by Kennedy Smith
coffee filter manufacturer needs to measure the diameter of laser-applied holes in its product. An automotive-painting company wishes to measure the thickness of a clear protective topcoat. A paper company wants to see how much ink bleeds on a magazine cover. A medical equipment manufacturer needs to see just how sharp its eye surgery needles are.
These issues are common in the manufacturing industry, and microscopes provide a common solution. Put simply, if you need to look at something up close--very close--you need a microscope.
"From the standpoint of all things on Earth and investigations into their minute structure, all microscopes can actually be considered quality control/quality assurance
instruments," explains Gary Czarnecki, Finescope product supervisor at Mitutoyo America Corp. "Regardless of the sample, microscopes reveal micro-structural detail that is used in an
extremely wide range of qualitative and quantitative report criteria, from blood samples to metallurgy."
One difference between a microscope used in quality control compared to one
used for biomedical and scientific purposes is the sample itself, notes Czarnecki. In the quality assurance laboratory, many of the specimens (i.e., steel and silicon wafers, semiconductors, tool
edges, etc.) are not transparent, so viewing them is fairly simple: You shine a light on them. However, in the biomedical research lab, the samples are often transparent, such as living tissue
and cells. So the researcher needs to incorporate more elaborate illumination techniques to properly observe the object. This is not to say, however, that quality control inspectors don't also
incorporate sophisticated illumination techniques in their applications.
Anatomy of a microscope
A typical microscope
consists of a tube extending down toward a flat plane called the stage and up toward the eyepiece, through which the user views an object. At the distant end of the tube is a turret of objective
lenses. Another lens near the eyepiece further magnifies the image, so that full magnification is determined by multiplying the power of both lenses. For example, if both lenses magnify to the
10th power, the total magnification is 100X. A light source on or around the stage allows the user to see specific features of the object being viewed. This basic structure has been manipulated
over time to create specific types of microscopes for particular uses.
Pick a microscope, any microscope
microscopes work the same way, and it's important to note the differences in order to determine which one is best for a given application. Two common types of microscopes are stereo microscopes
and compound microscopes. Stereo microscopes display images in 3-D; compound microscopes don't.
Stereo microscope. "Stereo microscopes are ideal for inspecting 3-D objects," notes Rick Coyle of Vision Engineering Inc. Because a stereo
microscope has two separate optical paths, the user sees a 3-D image. "For example, it's ideal for looking at a part for scratches or if a solder joint has the
proper flow of solder," says Mike Metzger, measuring department manager at Nikon Instruments. The stereo microscope typically has a significant amount of
working distance between the lens and the part, so the user can easily manipulate the part by hand.
Compound microscope. The compound microscope, when used for metallurgical work, is called a metallograph. For metallurgical applications, the
compound microscope would be used to look at the grain structure of metal or the structure of composite materials. "It was mentioned in all the hype around the
Titanic," notes Metzger. "They used the metallograph to test the quality of the steel."
Because compound microscopes have higher magnification capabilities, they're
also used to examine very small features on very small parts. For example, an inspector may use a compound microscope to discover patterns or minute flaws on semiconductors.
Measuring microscope. This is a compound microscope with measuring
capabilities. Often referred to as a toolmaker's microscope, the measuring microscope can be used with the compound microscope lenses or with special
toolmaker's lenses. The difference between the typical compound or stereo microscope and the measuring microscope is that the stage is equipped with linear
scales to measure the stage motion. Not only does the user see a magnified object, but he or she also can move the stage and measure positional differences.
"Incorporating a linear scale into the microscope was a significant accomplishment for improving measuring accuracy," says Metzger.
They're called toolmaker's microscopes because of early applications that required a toolmaker to closely examine the edge of a cutting tool or examine the
outcome of using that tool. When a toolmaker looked at an edge created by a cutting tool and saw that it was accurate and reasonably smooth, he or she was
then able to determine that the instrument had been correctly fashioned.
Measuring with a microscope
Collecting data from a measuring microscope is similar to the process used with optical comparators. Linear scales are built into the stage and often into the
focusing mechanism of the microscope. With a computer attachment, the scales automatically measure and record the distance each time movement occurs. "It's
the ability to measure that motion that makes this tool not only an important imaging device, just like a compound microscope, but also a measuring device," explains Metzger.
For further data collection, an image capture device, such as a video or digital camera, can be mounted onto the microscope. "I'd say that 50 percent or more
of our microscopes are sold with a digital camera for documentation," adds Coyle. The captured images can be stored, compared against other images,
e-mailed or projected on a screen for viewing by more than one person at a time.
Integrating of software is another way to examine data. Once a video or digital
camera captures an image, it can be incorporated into measurement software for analysis. "Special programs even allow us to measure and record material under
stress over a period of time," notes Douglas Knight, senior R&D consultant and former chairman of Questar Corp. Combining video systems with microscopes is
one way of monitoring subjects with a timeline. "You can easily inspect an object over a period of an hour or compare the repetitions of a motion," he says.
Another important feature of a measuring microscope is the capability to measure the Z-axis, whereas an optical comparator only measures X and Y.
How microscopes measure up
A measuring microscope allows higher magnification than an optical comparator
and enables the user to see not just a shadow of the object, but details on the object's surface. With an optical comparator, you can project the image on to a
ground-glass screen, but with a measuring microscope, you can project the image to a video camera or digital imaging device. "This bridges the gap between an
optical comparator, in which you can only look at the screen, and a vision system, where you can only refer to the digital imaging device," says Metzger. "With the
microscope, you can bring the image up to your eyepiece or you can take the image up to the computer."
The difference between a microscope and a vision system is readily apparent
when using a stereo microscope for inspection. Once a camera is hooked up to the system, you lose a dimension. "As long as you're looking at a video screen,
you're looking at two dimensions," explains Coyle. "You might find a fault, but you'd have to go back to the eyepieces to see it again in 3-D."
Advanced optical capabilities
Basic microscope illumination. Light is shone directly onto the objective.
Light is shone indirectly onto the objective. Light shines at an angle to enhance contrast and reveal scratches, bumps and contaminants.
A variation of darkfield lighting. A series of colored filters creates optical staining of the specimen, providing rich color and contrast between the objective and background.
Light is separated, and when recombined, the interference produced by the two light paths produces contrast between dense structures and the background.
Differential interference contrast (DIC): Polarizing filters and prisms separate and recombine light paths, giving a three-dimensional appearance to the objective.
Hoffman modulation contrast:
Similar to DIC. Plates with small slits in both the axis and off-axis of the light path produce two sets of light waves, and a 3-D image appears.
Polarized light is positioned perpendicular to one another on either side of the specimen so that only light that passes through the specimen reaches the eyepiece. Features within the specimen itself rotate the light, which passes through a second polarizing filter. These areas appear bright against a dark background.
High-energy, short-wavelength light excites electrons within the specimen, causing the electrons to heighten their orbits. When they return to their original energy levels, they emit lower-energy, longer-wavelength light, which forms the image.
Source: Nikon's MicroscopyU, www.microscopyu.com
"People try to replace the measuring microscope with vision systems, but when you
need advanced optical techniques, there's no other tool that you can select," says Metzger. Unlike an optical comparator or video system, a microscope can offer several ways of looking
at an object to highlight specific features. By using specialized lenses or manipulating the projected light, the user sees things otherwise invisible under basic microscopic conditions.
"The greater magnification, the more illumination you need," explains Knight. "There are some very high-intensity lighting techniques,
along with various positions you can put lights in. You can light from behind, from the side at different angles, depending on what features you want to observe."
Here are three common optical techniques used with microscopes:
Brightfield. This directly illuminates the observed part.
Darkfield. This technique doesn't admit light directly to the objective. It's used to examine specimens that can't be distinguished from the background.
Differential interference contrast. This is a polarized-light technique. It yields an image
with a shadow relief, so that variations in optical thickness appear as variations in the brightness of an image. It's often used to examine semiconductors, where it's difficult to develop
contrast between patterns. "This technique can give a 3-D look to a very two-dimensional part," comments Metzger.
"Video microscopy is highly important because it gives you a record," says Knight. "In fact, we very often use video microscopy because in our
customers' terms of quality control, a record of successive events would let you measure changes in a situation."
"Video microscopes are not the standard,"
says Bill Miller of Hi-Scope Systems. "However, the vast majority of microscopes have a video attachment because it's so much easier on the eyes and allows everybody to
look at the same thing at once. You can call your colleague over and you both know that you're looking at the same thing."
Hi-Scope Systems Inc., developer of video
microscopes, manufactures equipment that's slightly different from traditional microscopes in that there is no human eye involved in the optics. Instead, the optical path consists of a
hand-held or mounted device that is held up to an object that is projected onto a video monitor.
Typically, data collection is accomplished through a screen-capture feature, which can be
translated into an electronic file. The file can be incorporated into software programs that take measurements of the part. Or, the video monitor itself can be equipped with an overlay
to check tolerances on a part immediately, much the way an optical comparator functions.
For hundreds of years, the basic structure of the microscope has remained pretty much the same. However, constant improvement has added functionality. Experts
note the most recent improvements in microscope technology:
"The biggest improvement in microscopes is the change from fixed tube length to
infinity-corrected optics," notes Miller. In the past, microscopes had a fixed optical length of about 160 mm from the objective lens to the eye. This meant that
light had to hit the object at just the right angle and distance from the lenses or the user wouldn't get a clear picture. In simplest terms, infinity-corrected lenses
enable the light to emerge parallel or "focused to infinity," within the microscope. And, within this parallel range of light, devices such as beam splitters, polarizers
and prisms can easily be added without the user losing optical quality.
Another improvement is the quality of lenses. "Now, with computers designing
the lenses and computers controlling the machines that actually make the lenses, they are better than ever before," says Metzger. This has paved the way for brighter, clearer imaging.
"The digital age is having an influence everywhere," says Coyle. "We now have the ability to take images from a microscope and input them into a computer
where you can work with measurements."
Miller agrees. "I think more and more microscopes will include video and digital
capabilities," he says. "There's going to be higher resolution, digital output and software integration."
One area of microscopes that has been a continuous issue is the ergonomics of the systems. Simply put, it's tiring on the eyes, neck and back to stare into an
eyepiece all day long. Many traditional microscopes now have a tilting head feature that can be adjusted depending on where the user is seated, the height of
the bench or the angle of the head. Although ergonomics have improved, over time most experts agree there's a long way to go.
With the issue of ergonomics in mind, microscopes at Vision Engineering Inc. have been designed with a special focus on user comfort. The optical image that
the user would normally see through eyepieces is instead projected onto a screen. Unlike a video system, this image is not electronically reproduced but is an actual
representation of a microscope's optical image. "You get the resolution of a microscope with the fatigue-free view of a video system," says Coyle. "It's much
more ergonomic than a traditional microscope."
Seeing the bottom line
Prices for the products discussed in this article range from $1,000 to $45,000. Stages can range from $5,000 to $20,000, which represents a significant
difference in measurement performance. "If a customer needs high magnification and eyepieces that a measuring microscope provides, but they only need low
precision and staging, they can save some money," says Metzger.
What does a quality inspector look for in a microscope? Coyle lists some key considerations when choosing one.
"Can you change lighting easily, should your applications change? Can you add a camera to it? Can you hook it up to a computer?"
Czarnecki explains that the type of microscope required is highly dependent on its intended application. In general, he notes a modern, quality inspection
microscope should offer the following:
A large stable base, with built-in power for a 12V/100 quartz halogen
Capability of both reflected and transmitted light techniques
Bright and sharp images at all magnifications from high to low power
Wide field, high eye point eyepieces that have a field number greater than 22 mm
A wide selection of long-working distance objectives to accommodate a variety of applications
Capability of performing various illumination techniques, including brightfield, darkfield, interference contrast and polarized light
Comfortable viewing position, ergonomic tilting viewing bodies
A large, stable specimen platform with coaxial adjustable X and Y positioning knob
Ability to adapt a camera/image collection system
Aperture and field diaphragm controls for both reflected and transmitted light illumination
Right and left coarse and fine coaxial focusing stage mechanism with a minimum of 0.2 mm accuracy per fine focus revolution
The most important consideration, notes Miller, is that the microscope does the job and does it right. "It doesn't have to be the best microscope or the highest
resolution," he explains. "Ultimately, at the end of the line, you need something that easily shows what you need to see."
About the author
Kennedy Smith is Quality Digest's assistant editor. E-mail her at
firstname.lastname@example.org . E-mail letters to the editor regarding this article to email@example.com .