Illumination choices for machine vision system designers now include halogen, incandescent, fluorescent, compact-fluorescent, plasma-type, high-intensity discharge (HID), Xenon, and light-emitting-diode (LED), fiberoptic, and structured light sources.

Vision System Design’s online Buyer’s Guide lists well over 100 different suppliers of lighting and illumination products. This is unsurprising when we consider how often the importance of lighting as a factor in the success of a machine vision application has been discussed thus far.

LEDillumination

beetle: An order of insects known as Coleoptera, containing at least 350,000 different species. Adults tend to have hard and/or horn-like “forewings” which covers the wings used for flight.

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Fluorescent lighting sources are relatively inexpensive. They are available in many shapes and sizes and provide diffuse light that minimizes glare. The limiting factors here are that fluorescent lights can’t be strobed, they degrade over time, and it’s difficult to dim them. Fluorescent lights also flicker due to the frequency of the AC power source, which can affect the image quality of vision inspection applications. Using high frequency ballasts can help reduce the amount of flicker but can’t eliminate it entirely.

Sunlight reflecting off water, for instance, can become polarized. The incoming light waves from the sun wiggle all different ways. But the light waves that bounce off the surface of the water are all mostly wiggling at the same angle. Pavement is another surface that can polarize reflected light.

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Without tools like sunglasses, humans typically can’t detect the polarization of light. But other animals can. Examples include mantis shrimp, fiddler craps and some beetles, birds and fish.

This type of lighting is distinguished from full bright field in that partial bright field is directional, typically from a point source. Because of its directional nature, partial bright field lighting is a good choice for generating contrast and enhancing topographic detail. It is much less effective, however, when used on-axis with specular surfaces, as this generates “hotspot” type reflections within the image.

Many strobing systems that are older employ Xenon lights. Xenon is ideal for strobing because it can provide very high intensity over a short period of time, thus creating the impression of stopped motion. The intensity of Xenon strobes decreases over time, however.

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mantis shrimp: A marine animal related to crabs and lobsters. Mantis shrimp use armlike body parts to kill prey. They are often multicolored and have a very complex vision system.

Dark field lighting relies on light that is incident at low angles and reflects back from small surface imperfections. Dark field lighting is characterized by low or medium angle of light incidence, typically requiring close proximity to the sample.

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A lighting technique is a combination of a light source and how it is placed respective to the part and the camera. A specific technique may enhance your image by negating some features within the image or accentuating others.

For example, silhouetting a part to allow measurement of edges while negating the surface details, such as in back lighting, generates instant contrast as it creates dark silhouettes against a bright background. The most common uses for silhouetting are detecting presence / absence of holes and gaps, part placement or orientation, or for measuring objects.

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filter: (n.) Something that allows some materials to pass through but not others, based on their size or some other feature. (v.) The process of screening some things out on the basis of traits such as size, density, electric charge. (in physics) A screen, plate or layer of a substance that absorbs light or other radiation or selectively prevents the transmission of some of its components.

polarization: (in physics) The condition — or creation of a condition — in which rays of wavelengths of light exhibit different properties when viewed from different directions.

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refraction: The change in direction of light (or any other wave) as it passes through some material. For example, the path of light leaving water and entering air will bend, making partially submerged objects to appear to bend at the water’s surface.

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A better alternative is to use light emitting diodes (LEDs). These sources are the most common ones for machine vision applications today. They last a very long time and can be strobed or left on continuously without heat issues. LEDs are also very safe to use, efficient, and can be sourced in virtually any color or specific wavelength.

Any imaging system that has thresholding or is looking for subtle features such as surface defects or color inspection will benefit from improving the repeatability of the light levels. This is where lighting controllers may be useful as they provide consistent illumination.

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Light can become polarized when it bounces off of a surface. The amount that it is polarized depends on the angle at which the light hits the surface. It also depends on what the surface is made of.

In some applications, such as line scan inspection systems, it is necessary to continuously illuminate the product being inspected. In other applications, where an area array camera must capture images of objects at high-speed, the LED illuminator may need to be strobed.

Proiettore in ABS bianco completo di base cromata. Orientabile e sfilabile dalla base con un cavo estensibile fino a 2 metri è completo di spina. Bulbo Ø mm.

Another way to polarize light is to pass it through a filter. Such filters are made up of molecules that are all aligned the same way. This allows only light waves wiggling at specific angles to pass through. For example, some sunglasses contain polarizing filters. These glasses help reduce the amount of reflected light — or glare — that reaches a wearer’s eyes. That can block out glare from roadways or puddles to help protect drivers’ eyes and allow them to see more clearly.

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Light sources from lamps to stars typically give off light waves that wiggle at a range of different angles. This jumble of waves is known as unpolarized light. Polarized light, on the other hand, is unjumbled: its waves all wiggle the same way, at the same angle. (For help picturing this, check out the artist’s depictions at 1:30 in this PBS video.)

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Diffuse, or “full bright field” lighting, is most commonly used on shiny specular or mixed reflectivity samples where even but multi-directional light is needed. Diffuse lighting is commonly implemented with a hemispherical dome or cylinder, or by aiming the lighting on-axis. Diffuse dome lights are very effective at lighting curved, specular surfaces, and are commonly found in the automotive industry.

molecule: An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

Dark field lighting

In other applications, it may be necessary to illuminate objects with multiple LED lights or with varying illumination intensity to extract the relevant features from the object being imaged. In such cases, intelligent lighting controllers that incorporate selectable intensity, pulse control, speed, and programmable illumination sequencing can meet these demands.

A "Green Machine" model can have one or more of the following parts painted in green: Body, chassis, wheels, interior, engine, tinted glass and tires.

glare: Also called direct glare, it is light that travels directly from a light source and into someone’s eyes. It reduces the ability to see at night. Direct glare is one of three types of light pollution.

angle: The space (usually measured in degrees) between two intersecting lines or surfaces at or close to the point where they meet.

For machine vision applications, lighting controllers that provide constant-current sources are offered by most LED lighting and controller manufacturers since the current flowing through the LED, not the voltage, determines LED brightness. While voltage controllers are available, they are not recommended for machine vision applications since small voltage changes result in large current changes and hence a large change in light intensity. By regulating the current through the LED, constant-current sources ensure that the absolute maximum rating of the LED being driven is not violated and that a predictable and stable luminous intensity is therefore obtained.

Lighting is a pivotal factor in the design of a machine vision system because these systems create images by analyzing the reflected light from an object, not by analyzing the object itself. For this reason, it is important to understand how the material, size, shape, color, and surface finish of the object you are illuminating will affect light reflected off the object, and what challenges these physical characteristics of the object(s) under inspection may pose.

Light is made up of waves. They wiggle as they zip through space. They can wiggle up and down, left and right, and any angle in between. Light is polarized when its waves all wiggle at the same angle.

Light can also become polarized as it passes from one type of material into another. This is a process called refraction. And light can become polarized when it bounces, or scatters off atoms as it passes through a substance. This happens to moonlight and sunlight that travels through Earth’s atmosphere.

Halogen lights are good for applications where very bright lighting is required. Halogen lights are also limited by their poor strobing capability. They tend to run very hot and lose intensity over time. If halogen light is to be used for an inspection application, it would be beneficial to run the lights at 80% of their maximum power to extend their life, keeping the voltage constant to avoid color changes, and using fiber optics to enable mounting the lamp remotely if the application is sensitive to the heat produced.

Illuminationtechniques in slit lamp

Lasers are highly collimated light sources which have high intensity, allowing them to be mounted a distance from the object being inspected. A special type of laser called a “diode laser” can be strobed. Lasers are often used to make distance measurements, sometimes in three dimensions, but may not be capable of high accuracies. Lasers are traditionally very expensive and require special safety precautions as they present potential safety hazards. Fiber optics may provide a safer option to provide collimated light if an application requires it.

In addition to types of lighting and lighting techniques, machine vision system designers may also want to consider lighting controllers.

Light can become polarized when reflecting off flat surfaces like puddles and pavement. Polarized sunglasses can then specifically block out those polarized light waves, reducing the glare that reaches a driver’s eyes.

On-axis lights work in similar fashion for flat samples and are particularly effective at enhancing differentially angled, textured, or topographic features on relatively flat objects. To be effective, diffuse lights, particularly dome varieties, require close proximity to the sample.

Partial bright field lighting is the most commonly-used vision lighting technique. It is also the most familiar lighting we use every day: sunlight is partial bright field lighting.