Since the early 1900s, people in science and industry have used optical measuring and inspection techniques to get enhanced views of their products. These techniques enable users to observe, measure and analyze parts for quality control, quality assurance and other purposes.
Readers familiar with optical measurement will find the following outline of the basic material a useful review, while those who are using or plan to purchase optical measuring equipment for the first time will find the information helpful in pointing out issues relevant to selecting and purchasing the appropriate instrumentation to meet their needs.
Selecting the right measuring techniques
Manufacturing and engineering facilities usually employ both inspection and measurement in evaluating samples. The two evaluative methods give quite different information.
Inspection is designed to answer qualitative questions such as, Is this flat enough? Are there too many pits in this tool? Do the grain and color look right? or Is this contaminated?
Measurement is designed to answer quantitative questions such as, How long is this piece? What is the thread pitch of this screw? or How deep is this hole?
Some questions probe for information that can be considered semiquantitative, such as, Does this match my overlay? or Does this line up at the right place?
Optical measuring uses the human eye to compare specific sample features against standards that are established in advance. This measurement often is just one of many evaluative techniques used in a facility; others include various test, inspection and calibration techniques. Optical measuring is especially significant, however, because in most manufacturing and engineering environments, there is no substitute for visual information in ascertaining future performance potentials for a product or material.
An effective measuring technique will reflect the users' specific needs. Depending on the sample size or the need for color, 3-D accuracy, computer interface, tolerances allowed or throughput required, users should select appropriate techniques for handling their measuring needs.
In many cases, microscopes offer the highest accuracy, resolution and color fidelity of any optical measuring method. Other important technologies include optical comparators and video-based measuring systems. Comparators often are chosen because of their ease of use and cost-effectiveness for measuring parts too large for a microscope. Video measuring systems, though often more expensive than the other two, can present the best solution to users with more complex needs. Video measuring combines a comparator's ease of use with a microscope's reliability, accuracy and precision.
The basics of magnification
Measuring microscopes, also called toolmakers' microscopes, typically examine small objects such as machined or molded parts. Before discussing microscopes, let's review some of the relevant terminology:
Magnification refers to the degree to which a part has been enlarged. Higher magnification does not necessarily mean better image definition, which is a factor of resolution. Higher magnification is commonly and incorrectly assumed to equate to higher resolution and better accuracy.
Resolution is the ability to distinguish between two discrete objects in an image.
Field of view is the visible area when a part is magnified.
Depth of focus refers to the range above and below the point of focus that still appears to be in focus.
In its most basic form, a microscope consists of two simple magnifying lenses placed a specific distance apart. Light illuminates the sample, creating an image that passes through the first lens and is magnified. The magnified image passes through the second lens, which magnifies it again. Then it continues on to the user's eye.
Typical optical magnifications of toolmakers' microscopes range between 10X and 3,000X, with combinations of objective magnifications of 1X to 150X and eyepieces of 10X to 20X.
However, a light source and lenses are just the beginning. A magnified image also must truly represent the specimen in detail, shape and color. So toolmakers' microscopes have filters, diaphragms, lens coatings, special accessories and techniques for observation. These all affect the raw image to allow the user better, more accurate information.
Many techniques exist for illuminating measurement samples. Brightfield provides flat, even illumination of the field of view. In brightfield, it's possible to see clearly where things are located, how they attach to each other and how the sample's color and grain appear. Darkfield illuminates the specimen surface from an oblique angle so that pits in the finish, deviations in flatness and other surface problems are strikingly clear. When examining a machined metal part, brightfield might be used to measure how far two holes are drilled from each other. Darkfield might be used to ensure that the part's metal finish is smooth and free of contaminants.
Special measurement issues
A toolmakers' microscope presents two basic measurement forms -- field of view and stage movement. Some of the best systems incorporate both techniques in one instrument. With field-of-view measurement, precise eyepiece reticles superimpose a pattern or scale over the image. If the entire region to be measured is visible at one time, this method offers a quick, precise way of measuring without the potential inaccuracies that can result from mechanical motion. Special attention should be given to the optics when performing field-of-view measuring; superior optics should provide a flat image field and be free of aberrations and distortions.
Stage movement measurement is used when the feature to be measured doesn't entirely fit in the field of view. Typically, a linear scale or a rotary encoder inside a drum micrometer measures stage displacement as the object on the stage moves past a reference point, usually a crossline, in the eyepiece.
Whatever kind of measurement is required, it is important to use the best optical resolution for feature detection. In addition, the microscope stand should be massive and stable to eliminate environmental variables such as heat, humidity and vibration.
Microscope measuring offers the best resolution, the greatest optical versatility and the broadest magnification range of any optical measuring technique. In addition, microscopes usually are assembled in modular, building-block designs, allowing a company flexibility in putting together its inspection and measuring systems.
Measuring microscopes also have unparalleled documentation capabilities, both for photo and video, and most microscope systems can be upgraded for video after purchase, if desired. When purchasing a toolmakers' microscope, consider an instrument that includes ergonomic features and eyepiece adjustments that allow for fatigue-free viewing over an extended period of time.
Profiling a part
Optical comparators, also called profile projectors, are probably the most common optical measuring tool in the manufacturing industry. In their early days, they operated by projecting an image on a screen with back lighting -- "profiling" the part under evalua- tion -- hence, their name.
Today, these easy-to-use instruments still project an image on a ground glass screen, but they now offer both profile and surface illumination. The images can easily be viewed on the screen in a bright room. Optical comparators are used for every kind of measuring and inspection, from quality control of pasta in packaged macaroni and cheese to measuring disk drive assemblies.
Optical comparators' greatest advantage is their large ground glass screen, which affords easy viewing. A typical optical comparator with a 12" glass screen will image 1.2" of a part being inspected with a 10X projection lens. Standard magnifications range from 5X to 500X, with a measurement accuracy of 0.0001".
The best optical comparators offer parfocal, telecentric lenses mounted on rotating turrets. Features include through-the-lens (on-axis), profile (creating a shadow) and oblique (side-angle) illumination. Versatile comparators also feature built-in digital readouts along with interchangeable stages. Comparators come in all sizes and price ranges, so it's important that the unit's screen size is right for your company's needs and that the optical system is the best and the brightest you can afford.
Aside from the difference in the part sizes that can be accommodated, three major points should be kept in mind when comparing toolmakers' microscopes with optical comparators:
Optical comparators work best when measuring in two dimensions. Toolmakers' microscopes are a better choice when Z-axis height information also is needed.
Toolmakers' microscopes can easily adapt to cameras and CCTVs for photo-documentation. This is not as easy with most optical comparators.
Toolmakers' microscopes offer a number of different optical techniques and can use optics that offer higher resolution for more measuring accuracy.
The future of optical measurement
In some ways, video imaging offers the best features of both toolmakers' microscopes and optical comparators. Video-based measuring systems are becoming increasingly prevalent in government and industry, and may one day serve as the standard for taking measurements.
Video measuring equipment includes stable, heavy-duty stands and video monitors for easily viewing parts. With their excellent online documentation and archiving capabilities, repeatability, and image-processing and manipulation capacities, video-based systems offer extraordinary flexibility.
A fully automated video measuring workstation will create a workpiece coordinate system, adapt parts to their varying orientations, import and export files, and take advantage of computer capabilities such as dynamic data exchange. High-end units provide profile, through-the-lens and oblique illumination as standard features and can operate in manual, semiautomated or fully automated modes.
Video offers other advantages as well. For instance, it can be used not only for measuring but for inspection, teaching and documentation. Also, because users look at monitors, these systems are comfortable to use over long periods of time. Finally, they can include a graphical part display -- a "road map" for measuring sequences.
Shopping for a measuring system
Not surprisingly, a relationship exists between price and performance with measuring systems. An instrument can be purchased for as little as $1,000 or as much as $200,000. Typical sales for precision measuring systems with digital readouts range from $5,000 to $50,000 or more; video measuring systems range from $8,000 to more than $100,000.
Here are some questions to ask when comparing measuring systems:
What measurement tolerances do the parts have? The answer to this question will help distinguish the resolution required. Work with a knowledgeable sales engineer or other person who can help you select an optical system and magnification that fits your needs.
Is it necessary to measure Z heights? If the answer is yes, then consider either a toolmakers' microscope or a video system that includes Z-axis measurement capability.
Is photo-documentation necessary? If so, a toolmakers' microscope or video inspection system should be considered.
What is the X-Y-Z envelope size of the samples? Measurement will be limited by the distance the instrument's stage can travel in each direction.
What are the company's current and future needs? Look for a modular system that can be upgraded. Though you may only need transmitted light right now, eventually you might want to add reflected light. Likewise, documenting findings might not be necessary now, but eventually you might want to have photomicrographic or video records.
Is the manufacturer widely respected? Is the local dealer reliable? No one company makes the best of everything. Look for a manufacturer whose instruments have a reputation for top optical quality and can be retrofitted to meet your changing needs. Also, make sure the dealer's salespeople have the service orientation and the field application knowledge to become your partner.
The essence of a measuring system is its optics, and there is never a substitute for optical integrity. Toolmakers' microscopes, optical comparators and video-based systems can all be effective tools for quantitative measurement in research and development applications. Buy the best optics you can afford. True measuring optics are highly corrected for both spherical (shape) and chromatic (color) aberrations, and will introduce no errors into a quantitative measuring system.
Finally, the instrument you select today should be optimized for the tasks for which you intend to use it. An instrument with a well-conceived design will not only fulfill current requirements but can be easily upgraded as your needs become more demanding.
About the authors
Brad Bartmess is product manager for measuring instruments, and Jack Isaacson is field sales engineer, video measuring, with Nikon Inc. The company offers a broad line of world-renowned optical metrology equipment through its nationwide network of dealers and also provides hardware and software for metrology and inspection in research and manufacturing facilities. Products include measuring microscopes, video metrology systems, optical comparators, digital micrometers and metallurgical microscopes.
Contact Nikon at 1300 Walt Whitman Road, Melville, NY 11747. Readers can request literature by calling (800) 526-4566, ext. P8035. Visit Nikon's Web site at www.nikonusa.com.