Test & Inspection: Advanced Optical Technology Opens New Doors
May 31, 2011
The basic technology for optical measurement of manufactured parts has been commercially available for more than 65 years. As optical technologies have advanced, so have the capabilities of optical inspection and measurement systems. Today’s optical comparators, digital measuring machines and multisensor coordinate measuring machine (CMM) systems are more capable and more accurate than ever before. Recent advancements in optical technologies are enabling manufacturers to perform dimensional measurements on more kinds of parts, more quickly and more accurately.
Lighting the WayThe fundamental principle of optical measurement is that light is manipulated to form a precisely magnified image of an object, and the image is measured, rather than the object itself. So first and foremost, there must be enough light to create an accurate image. In a simple microscope, providing sufficient illumination to image an object is relatively easy: the distance between the object and objective lens is very short, the optics are small and of good quality, and the image sensor-the human eye-is very sensitive, even in low light situations.
In practical industrial applications, providing sufficient illumination is not as easy. Measurement systems must be designed to accommodate a wide range of part sizes and geometries, and typically offer a longer working distance between object and lens. This typically requires much stronger illumination. Optical comparators use several additional lenses to relay the image to the viewing screen for measurement. Even the best quality optics have some degree of transmission loss, and the higher the magnification, the greater this effect can be.
For years, optical comparators have used mercury arc lamps to provide high intensity light, which could project over long distances. While mercury arc lamps still have their uses today, they are costly, they create environmental and disposal concerns, and their intensity degrades with time. Arc lamps are more difficult to automate too because their intensity cannot be controlled directly by varying current or voltage. Optical filters must be interposed to lower the intensity to suit the situation. Most significantly, mercury arc lamps generate a significant heat load which, if transferred to the part under inspection, can cause substantial dimensional changes.
In the 1980s tungsten halogen lamps became commonplace in optical comparators and the new generation of video measuring systems. Tungsten lamps offered the advantage of cooler operation, lower cost and very consistent brightness over their service lifetimes. The performance of tungsten-halogen lamps has improved considerably over the years. However, the typical 12V or 20V halogen lamp is still limited in its lifetime and brightness, and will generate appreciable heat, requiring lamps to be remotely located to avoid heating of parts being measured.
In the 1990s, LED light sources became readily available and quickly found their way into many video-based optical measuring systems. These early LEDs were not very bright, producing less than 100 lumens at normal output, and could be costly and difficult to replace in the event of a failure. While adequate for backlit video measurements, those early LEDs lacked the output to be practical for surface measurement of diffuse materials, and never achieved the brightness to illuminate the screen of an optical comparator.
Today’s high-brightness LEDs resolve essentially all of these drawbacks. They are very bright and produce very little heat load. They are reliable, compact and can be arranged into arrays allowing control in myriad combinations and intensities to suit the application at hand.
Small LED arrays can produce more than 1,600 lumens of intensity in broad spectrum or monochromatic wavelengths. They can be multicolored, enabling use of color discrimination to improve image processing capabilities.
The technology of LEDs has advanced so that high brightness LEDs can now be used as the exclusive light source in optical comparators, vastly simplifying the operation, maintenance and safety of comparators, even at high magnifications.
Seeing the Big PictureVideo camera technology also has made big advances in the past few years. The cameras used in early video measuring systems were far less sensitive than those available today, and whole classes of materials-such as black plastic-were difficult to illuminate brightly enough to make automatic measurements. Elaborate processing schemes were needed, such as storing and summing-up successive frames of video to “boost” image brightness.
Today’s imagers not only work in lower light, they also have substantially more pixels packed into the same size imaging array. Advanced fabrication technology enables very high resolution silicon-based cameras to be produced quickly and cheaply. However, metrology cameras have lagged behind consumer cameras in terms of pixel resolution for good reason: metrology cameras have far more stringent requirements than ordinary imagers.
Cameras well-suited for measurement must have very symmetrical pixels, minimal spatial variation and be thermally stable. They must offer automatic gain control, and have high-speed interfaces, such as FireWire or GigE, to enable fast transfer of their massive dataloads to the image processing system. Most important, cameras for precision measurement must have proven stability over time. Thus, evaluation and selection of metrology cameras takes considerable time and care.
Until recently, VGA format cameras (640 x 480 pixels) were the industry standard for metrology, but increasingly, megapixel format cameras-1.0, 2.0, 5.0 or higher million pixels-are becoming reliable and reasonably priced alternatives. The improvement in pixel resolution offers a number of practical benefits for measurement.
First, with improved pixel resolution comes the potential for greater measurement resolution. Because most image processing algorithms are capable of 10:1 or better sub-pixel resolution, working with a smaller pixel enables even finer resolution. A more subtle benefit is that virtually all image processing algorithms rely on having some minimum number of pixels in transition to accurately determine an edge location. With smaller pixels, the minimum pixel requirement can be met with a more sharply focused image, making these cameras better suited to use with high-magnification microscope optics for very small feature size measurements.
At the other end of the scale, a higher-resolution camera often allows use of a lower magnification for larger feature size measurements, thus providing a larger viewing area with equal or better resolution than would be afforded by a standard format camera.
Sharpen the FocusVery high resolution cameras place other demands on an optical system designed for dimensional measurements. In most cases, it is not sufficient to simply put a multimegapixel camera on a standard optical system. Much like point-and-shoot digital cameras, the quality of the optics is equally or even more important than number of pixels when it comes to image quality. To take advantage of the new megapixel camera technology, the imaging optics must deliver image resolution that is matched to the camera resolution.
Higher optical resolution is achieved by increasing the numerical aperture (N.A.) of the optical system, and this is more easily done by increasing the magnification. However, increasing magnification is not always practical in industrial applications. The higher the magnification, the smaller the viewing area, so more stage motion is required to measure large features. Such motion not only takes time, it also adds more variability in the measurement accuracy. Thus, the challenge to designers of optical systems for precision measurement is to provide high resolution with a reasonably large field of view and practical working clearance
High-Speed ImagingThe combination of very high intensity lighting, high-resolution optics and cameras presents another opportunity for optical metrology-the ability to measure objects in motion. While speed has always been a hallmark of optical measurement, throughput is still limited by the need to move the part stage so that the desired feature is in view, then stop and settle before a video snapshot is acquired. While this technique is adequate for many classes of parts, those with a very high feature density, such as wire-bonded chips or printed circuits have always presented a dilemma-either compromise the number of sites measured, or take the time to measure all of them. Faced with this choice, all too often manufacturers are forced to accept the compromise of a smaller sampling rate to be able to acquire data within a reasonable time.
The availability of cool, high intensity lighting and high speed, high resolution cameras changes this picture. Combining these technologies offers the ability to continuously acquire images without actually stopping the part stage. This technique, known as continuous image capture, can offer as much as 100 times faster measurement speed compared to traditional move-and-measure techniques. However, implementing this technique is not simple. Because the part is never still, the illumination must strobe with short bursts of very high intensity to prevent “smearing” of the image as frames overlap. In theory, the light pulse must be infinitely bright for an infinitely short period of time.
High brightness LEDs make this possible, but the LEDs themselves must be capable of being driven very hard, and the drive electronics must have fast switching speed and high power output. Only recently has LED technology reached the level that reliable, low-cost devices are available that meet these requirements. In addition to the LEDs, the video, lighting and stage servo motion controls must be synchronized such that each frame can be processed without smearing and with sufficient overlap so that there is no gap when the strobed images are stitched together. For maximum efficiency, the image processing software must be capable of stitching the strobed images into one large image, and processing the indicated measurement locations in parallel with data acquisition.
Direct from CADAnother optical technology that has advanced the state of the art is image projection, based on the digital light processing (DLP) technology invented by Texas Instruments. Traditionally, optical comparators use custom-made chart gages to overlay dimensions and tolerance zones on the projected image of a part. These go/no-go charts enable an operator to quickly determine a part’s tolerance condition and clearly see where there is too much or too little material. While highly accurate, a special chart is needed for each and every part inspected, at each inspection magnification. For parts larger than the viewing area, multiple charts are required.
Now imagine if it were possible to directly project an overlay chart from the computer-aided design (CAD) model, which could move as the part stage is moved. Indeed, today’s most advanced optical comparators can do just that, using DLP technology. Projecting an accurate, undistorted CAD image is not simple, but with attention to the right details in the optics, and with software correction for certain effects, very accurate overlays can be projected directly from CAD.
Benchtop ConvenienceThe availability of these new optical technologies is enabling a new class of measuring machine known as the digital measuring machine (DMM). These compact, benchtop measuring systems combine high-resolution cameras and high brightness LEDs with unique optical systems offering a very large viewing area, to enable virtually instantaneous measurement of complex parts. Because DMMs typically do not have moving stages, very little effort is needed to set up automatic measurement routines.
The key technology in the DMM is the telecentric, long working distance optical system. With this type of optical arrangement, all of the measurements on a part more than 3-inches wide and 2-inches thick can be performed in one video snapshot without the need to fixture the part.
What makes this large viewing area practical is telecentric design with very low distortion across the entire optical field. Unlike conventional vision systems which require all feature measurements to take place at or very near the center of the field of view, the DMM optics allow a feature located anywhere in the viewing area to be measured with the same resolution. Digital zoom provides multiple levels of magnification, enabling processing of the entire area image with high resolution and small effective pixel size, all with no moving parts.
Also key to the capability of these DMMs is their measuring software. The biggest hurdle to overcome in making advanced optical technologies universally accessible is to simplify the setup and operation of automatic measurement routines. Advances in image processing and metrology software now enable measurements to be made without explicit programming steps. For example, the capability for feature extraction-the ability to automatically identify and measure primitives such as lines, arcs and circles from within a video snapshot-have existed for many years. A recent advancement over simple feature extraction is the technique known as auto-correlation, which “remembers” groups of primitives and their relationships within a scene, enabling recognition of parts, regardless of orientation.
While many of the basic technologies outlined here have existed in various forms for some years, recent times have seen considerable refinement of the raw technology into practical, user-oriented measuring systems that are easy to use and cost effective. Manufacturers today no longer have to compromise accuracy for speed, or resolution for flexibility when selecting an optical measuring system. All these technologies are available in one package, ready to transform the art of measurement into the science of process control. Q
Tech TipsToday’s optical comparators, digital measuring machines and multisensor coordinate measuring machine systems are more capable and more accurate than ever before.
Today’s LEDs are reliable, compact and can be arranged into arrays allowing control in myriad combinations and intensities to suit the application at hand.
To take advantage of the new megapixel camera technology, the imaging optics must deliver image resolution that is matched to the camera resolution.