Simon Stanley, director of technology at ProPhotonix (Salem, NH, USA) also provides an example of how a customer in the medical device industry addressed the issue of color. “The customer was inspecting data matrix codes on different colored capsules. They were limited to using a high-resolution, but monochrome, Cognex In-Sight 9000 camera in line-scan mode; and a white line light and found that for some of the colored capsules, the images did not have adequate contrast.”

Electric polarization

“By using cross-polarization, a technique where a linear polarizer is placed over a light source (transmitter) and another rotating linear polarizer is attached over the camera lens (receiver), the light rays coming from the surface of the item are reduced while the part of the light energy that penetrates the surface is passed through the receiving polarizer. Since the cross-polarization effect reduces the light from the surface, glare and other specular effects from glossy plastic surfaces is reduced,” he says.

In general, according to Advanced Illumination, a light that is of a similar color as an object will reflect and thus brighten the object while a light on the opposite side of the color wheel will darken the object.

Unpolarizedlight

Common terminology when discussing light reflected from a surface is to refer to 'p-polarized' and 's-polarized' rays. Controlling reflections is an ...

(More information from Vision Systems Design on illumination techniques for machine vision applications in logistics is available here.)

On the other hand, the polarization state of the laser output can be disturbed e.g. by random (and temperature-dependent) birefringence, such as occurs e.g. in optical fibers (if they are not polarization-maintaining or single-polarization fibers) and also in laser crystals or glasses as a result of thermal effects (→ depolarization loss). If the laser gain is not polarization-dependent, small drifts of the birefringence may lead to large changes of the polarization state, and also a significant variation in the polarization state across the beam profile.

Lighting controllers also allow for flexibility, particularly in situations in which more than one lighting solution is deployed, explains Sullivan at CCS America. Some examples: multiple lights are on at once but at different intensity settings to avoid overexposing an image or multiple lights go on/off in sequence to capture multiple images. “For example, a controller can turn on a flat dome light for a bright field image, then a low-angle light for a dark field image,” she says.

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The company’s Modular Tiny Bar Light (M-TBAR), which is designed specifically for confined spaces such as in robotics applications, offers flexibility, the company says. The product allows users to change the beam angle with the company’s angle changers, which are micro-optic diffusers that can be used to widen the beam angle. The light also offers strobe overdrive illumination.

The polarization state of monochromatic light is often described with a Jones vector, having complex electric field amplitudes for <$x$> and <$y$> direction, if propagation occurs in <$z$> direction. That Jones vector may be constant over some area across the beam, or it may vary, for example for a radially polarized beam (see above). The effect of optical elements such as waveplates, polarizers and Faraday rotators can be described with Jones matrices, with which the Jones vectors can be transformed by multiplication. (One assumes a linear relationship between input and output amplitudes.) A whole sequence of such optical elements can be described with a single Jones matrix, which is obtained as the product of the matrices corresponding to the components.

EFFI-FLEX2 from EFFILUX (Les Ulis, France) also offers flexibility, allowing engineers to adjust the emitting angle, uniformity, and operating mode (overdrive, adjustable strobe, and dimmable continuous mode).

To choose from so many options, it is important to consider the characteristics of the object to be imaged, such as size, shape and color; the spatial relationship between the object, light and camera; and how the light is reflected or absorbed by the object and background. (Vision Systems Design offers more explanation here.)

polarization中文

Of course, the polarization can have any other direction perpendicular to the beam axis. Note that a rotation of the polarization by 180° does not lead to a physically distinct state.

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In the simplest case, a light beam is linearly polarized, which means that the electric field oscillates in a certain linear direction perpendicular to the beam axis, and the magnetic field oscillates in a direction which is perpendicular both to the propagation axis and the electric field direction. The direction of polarization is taken to be the direction of the electric field oscillations (i.e., not the magnetic ones). For example, a laser beam propagating in <$z$> direction may have the electric field oscillations in the vertical (<$y$>) direction and the magnetic field oscillations in the horizontal (<$x$>) direction (see Figure 1); it can be called vertically polarized or <$y$>-polarized. In a different perspective, this is also shown in the second part of Figure 2.

A circular polarization state can mathematically be obtained as a superposition of electric field oscillations in the vertical and horizontal direction, both with equal strength but a relative phase change of 90°. Effectively, this leads to a rapid rotation of the electric field vector – once per optical cycle – which maintains a constant magnitude.

Linear polarization

As the earth's atmosphere absorbs most of the space (solar) UV radiation, any UV application is based on man-made UV sources. In the solar spectrum, the high- ...

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Reading those barcodes accurately has become increasingly complicated, requiring a creative approach to lighting. As Steve Kinney, director of training, compliance and technical solutions for Smart Vision Lights (Norton Shores, MI, USA), explains, “Today’s scanning portal must read through the clear outer packaging on individual items, through poly bags to see the item inside, through clear films placed over shipping labels for weather resistance, or even through shrink wrap holding a pallet together to read the barcodes of the boxes comprising the pallet. In these demanding instances, not only do the various films and packaging cause problems seeing the barcodes to be read, but they also have uneven glossy surfaces that make the imaging even harder.”

Sometimes the best choice is to darken the object—particularly if the goal is to read printed codes or words. By illuminating red print on a white envelope with a green light, you’ll get the clearest image of the print, Advanced Illumination says on its website.

There are also azimuthally polarized beams, where the electric field direction at any point is tangential, i.e., perpendicular to a line through the point and the beam axis.

However, brightness would be an advantage if the goal is to brighten or enhance the contrast between an object and its background, such as to detect wrapped candy on a conveyor belt.

(More information from Vision Systems Design on addressing lighting challenges in medical device and pharmaceutical inspection is available here.)

Numerous experts agreed on the most common pitfall: Waiting until the tail end of the design process to address lighting for a machine vision application.

As Brett Thrailkill, application support and inside sales representative at Advanced Illumination (Rochester, VT, USA), explains, “Unfortunately, many users have the misconception that proper machine vision lighting doesn’t need to be considered until the end of the system design process. This often leads to cost overruns, inferior lighting, and delays in production timelines. To prevent this, we encourage all application engineers to consider lighting options early in the development process.”

The most recent addition, introduced in 2018, is the Washdown Monster Dome Light, which is designed to remove glare from product images without a loss in intensity.

Circular polarization

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TPL Vision (Perth, Scotland, UK) Product Manager Jack McKinley says choosing flexible lighting products reduces risks to an application’s feasibility posed by unknown factors in the current or future production environment.

R Paschotta · 2 — In the simplest case, a light beam is linearly polarized, which means that the electric field oscillates in a certain linear direction perpendicular to the beam ...

Mark Kolvites, sales manager at Metaphase Lighting Technologies (Bristol, PA, USA), says, “Another pitfall is overlooking the fact that the lens and camera sensor are also part of the optical path for the captured image, which could affect the illumination implemented. For example, the wider the angle of the camera lens, the more optical distortion around the perimeter of the field of view, making the image look non-uniform, even though the light itself might be uniform.”

In the previous cases, the direction of the electric field vector was assumed to be constant over the full beam profile. However, there are light beams where that is not the case. For example, there are beams with radial polarization, where the polarization at any point on the beam profile is oriented in the radial direction, i.e., away from the beam axis.

Your first plot shows the magnetic and electric field in phase – which is wrong. The magnetic field is made from the changing electric field. The two fields swap energy back and forth. Hence the magnetic field is at a maximum when the electric field has the largest rate of change, that is, at zero E field. The magnetic field zeros in strength when the electric field rate of change is zero, at it's peak. These are a simple consequence of Maxwell's Equations and is covered in most any text on E&M. The worst error I have found in years of use of your marvelous resource!

If the oscillations of the horizontal and vertical electric field vector do not have the same strengths, one has the case of an elliptical polarization, where the electric field vector, projected to a plane perpendicular to the propagation direction, moves along an ellipse.

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Fully polarized states can be associated with points on the so-called Poincaré sphere. Partially polarized states correspond to points inside that sphere; unpolarized light is represented by the point at its center.

Kinney adds that lighting manufacturers have developed specialized lighting products that feature overdrive, “where the light is operated at multiples of the steady state current for short periods to create a brighter strobe. This allows the use of polarization needed to effectively work in portal systems while still having enough light for the short exposure periods required to image without motion blur at the item passes at high speed through the system.”

Hardy notes that the company’s IP68 Washdown series of LEDs is designed for these types of environments. The series includes ring lights, dome lights, spotlights and linear lights.

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Note that radial or azimuthal polarization state requires a zero electric field strength and thus also a vanishing optical intensity on the beam axis; it is not compatible with a Gaussian beam, for example. Radially polarized beams frequently exhibit a kind of donut profile.

Whether an application involves reading barcodes on packages or inspecting products, these experts say they’ve described some of the most common missteps in designing and implementing illumination solutions for machine vision applications.

As explained above, a waveplate or other birefringent optical element may rotate the direction of linear polarization, but more generally one will obtain an elliptical polarization state after such an element. True polarization rotation, where a linear polarization state is always maintained (just with variable direction), can occur in the form of optical activity. Some optically active substances such as ordinary sugar (saccharose) can produce substantial rotation angles already within e.g. a few millimeters of propagation length. Optical activity can be accurately measured with polarimeters.

Circularly polarizedlight

Overlooking the Impact of the Sensor and Lens on Illumination Mark Kolvites, sales manager at Metaphase Lighting Technologies (Bristol, PA, USA), says, “Another pitfall is overlooking the fact that the lens and camera sensor are also part of the optical path for the captured image, which could affect the illumination implemented. For example, the wider the angle of the camera lens, the more optical distortion around the perimeter of the field of view, making the image look non-uniform, even though the light itself might be uniform.” This is why Metaphase developed “light sources where, for example, the end of a linear bar light will be brighter at the ends compared to the middle of the light to compensate for lens image attenuation around the perimeter of the field of view,” he explains. An example is the company’s Multispectral Exolight v2.0, which is available in versions covering up to six wavelengths including the short-wave infrared (SWIR) spectrum.

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This is why Metaphase developed “light sources where, for example, the end of a linear bar light will be brighter at the ends compared to the middle of the light to compensate for lens image attenuation around the perimeter of the field of view,” he explains.

Outside the realm of inspection in manufacturing, lighting challenges abound in machine vision applications in logistics and warehousing. One example involves a portal system, which scans barcodes in automated or smart warehouses.

Linda Wilson joined the team at Vision Systems Design in 2022. She has more than 25 years of experience in B2B publishing and has written for numerous publications, including Modern Healthcare, InformationWeek, Computerworld, Health Data Management, and many others. Before joining VSD, she was the senior editor at Medical Laboratory Observer, a sister publication to VSD.

Available in ring and linear formats, the new lights offer direct connection and control through a camera’s trigger output.

I would have been glad to finally remove a serious mistake, but I believe my equations are correct. They agree with those in various textbooks and e.g. also in Wikipedia. Your argument concerning energy swapping back and forth between electric and magnetic fields looks somewhat plausible but is not accurate.

A radially polarized laser beam may be generated from a linearly polarized beam with some optical element, but it is also possible to obtain radially polarized emission directly from a laser. The advantage of this approach, applied in a solid-state bulk laser, is that depolarization loss may be avoided [4]. Furthermore, there are applications benefiting from radially polarized light.

There are cases where polychromatic light can be described with a single Jones vector, since all its frequency components have essentially the same polarization state. However, the polarization state is substantially frequency-dependent in other cases.

Lindsey Sullivan, technical marketing manager at CCS America (Woburn, MA, USA), agrees, saying, “It is important to test and solve the application at the beginning of the design process. Once the required lighting is understood, the design of the inspection system can account for the positioning of the sample, size, working distance, color (and) geometry of the light.”

David Hardy, vice president and design engineer for Spectrum Illumination (Montague, MI, USA), says that white lights also provide the most flexibility over time if a product’s color changes in a future iteration. “For example, the part under inspection is red or white, and a red light is chosen. But if a part color of green or blue is added later, it can appear quite dark as green and blue colors are somewhat opposite red on the color wheel. In this case, using a white light initially would have been better, or even an infrared light to remove the color.”

Underestimating the speed necessary for inline inspection in some environments also could lead to illumination challenges. Advanced Illumination updated its BL2 Backlights to increase brightness, allowing for higher throughput. The lights are also designed for scalability, expandable in 1 in increments up to 46 in in length or width. They can be operated in operated in continuous or overdrive strobe modes.

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polarization极化

The degree of linear polarization is often quantified with the polarization extinction ratio (PER), defined as the ratio of optical powers in the two polarization directions. It is often specified in decibels, and measured by recording the orientation-dependent power transmission of a polarizer. Of course, the extinction ratio of the polarizer itself must be higher than that of the laser beam.

An example of this type of lighting solution is the Lightgistics series from Smart Vision Lights. The lights feature Dual OverDrive, which combines SVL’s Deca OverDrive and standard OverDrive engines, allowing end users to attach polarizers while retaining light output to handle any speed.

For example, he says TPL Vision’s customers often encounter different application conditions than what was tested in the lab. As a result, “higher brightness may be required (solved by overdrive products), different ambient conditions may need a different lighting setup, or integration may be different than anticipated. This is precisely where flexibility can be beneficial,” he says.

Polarization oflight

Note that a very small gain or loss difference for the two polarization directions can be sufficient for obtaining a stable linear polarization, provided that there is no significant coupling of polarization modes within the laser resonator.

Proper lighting is crucial to the success of machine vision applications. Without it, images will not show enough visual information about the important features of an item.

Further, we have many interesting case studies on the same page, with topics mostly in fiber optics. Concrete examples cases, investigated quantatively, often give you much more insight!

A light beam is called unpolarized when the analysis with a polarizer results in 50% of the power to be in each polarization state, regardless of the rotational orientation. Microscopically, this usually means that the polarization state is randomly fluctuating, so that on average no polarization is detected. Note that such fluctuations are not possible for strictly monochromatic light.

Linearly polarized light can be depolarized (made unpolarized) with a polarization scrambler, which applies the mentioned random polarization changes, or at least quasi-random changes.

“There are also a variety of other optical accessories that are easy to install on the light. This means you can get a different lighting geometry without needing to purchase a new light,” saving money and time on redesign work that can cause delays in projects, explains Mariann Kiraly, US sales manager for EFFILUX.

CCS America’s PD4 Series lighting controller is designed for these types of situations, allowing engineers to program up to 16 steps of on/off for the lights in an application.

In many respects, light can be described as a wave phenomenon (→ wave optics). More specifically, light waves are recognized as electromagnetic transverse waves, i.e., with transverse oscillations of the electric and magnetic field.

Hardy at Spectrum says another pitfall is not considering the production environment during the lighting selection process. “We also see many customers choosing the wrong type of light for the environment. For example, using a standard backlight or dome light near a CNC (computer numerical control) machine where coolant splatters on the light and over time will wick inside causing internal corrosion and eventually failure.” When designing a machine vision application for harsh environments, choosing a light with a higher IPrating is a better option, he explains.

Common machine vision applications in which illumination is part of the solution involve inspecting products for defects before they leave the manufacturing plant or reading barcodes or other alphanumeric data on products or packages as they move through the logistics pipeline.

One distinguishes left and right circular polarization (see Figure 2). For example, left circular polarization means that the electric (and magnetic) field vector rotates in the left direction, seen in the direction of propagation. For an observer looking against the beam, the rotation of course has the opposite direction.

With so much to consider, missteps abound. To get insights on how to avoid, or solve, common pitfalls in illumination planning and execution, Vision Systems Design asked experts at a handful of lighting companies to share their thoughts. We’ve included their comments here.

J Schmit · 2002 · 96 — Introduction. Interferometric techniques for shape measurement such as white-light interferometry (WLI),[1]-[3] phase-shifting interferometry (PSI) ...

An example is the company’s Multispectral Exolight v2.0, which is available in versions covering up to six wavelengths including the short-wave infrared (SWIR) spectrum.

This is why engineers choose white lights, which comprise both warm and cool colors, for many applications, and especially those involving a color camera. Colored lights are used with monochrome cameras.

Of these, LEDs are the most common choice for machine vision applications because they have long life spans, can be left on continuously, and come in many sizes, shapes and colors. LED lighting choices also span the visible, ultraviolet (UV), narrow-band and broad-band visible and infrared (IR) spectrum.

Jones vectors can be used only for fully defined polarization states, not for unpolarized or partially polarized beams (see below) having a stochastic nature.

To solve the problem, he says ProPhotonix sold the company the RGB-White COBRA MultiSpec LED, which has individual color and intensity controls that engineers access through a graphical user interface (GUI). This allows them to “tune the light to optimize contrast across the whole grayscale image resulting in accurate image acquisition regardless of the color of the capsule.”

201873 — With the right amount of supplemental light, a grower can expand their growing season, growing 24/7 or, in some cases, speed up production time.

The polarization state of light often matters when light hits an optical surface under some angle. A linear polarization state is then denoted as p polarization when the polarization direction lies in the plane spanned by the incoming beam and the reflected beam. The polarization with a direction perpendicular to that is called s polarization. These indications have a German origin: s = senkrecht = perpendicular, p = parallel.

There are also partially polarized states of light. These can be described with Stokes vectors (but not with Jones vectors). Further, one can define a degree of polarization which can be calculated from the Stokes vector and can vary between 0 (unpolarized) and 1 (fully polarized).

There are thousands of lighting options to choose from including those that are halogen, incandescent, fluorescent, compact-fluorescent, plasma-type, high-intensity discharge (HID), Xenon, light-emitting-diode (LED), fiber optic, and structured light.

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While optical activity usually results from the presence of chiral molecules, with a concentration difference between the two possible enantiometers, it can also be induced by a magnetic field in a substance which is not naturally optically active. That is called the Faraday effect, and is exploited in Faraday rotators and Faraday isolators.