HOW TO PUT machine vision IN THE BEST LIGHT
By Dave Wilson, Contributing Editor
Many a well-thought-out vision system has failed for lack of appropriate lighting. Vision-system applications that function well as prototypes do not necessarily work well on the manufacturing floor, where fluctuating ambient light can distort images. Although lighting is critical in every machine-vision application, the correct lighting is often overlooked or low on the list of developers` priorities.
When choosing lighting sources for machine-vision systems, integrators have many options (see the listing of lighting vendors, p. 54). Most vendors will deliver impartial advice on the lighting that is most appropriate for any given application. Even if you are unsure of your particular requirements, most vendors will provide an overview of available technologies.
Describing the differences between illumination products is easy, but making the right choice for an application is difficult. In most cases, lighting choices are very application-specific. Designers must consider camera types, ambient lighting, and reflective-surface characteristics in choosing lighting sources. Before designing a machine-vision system, lighting must be made a priority. Lighting suppliers can offer help with your design, and others will suggest more cost-effective ways of attacking your specific problem.
The right light
"Several lighting options must be examined before deciding on the right type," says Mike Muehlemann, president of Illumination Technologies (East Syracuse, NY). These include spectral output, lamp cost, stability, lifetime, and efficiency (see "Illuminating lighting choices for web inspection," p. xx). These types of illumination include tungsten-halogen, xenon, and metal-halide lamps and LEDs.
Tungsten-halogen lamps can be well controlled and regulated with a dc supply. The spectrum of the tungsten-halogen lamp is pure, and the spectral response is linear. What`s more, spectral response remains consistent throughout the lamp`s life. Color temperature does not shift, and light can be focused onto small spots or injected into fiber.
"The lifetime of tungsten-halogen lamps is never long enough for most users," says Meuhlemann, "and there is little warning that the lamp is going to fail." It is a fairly inefficient light generator and limited in the amount of blue light it creates. While the lamp appears to be white, there is a lot more red and a lot less blue on the spectral output.
Alternatively, xenon is a stable plasma lamp with a linear spectral curve, a flat response from blue to red, which produces high-intensity white light. Although the lifetime of xenon lamps is about the same as tungsten-halogen lamps, intensity and efficiency is much higher. When used in conjunction with backlighting, xenon lamps provide uniform lighting for vision inspection systems (see Fig. 1).
"LED-based strobe technology complements xenon but provides monochromatic light and cannot be used in color applications," adds Muehlemann. Other technologies include metal-halide and high-pressure sodium lamps. While metal-halide lamps produce a very white light and have a long lifetime, they cannot be operated with dc power. High-pressure sodium lamps are the most stable, highest-lifetime lamps available on the market, but, because the temperature of these lamps is yellow, they cannot be used for color or fiberoptic applications.
According to Joseph Muratore of Dolan-Jenner Industries (Lawrence, MA), choosing one lamp over another is not simple. "Applications," says Muratore, "are based on detector physics, reflection, and absorption and transmission properties of the illuminated surface." Typically, xenon lighting is used with specific detector types. For example, CCDs are sensitive to the red component of light and insensitive to blue, so it is necessary to mix and match detectors to meet application demands.
LED lighting is used at short distances for monochrome imaging applications. But because LEDs are weak light sources, they need to operate close to inspected surfaces. "That`s why," says Dolan-Jenner`s Muratore, "LEDs are used at short-working-distances, mainly in the semiconductor industry. "
One common advantage of LED lighting is cited by Chuck Bourn, director of customer support of PPT Vision Systems (Eden Prairie, MN). " Unlike white-light systems, LED lighting does not cause image aberration and results in a sharper and more consistent image quality," he says. Here again, the choice is application-dependent, and LEDs are not likely to replace conventional illumination where large areas must be illuminated.
"For faster rates, it would take a cumbersome bank of LEDs to illuminate something one foot wide," says Dick Peters, product manager of Monarch Instrument (Amherst, NH), "and LEDs could never be made bright enough." Nevertheless, LEDs should not be disregarded completely. In many applications, such as illumination of drug bottles on production lines, they are adequate. However, for high-throughput systems with short flash durations, other technologies should be considered.
According to Timothy Ormsby, sales and marketing manager at Fostec (Auburn, NY), delivery of light via multilegged fiberoptics has allowed systems developers to increase processing speeds in a cost-competitive fashion. Shawn Pearsall, regional manager at Danfoss Videk (Rochester, NY), cites document inspection as an example. "If a bank check was imaged with one strobe light, there would be a greater amount of light in the center and dimmer light toward the edges. But several areas of the check--a barcode or digits--may need to be examined.
To solve this problem, Danfoss Videk uses multiple fiberoptic legs emanating from a single light source, enabling even illumination to be guided to a number of individual target areas with one strobe light. Processing throughput is increased because several areas can be inspected simultaneously, and the end user does not have to pay for multiple strobe capability. Fiber optic lightlines are also used in web-inspection applications to guarantee uniformity across large areas (see Fig. 2).
Jeff Synder, manager of applications engineering at Acuity (Nashua, NH), says that when two or more machines are deployed on the factory floor, it is important that they digitize images in the same way. Uniformity can be enhanced by using fiber-bundle lighting. In response to such needs, Fostec recently introduced multileg bundles calibrated to within 5% leg-to-leg uniformity and within 10% unit to unit .
Area and speed
Before making any choice of lighting, it is important to define what area needs to be illuminated, says Monarch `s Dick Peters. "The area illuminated determines what type of lighting needs to be used," he says.
As an example of the scale of some applications, Peters cites one developer involved with illuminating helicopter rotors. By spraying water droplets onto a rotor in freezing weather conditions, the effect of icing on the rotors can be studied. "When designers need to illuminate large areas, parabolic reflectors are a good solution,"© says Peters. Alternatively, if the application calls for illumination on a microscopic scale, fiberoptic bundles are the most appropriate choice. "To look at blood flowing through the arteries of animals, a fiber-based strobe can be used to effectively stop motion," he says.
It is important to determine the area of illumination at the flash rate needed to capture the image. "Speed is important," says Peters. "For ultrahigh-speed photography, where 400-1000 frames per minute are required, a 20-30-ms flash duration will suffice," he says.
System developers should also not overlook ambient lighting conditions. "Many people using a strobe cannot eliminate factory lighting so it is important to incorporate other light sources into the equation. Where ambient light sources interfere with strobed lights, it may be necessary to enclose the imaging area to keep out other light sources," says Peters.
According to Fostec`s Timothy Ormsby, demands of machine-vision-systems designers have led to the development of more regulated dc light. Today, higher demand coupled with newer light sources has enabled fiberoptic companies to offer dc-regulated light level at a price comparable to ac-regulated lighting. With such choices, the future seems bright for vision-systems designers and lighting suppliers.
FIGURE 1. When back lighting is used, light must be uniform. Otherwise, an inspection system may not detect the difference between a reduction in light intensity and a defect. To overcome these obstacles, High Tech Control Systems used xenon lamps in conjuntion with two Fostec 27 ¥ 27-in. back lights in a vision system to inspect consumer products.
FIGURE 2. Variations in surface color or plating can cause blooming when white light sources are used for illumination. In such applications, objects are better illuminated with monochromatic light-emitting diodes (LEDs). Cost-competitive with xenon, halogen and fluorescent sources, LED lighting has a lifetime measured in years. In inspecting stamped parts, 25%-30% greater measurement repeatability can be achieved using LED as opposed to xenon back-lighting. Fig a and b shows a watch battery illuminated by a fluorescent and an LED ring-light, respectively. LED light provides sharper reflected image edges and improves measurement accuracy because of consistent light output from flash to flash.
FIGURE 3.Many systems integrators have addressed Web-scanning applications with small fiberoptic light lines placed side by side below the bottom roller to create long strips of light. But a dip in the light, at each point where the units meet, requires the use of multiple line-scan cameras (not shown). In response, new longer continuous light lines from Fostec, up to 40 in. long, have been developed to provide uniform light in applications such as this photographic-film scanner from Data Network Systems.
Illuminating lighting choices for web inspection
System repeatablity presents a significant problem in harsh industrial applications such as web inspection. To successfully light such systems, illumination must be repeatable and reliable. To accomplish this, systems must incorporate real-time light-feedback controls to preserve lighting-intensity levels in conditions of changing temperature, power-line fluctuations, and lamp degradation.
Many different lamp technologies are available, each with their own trade-offs. Factors that must be considered in specifying lighting technology are lifetime, color, frequency, intensity, stability, reliability, and cost. Priorities vary depending on application type. In some industries, speed is the most important factor, and, therefore, intensity is of highest priority. In color web applications, spectral content and stability is of major importance. In almost all cases, lifetime and reliability rank highly.
Several lamps are used in web-inspection applications, including metal-halide, xenon, tungsten-halogen, fluorescent, and high-pressure-sodium (HPS) lamps. Metal-halide offers a very white source (5600 K) and has a lamp lifetime of approximately 10,000 hours. Its 50-kHz output frequency makes it a good choice for color line-scan applications. It is not an overly expensive option, but the color temperature of metal-halide lamps will shift approximately 300 K or more during its lifetime.
Xenon is a very white source (4000 K) with excellent focusing capabilites. Xenon is the best choice for high-intensity applications that require fiberoptic lighting. Because the lamps use a single-discharge gas, color temperature stability is superior to that of metal-halide lamps, making xenon a better choice for high-performance color applications. However, lamp lifetime is only on the order of 3000 hours, replacement costs are high, and power supplies are expensive because the lamps require high current at low voltage.
Tungsten halogen is an inexpensive technology that provides a stable spectral output that can be focused into fiberoptic assemblies to create high-intensity light. Because of its low cost and ease of regulation, the tungsten halogen lamp is the workhorse of many of today`s vision systems. Shortcomings of this lamp technology include relatively short lamp lifetime, poor blue spectral output, and 8% overall efficiency.
In fluorescent systems, an apertured fluorescent configuration enhances light into a strip that increases the intensity of light by a factor of four over ordinary fluorescent systems (see figure). Although the technology is inexpensive, it provides excellent uniformity over long web widths. However, such flourescent systems are extremely temperature-sensitive, and the color temperature drifts with age. The lamps can be difficult to obtain because the longer lengths are special order. They also must be placed close to the web, and, because they are fragile, they cannot be used in many industrial applications.
Of all these lamp technologies, high-pressure sodium offers the highest efficiency available and lifetimes in excess of 25,000 hours. As it is a long-arc light source, it is difficult to focus into fiberoptic waveguides, which translates to lower intensities. In addition, the lamp has very low color-rendering index, which makes it useful only in monochrome applications where color and intensity are not major priorities.
Illumination Technologies Inc.
Capable of providing 25 times the intensity of an apertured fluorescent system, the structured nature of the light line allows small defects such as scratches, smears, pits, cracks, and stretch marks to be detected by a machine-vision system in difficult environments.