Structured light. With structured lighting, a straight bright line or series of lines are projected onto the object. The bright lines will change depending on the object’s shape. Structured light is useful when the object has black-on-black parts. Black absorbs all the light and it is not visible to the camera. By using a concentrated beam of light, the camera sees the line of light and so can detect a black O-ring on a black plastic part, for instance. If the O-ring is there, the line will be in a certain position in the image, and if the O-ring is missing, the bright line will be in a different position.

Two red arrows representing light rays approach the light purple box at an angle from the top left. When they enter the light purple box, the lines become dotted and then disappear inside it. Illustration is labeled “Absorption: Light can be absorbed.”

Electromagneticwaves

Some light is absorbed and transformed into other forms of energy. Asphalt is black because it absorbs all colors of visible light very well. It heats up quickly in direct sun because a lot of that light is transformed into thermal energy (which is then emitted back out as invisible infrared light). Plants absorb mostly red and blue wavelengths of sunlight and turn them into chemical energy to live and grow.

By way of example, select a light source in the room, the object to be photographed, and use your eyes as a camera. Now keep two of those items in the same place and only move the third. For example, look at an object and keep your head still while paying attention to where an overhead light is positioned. Now move the object slightly. By doing this you can see certain features on the object highlight or disappear. The steps below expand on this concept.

The important thing to keep in mind when it comes to spectroscopy is that wavelength and energy are effectively the same thing. Human eyes detect differences in wavelength and energy as differences in color.

An xy graph shows the displacement of matter on the y-axis versus distance on the x-axis. The y-axis is a vertical red arrow pointing upward. The x-axis is a horizontal green arrow pointing to the right.

Light interacts with matter. When light encounters matter, a lot of things can happen. A few are particularly important to keep in mind when it comes to spectroscopy:

General dark field illumination. The camera looks straight at the object, but the lighting comes in from the very lowest angles. Think of the part as the horizon and the lights as coming in right above the horizon. The light skims across the object and away from the camera. Any bump or dent is highlighted because the light hits it and bounces toward the camera. This method is very effective for highlighting objects with direct part markings.

Dome, or diffuse, illumination (also called cloudy-day illuminator). Think of dome lighting as a big bowl with a hole in the middle. The bowl sits upside down, so the camera is looking through the hole, and the light shines up into the bowl and scatters in every direction. Dome lighting is also called “cloudy-day illuminator.” Imagine being outside on a very cloudy but sunny day. Everything is bright and lit up, but there are no shadows because the clouds diffuse and disperse the light all over the place. This method works great for objects that have a lot of different angles, such as packaging (blister packs and poly bags).

The graph consists of two perfect sine waves oriented perpendicular to each other. Both curves begin at the origin, the intersection of the three axes, where strength of the electric field, strength of the magnetic field, and distance all equal zero. The wave plotted on the xy-plane is shown in yellow and is labeled, “electric field.” The wave plotted on the xz-plane is shown in blue and is labeled, “magnetic field.” The curves intersect at the points where they cross the x-axis.

Gamma rays, X-rays, ultraviolet light, visible light (the visible rainbow), infrared light, microwaves, and radio waves are all forms of light, also called electromagnetic radiation. Together, they make up the electromagnetic spectrum. (That’s right, the radio waves that carry music from the station to your radio, the microwaves that heat up your food, and the X-rays dentists use to detect tooth decay are all forms of light.)

These four short wave infrared (SWIR) pictures show how machine vision designers can use this spectral band to see through paint and differentiate between materials. The two bottles show in images 1 and 2 show alcohol and water, with clear alcohol clearly differentiated in the second image. In the second pair of images, SWIR radiation is able to penetrate the paint on the plastic card, revealing the patterns underneath. This technique is widely used in ports for detecting altered storage containers.

A red arrow representing a light ray points straight down from the top and passes straight through the light purple box, exiting below it. The arrow does not change as it moves into, through, and out of the box. Illustration is labeled “Transmission: Light can pass through.”

Light also behaves like a particle. A particle of light is called a photon. Each individual photon has a very specific amount of energy (no more, no less), which corresponds to its wavelength. Blue photons carry more energy than red photons. Ultraviolet photons carry more energy than infrared photons. Sometimes photons are described as “packets of energy.”

Light wave

Protons and neutrons make up the nucleus of an atom, while electrons reside outside the nucleus. Although it’s actually much more complicated than this, you can think of each electron as occupying a particular energy level (sometimes referred to as an “orbital” or “shell”) around the nucleus. Electrons are a bit bizarre in that they can “jump” or “drop” (transition) from one energy level to another, but they can’t exist between two levels. (Why this is important? Keep reading or skip ahead.)

The wave is a perfect sine curve that begins at the origin, the intersection of the x- and y-axes, where both displacement and distance equal zero. The curve is exactly two wavelengths long, with two peaks and two troughs. The peaks and troughs have the same amplitude. A horizontal line marking the distance between two troughs is labeled “wavelength.”

Radio waves

Materials on Earth and in space are continuously emitting and interacting with light. What makes one material look different from another is the way that different wavelengths of light interact with it.

So, you can think of light as waves or you can think of it as streams of photons. Astronomers use both terms, depending on what they are trying to study or explain. (If you are confused as to how light can be both a particle and a wave at the same time, don’t despair. You are not alone.)

Wavelength symbol

Armed with the knowledge of what will happen to the light and the path of the light, technicians can decide on the type of light. There are six general ways to provide light to a part.

The second step involves looking at part geometry and imagining the path of the light from the source to the part and from the part to the camera. If there is no path to the camera, that part of the image will be dark. If light reflects to the camera, that part of the image will be bright. Changing the path of the light will brighten or darken different features. A couple of terms that are used frequently for lighting geometry are bright field and dark field. The “field” is the background or flat parts of the object. “Bright field” lighting refers to when the field is bright. In this case, the light comes from above the field and is reflected to the camera. “Dark field” is when the flat area is dark. This happens because the light is placed to the side of the field, which reflects the light away from the camera.

Lastly, we need to discuss color. Using colored lights or color filters with a monochromatic camera are a highly effective way of highlighting or hiding features in an image. Green objects are green because they reflect green wavelengths and absorb blue and red wavelengths. If you have not done so recently, please review a color wheel. It is helpful to know what colors are opposite each other on the wheel. You can use like colors to brighten an object and opposite colors to darken an object. For example, if you shine blue light at blue objects, they will appear white in the monochromatic image because they are reflecting all the blue light. The other colors absorb the blue light and will be darker. At the same time, CMOS imagers are more sensitive to red light, which also avoids potential UV eye damage issues from blue and shorter wavelengths. Color filters are used to exclude the noise. When filters are put in front of the camera they can cut out light wavelengths that are not needed or they can only allow the wavelength that is needed.

Coaxial illumination. The camera looks straight at the object, and light comes in straight in-line with the camera. All the light that is in-line and hitting a flat surface will shine directly back up at the camera. Anything flat will be bright. Anything not flat will be dark because if the light hits an angled surface it reflects away from the camera. This method works really well for highlighting scratches or other surface defects.

Wavelength is also what differentiates the various bands of light on the electromagnetic spectrum. When people talk about different “types” of light, they are referring to broad differences in wavelength. Gamma rays have the shortest wavelengths and radio waves have the longest. Visible light is in the middle. You can think of gamma rays, X-rays, ultraviolet light, infrared light, microwaves, and radio waves as bands of invisible color.

The first step involves looking at the part to determine how light interacts with the surface. Is it specular, matte, absorptive, refractive, or a little of everything? Reflective parts such as polished metal will reflect the light at the same angle as the light coming at the object. Matte parts will take light and scatter it all different directions. Some parts have material that will absorb light, such as ink on paper. Glass or liquid objects can change the direction of the light (refraction). Most objects have more than one of these properties. Technicians can take advantage of these differences by setting the light to highlight specific features of interest as part of an automated inspection or sortation system. In short, they can choose to highlight the signal and minimize the noise in any given image.

A red arrow representing a light ray points straight-up out of the middle of the light purple box. Illustration is labeled “Emission: Light can be emitted.”

An xyz graph shows the relationship between the strength of the electric field on the y-axis and the strength of the magnetic field on the z-axis versus distance on the x-axis. As in the graph of the mechanical wave, the y-axis is a vertical red arrow pointing upward and the x-axis is a horizontal green arrow pointing to the right. The z-axis is a blue arrow perpendicular to the xy-plane, pointing out of the page.

General bright field illumination. Imagine a ring of light surrounding the camera, looking straight onto the object. Features that are flat and reflective will appear brighter than those that are curved or absorptive. This is great for looking at print on objects.

Much of the latest news surrounding machine vision is about machine learning and the innovations regarding algorithms. But those algorithms need data to perform correctly. The data in this case is the images. It is imperative to capture the best image possible so that the algorithms can perform at their highest level.

Solid imaging begins by determining what is signal (the useful information) versus noise (the irrelevant, confusing information) in the image and using techniques to highlight the former by creating contrast.

When deploying any of the methods above there are a couple items to note because they can introduce noise. First, be aware of the ambient light. As much as lighting will help your image, unwanted light can hurt the image. Polarizers can reduce glare from non-metal parts. You should select a light that will be brighter than the factory lights. Be aware of any sunlight and if needed shroud the area to keep unwanted light away. Second, be aware that if you double the distance of the path of light the light is four times dimmer. The brightness of the lights is important because of the exposure time needed to capture an image. The exposure time is the time the camera collects light from the object. The longer the exposure time the fewer images a camera can collect in a given time. If the object is moving, the longer exposure will cause blur in the image. So having a dim light will likely have blurry features and reduce throughput. It is important to note in the instances where you need lights further away, consider using focused lights or lights that strobe (overdrive).

electromagnetic中文

Light that is not absorbed by matter can be reflected off it. Snow is white because it reflects all colors of visible light extremely well. Grass is green because it reflects a lot of green wavelengths of sunlight.

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Matter is the scientific catch-all word for stuff—anything that has mass and takes up space. Matter is made of microscopic particles called atoms. Atoms are made of even smaller, or subatomic, particles known as protons, neutrons, and electrons. Atoms can combine to form molecules.

Backlight illumination. This method uses a flat diffuse light that comes from behind the part and is aimed directly at the camera. The object will block the light so it will appear dark and the background white. This allows the image to have very clear edges. This method is great for measurements and checking the fill level of liquids.

Creating contrast requires understanding the flow of information for machine vision. First, the light source is scattered across an object, and the imager gathers that light to form a digital image. Only then can software begin extracting features (lines, edges, text) within the image to provide useful information (dimensions, part ID) and make a decision (pass, fail, sort). By manipulating the lighting, imagers can create contrast between the useful and irrelevant information.

Darkfield versus bright field: The “field” is the background or flat parts of the object. “Bright field” lighting refers to when the field is bright. In this case, the light comes from above the field and is reflected to the camera. “Dark field” is when the flat area is dark. This happens because the light is placed to the side of the field, which reflects the light away from the camera.

A red arrow approaches the light purple box at an angle from the top left. When it hits the top of the light purple box, it changes direction and moves up toward the top right. The two parts of the arrow form a V: The angle between the arrow and the surface of the light purple box of the reflected segment is the same as for the incoming segment. Illustration is labeled “Reflection: Light can bounce.”

Matter gives off light. Every object emits, or gives off, light of one sort or another simply because of its temperature. Glowing objects like stars, galaxies, light bulbs, and lava are all sources of visible light. Cooler objects like planets, dust grains, rocks, trees, animals, and icebergs don’t glow in visible light, but they do emit significant amounts of infrared light. Matter can also give off very specific colors of light depending on what it is made of and how it is interacting with other forms of matter and energy.

A red arrow representing a light ray approaches the light purple box at an angle from the top left. When it enters the box, it changes direction, moving through the box at a steeper angle. When it exits the box, it changes direction again, moving at the same angle that it entered the box. Illustration is labeled “Refraction: Light can bend.”

How is it possible to figure out such detailed information about materials on Earth and in space based only on color? Spectroscopy works because light and matter interact with each other in very specific and predictable ways. Before getting into the gory details, let’s review some relevant basics about light and matter. (If you know this already, feel free to jump ahead.)

The waves are identical to each other in wavelength and amplitude. Like the mechanical wave, each is two wavelengths long, with two peaks and two troughs.

Light waves are similar, but while mechanical waves cause oscillations in matter, light waves consist of electric and magnetic fields oscillating perpendicular to each other. Mechanical waves need matter in order to propagate, but light waves can travel through completely empty space as well as through matter. (If the idea of oscillating electric and magnetic fields does not make much sense, don’t worry. You don’t really need to know too much about it.)

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Light behaves like a wave. You are probably familiar with waves: water waves that ripple across a pond, sound waves that vibrate air and ear drums, and seismic (earthquake) waves that cause the ground to shake. These are all mechanical waves—energy that propagates through matter, causing it to move up and down, back and forth, or side to side.

Visiblelightwavelength

Light that is not absorbed or reflected by matter can be transmitted through it. Window glass is transparent, or “see-through,” because it transmits all colors of visible light. Strawberry jello is red because it transmits red light and absorbs all other colors.

Solids, liquids, and gases are all forms of matter. Planets, stars, nebulae, and galaxies are all made of matter. Rocks, water, air, dust bunnies, giraffes, viruses, spinach, coffee cups, and cowboy boots are all made of matter.

Electromagneticfield

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While that definition might make it sound like wavelength is a property that only a physicist could appreciate, most people are actually very familiar with the concept of wavelength: Human eyes recognize differences in wavelength as differences in color. On the visible part of the spectrum, shorter wavelengths look bluer and longer wavelengths look redder.

Electromagneticspectrum wavelength

Even though there are infinite possibilities of objects to inspect, with these steps you are well on your way to finding the right light to highlight the features needed for a great image. V&S

Graphic titled “Behaviors of Light” with five simple illustrations showing how light rays interact with matter. From left to right: absorption, emission, transmission, reflection, and refraction. In each illustration, light rays are represented by solid red lines with arrows pointing in the direction of travel. Matter is represented by a semi-transparent purple box.

One way to measure waves is by their wavelength. Wavelength is the distance between successive peaks. The wavelength of a light wave is the distance between peaks in the electric and magnetic fields.

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The strength of the electric and magnetic fields corresponds to each other: The peaks of the magnetic field occur at the same points on the x-axis as the peaks of the electric field.

Polarized filters help to reduce glare from highly reflective surfaces such as printed circuit boards. In the first image, multicolored lights are used to illuminate the PCB to show the various hot spots created by direct, unpolarized illumination. The second photo shows the same board with cross polarized light, reducing glare and revealing hidden codes, characters and features.