeared to have symptoms of camera shake. I later found that by keeping this switch turned on, VR / IS will look for camera vibration and when it doesn’t find it, it keeps looking. Hence in itself causing camera shake. So I went out the next night with IS turned off and captured the image seen below.

Polarization techniques can also be deployed in line scan cameras like the quad-linear Piranha4 from Teledyne DALSA (Waterloo, ON, Canada; www.teledynedalsa.com), which has four linear sensor lines, each of which has an on-sensor polarizer, with each sensor line set to a different polarization angle.

Long telephoto zoom lenses are perfect for photographing macro subjects. They allow you to stand further away from the subject so not to scare it away. This has its obvious advantages when shooting timid subjects like dragonflies and insects.

Polarization techniques Returning to the example of the sun reflecting off the surface of water and producing glare, a photographer may employ a polarization filter to remove that glare, improving the image by filtering light at the specific angle causing the glare. Similarly, in machine vision, artificial polarization techniques help developers select or restrict the direction of the light waves that enter a camera lens and strike an image sensor. The three basic ways to artificially polarize light are linear polarization, circular polarization, and elliptical polarization, with the latter two methods serving as extensions of linear polarization and not widely used in machine vision. Dichroic, thin-film, and wire-grid polarizers represent the most common components used in machine vision to linearly polarize light. Wire-grid polarizers specifically can withstand the power levels of lasers, which can be useful in factory and scientific environments. Polarization filters function by selecting or restricting light waves traveling in a single plane. The orientation of the filter determines the orientation of the light waves that can pass through the filter, thereby selecting one set of light waves to advance and preventing others from continuing (Figure 2). Use of multiple filters, each of which prevent the passage of light waves at different orientations, allows finer attenuation of light. A filter only allowing vertical light to pass, followed by a filter only allowing horizontal light to pass, would act together to block most of the light waves passing through the filter pair.

A normal focal length is known to be 50mm. When you shoot with a lens set at 50mm focal length, the subject will be photographed in size as you saw it in real life. If you zoom your lens out to 100mm focal length, the image will now be magnified X 2 so the subject appears much closer. Therefore the definition of a telephoto zoom lens is one that can be magnified higher than 50mm focal length.

Natural light, and virtually all artificial light (LEDs, incandescent lights, fluorescent lights, etc.) is unpolarized or weakly polarized. Natural light travels in any radial direction from the source of the light (Figure 1). Imagine a beam of light. Light waves oscillate 360° from every point along that beam. (An oversimplification, to illustrate the concept.)

By interpolating the images from each four-pixel block, the sensor gathers considerable information about the linear polarization occurring on the surface of the object being viewed. These interpolated pixels allow a very accurate definition of the exact direction and intensity of the polarization within each four-pixel group.

The polarizers in Sony’s sensor, a microscopic implementation of a wire-grid polarizer over every lens on the sensor, have 0°, 45°, 90°, and 135° polarization angles in four-pixel groups. The groups are interpolated, reducing the sensor’s overall functional resolution by 4x. In other words, each four-pixel block equates to one pixel of output.

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Even if the light projected onto an object is polarized in order to limit the potential number of reflective polarization angles, the reflective properties of the object may still create unwanted polarization when the light reflects off the object. Employing two filters allows for finer control of the light angles that enter the camera, ensuring the desired crispness of the image.