Metrology With White Light
Noncontact metrology uses 'white light' to measure products ranging from airplanes to contact lenses
By Bruce Morey
White-light metrology is a noncontact method that uses a familiar, safe light source—simple white light. Simplicity and accuracy may make white light the system of choice in many applications.
Structured white-light systems project shadows of lines from a 2-D lens onto a 3-D surface. The shadows are created by simple overlay slides or from digital projectors. Cameras view distortions of these 2-D lines on 3-D objects, and then apply sophisticated mathematics to provide point clouds over surfaces.
One such system is the Advanced Topometric Sensor (ATOS) provided by Gesellschaft für Optische Messtechnik (GOM; Braunschweig, Germany), which offered its first systems commercially in 1995, and has delivered 2500 systems worldwide through a network of 30 international distributors. "This equipment is not that new, but it's new to many people," explains Marc Demarest of Capture3D (Novi, MI), the North American distributor and integrator for GOM equipment. "GOM's equipment uses structured white light with a heterodyne phase shift algorithm to extract 3-D points. The sensor uses a central light projector flanked by two imaging cameras. This stereo arrangement helps with depth perception and protects process and data security," notes Demarest.
The ATOS technology comes in five configurations, capable of collecting from 800K points per scan up to 4M points, able to scan in less than 1 sec. Adjustments in scan volumes yield different point densities and accuracies. For example, the smallest scan field-of-view is 30 x 30 mm with a resulting point density of 0.015 mm. The largest scan field-of-view is 2000 x 2000 mm with a 1.0-mm point density. "Roughly speaking, for objects that fit in your hand, ATOS delivers measuring accuracies better than or less than 25 µm [1 sigma.] For objects as large as a car body, you can expect accuracies about 125 µm [1 sigma]," explains Demarest.
Alignment targets—white circles with black dots that stick onto an object.aid in extracting data more efficiently and compositing multiple views into a single data set. "With the recent developments of the ATOS software, targets are not necessary to collect data. Our system software can use shape data as well." Capture3D also offers fixturing frames with alignment targets where parts sit inside, eliminating the need to stick alignment targets on the object.
Like other scanning metrology systems such as laserscanning, Demarest notes that complicated or smoothly varying objects such as turbine blades, car bodies, many aircraft parts, or medical devices—even toys—are very well suited for this technology. He sayss that typical applications include first-article inspection, CAD comparisons, root-cause analysis, reverse engineering, and fingerprinting of stamping dies, among many others. While these are all typical uses of scanning-metrology-type technologies, he contrasts structured white light with laser scanners. "Data from structured white light tends to be smooth, without noise or texture, and typically scans faster. Sometimes when people see that data they mistake it for [originally authored] CAD data."
Structured white-light systems are, by all reports, used in both quality control and in production environments. Like many optical-scanning systems, they have difficulties getting into deep crevices and pockets due to line of sight limitations. To address the issue, GOM provides a hand-held touch-probe add-on with alignment targets for its systems.
Breuckmann GmbH (Meersburg, Germany) is another provider of structured white-light systems that uses a Miniature Projection Technique (MPT) to convert 'zebra stripe' patterns into 3-D metrology data. Their North American reseller, Exact Metrology (Cincinnati, OH), uses these systems as one tool among many. "We use 11 different metrology technologies here at Exact, including portable CMM arms, laser scanners, and optical trackers, as well as the Breuckmann structured white-light systems," says Matt Cappel, general manager for the service bureau at Exact Metrology.
With this experience, he agrees that structured white light is best for what he describes as 'organic' parts. This includes turbine blades or orthopedic medical devices—where many points are needed to accurately define complex shapes. "For prismatic parts with sharp straight lines or well-defined cuts and pockets, a touch-probe CMM is usually better and faster. Scanning methods like laser and white light typically have a little more difficulty with sharp prismatic edges."
Practically speaking, shiny surfaces, or objects with alternating colors also pose some problems for structured white light. "Typically, any structured whitelight system is best when the surface of the object is coated a gray or white color," says Cappel. He goes on to note that Breuckmann offers a stereo Scan 3D HE device especially designed for shop lighting and to accommodate colors on the objects scanned. Cappel also points out that alignment targets are also an option with Breuckmann equipment.
Exact Metrology typically delivers systems that include National Institute of Standards and Technology (NIST) certified artifacts for frequent calibration in-situ. "With structured white-light systems," explains Cappel, "I can calibrate every shift, even every lens change if I wish. With other systems, like lasers, you can determine if the sensor is okay, but, if it goes out of calibration, they have to go back to the manufacturer to adjust it." He reports the Breuckmann 5.0 Stereo Scanner delivers accuracies with field-of-view of ~ 775 x 775 mm to 50.8 µm to 3 sigma when measured in a full gage R&R study. For FOV of ~ 42 mm x 42 mm, it delivers 12.7 µm accuracy (3 sigma.) "The Breuckmann [high end systems] can collect as many as 6.6 million points per scan, which typically takes about 10 sec, depending on the application and the system."
Mark Bliek of Bolton Works (East Hartford, CT), a reseller for Steinbichler (Neubeuern, Germany), another maker of structured white-light scanning systems, says: "Structured white-light devices typically gravitate to meet high-resolution and high-accuracy requirements." He notes that structured white-light scanners are, in general, more accurate than laser scanners, because the white-light scanners are stationary at the time of measurement. "During data acquisition with a laser scanner, you need to keep track of where the sensor is relative to the workpiece," leading to some loss in accuracy. The advantage for hand-held laser scanners is they are typically more flexible and faster.
Do not count out CMMs yet, says Bliek. "CMMs are still the workhorses for geometric quality control," he explains. "The calibration of a CMM can be traced back to NIST standards, and therefore the industry is comfortable with the measurements made with a CMM. The measurement uncertainty of a CMM is in general lower than for a scanner. There are more variables in regards to scanning equipment that can influence the measurements—operator, light environment, surface reflectivity, angle at which a surface is scanned, and so on." He goes on to say that while this observation might strike a cautionary note, the advantages of using white-light scanning equipment are such that a bright future lies ahead—especially when prices come down. An operator does not need to understand the critical dimensions to collect useful data. The large volume of points collected by a scanner not only allows extracting dimensions; it also will help to pinpoint causes when dimensions are out of tolerance, as one can look at the surrounding area.
Interferometry is another instance where white light is used instead of lasers. For example, Zygo Corp. (Middlefield, CT) offers its NewView 700 and NewView 7000 series surface profilers built around scanning white-light interferometry (SWLI). "SWLI was developed after laser interferometry was introduced," says John Roth, a director with Zygo. "For rough surfaces, it turns out that laser interferometry tends to break down. SWLI is well-suited for small, precision-made parts, and laser interferometry is good for smooth and nominally flat surfaces, like glass." Their machines use fields-of-view that range from less than 3.5 to 25 mm with sub-nanometer precision. Typical machined parts they measure include precision automotive parts such as fuel injectors.
"Worldwide, a significant portion of our precision machining business is automotive, including superfinished parts for diesel engines or gasoline sealing surfaces," remarks Mike Schmidt, senior applications engineer, noting that Zygo developed an optical diverter to enable measuring inside deep holes or inlets, especially highpressure bores. In addition, options are available for combining machine vision to exploit the 3-D data from SWLI to provide accurate dimensional data as well. "In certain applications, SWLI provides better dimensional measurements [than machine vision] because we use the height information." He sees current trends involving delivering systems that take more measurements on a single instrument. "Customers place a value on single setup, on a case-by-case basis. Our systems are like Swiss army knives with a number of objective lenses, to measure roughness, waviness, or flatness in a single setup."
In a twist on white-light interferometry, Optical Gaging Products (OGP; Rochester NY) developed what Tom Groff, their metrology product manager, calls a quasi-white-light interferometer. "The Telestar TTL sensor is not really a laser because it uses and exploits multiple wavelengths," he explains. It uses interferometry to determine distances while functioning as a through-the-lens laser for the company's video-based multisensor measurement systems. Its advantages, according to Groff, include longer working distances than other noncontact sensors with similar accuracies. Used to measure transparent surfaces and employing light bounced off mirrors to reach recesses and internal features, the system's working distance ranges from 19 to 200 mm, and accuracies range from 2.5 to 4 µm. "You cannot as easily use mirrors with true lasers or other white-light technologies, because distortion from those mirrors affects their measurements," comments Groff. He describes this technology as especially applicable to working on complicated prismatic parts, such as measuring O-ring grooves in castings. Its small spot size of 2.4–10 µm lends itself to measuring small, deep holes that are inaccessible when using other probes and sensors.
Another use of white light is to capture fine measurements using the principle of chromatic aberration. This technique shines white light through a lens that acts as a prism, separating the light into colors. Each color will focus at a different distance from the surface. The detector determines color to measure distance. These sensors are typically offered as part of a multisensor system.
"We offer our Rainbow probe in a number of models," says OGP's Groff. "Our Rainbow CL3 offers an axial [height] resolution of 0.025 µm with a best accuracy of 0.2µm [3 sigma.]" One of the tradeoffs he is quick to point out includes consideration of measuring range—the Z distance within which good measurements can be made—which is a function of the focusing distance or focal range. Another consideration is the working distance, the distance from the receiving lens to the part measured. For instance, for the Rainbow CL3, the measuring range is 1.1 mm at a working distance of 12.7 mm. Probes with larger working distances and measuring ranges offer less resolution and reduced accuracy—the CL6 has an accuracy of 3 µm within its measuring range of 20 mm. "Rainbow Probes lend themselves to specialized applications. They detect transparent surfaces, such as glass and thin films. Since they detect both the top and bottom of such surfaces, they can measure film thicknesses without a step height."
Hexagon's (Elgin, IL) Optiv brand also offers a Chromatic White Light Sensor (WLS) based on the principle of chromatic aberration, for use in its Optiv multisensor metrology systems. Buck Burgraff, vision specialist for Hexagon, notes other advantages of this type of sensor. "Compared to lasers, our WLS gives you more flexibility in scanning. For instance, it scans faster than a singlepoint laser, scans steeper surface inclinations, scans over voids, and not only can you detect transparent surfaces, the device is also better than lasers on shiny surfaces or matte black." A WLS offers a measuring range/working distance that varies from 300 µm/4.5 mm to 10 mm/75 mm, and resolutions as fine as 0.01 µm. "We have used WLS mostly for orthopedic implants and some automotive applications. We have one customer using it to reverse engineer a bone screw."
This article was first published in the July 2010 edition of Manufacturing Engineering magazine.