Picking the right microscope for the job

Those looking to purchase an optical microscope for inspecting or measuring microparts should expect to do some research beforehand, as the technology has changed markedly the past 20 years and many styles of “mics” are available.

The first task is becoming familiar with the basics of how a microscope operates.

A little background

A microscope is used to perform several fundamental tasks:

• to observe or inspect;
• to measure physical objects and their features, including dimensional relationships with other features in and out of the field of view; and
• to document objects and their features—usually with a camera affixed to the mic—for later evaluation.

[SIZE=1]Inspection of a microtool on a stereomicroscope. Photo courtesy Carl Zeiss MicroImaging.

The basic components are the eyepiece, objective lens, stage (the platform the part being examined rests on) and illumination source.

The eyepiece is a lens at one end of the “tube,” and the objective lens is at the other end. The combination of the two lenses forms the magnification power of the microscope. With a 10× eyepiece and a 20× objective, parts can be observed at 200× magnification. A 10× eyepiece used with a 40× objective supplies 400× magnification.

The higher the magnification of an object, the smaller the field of view, or area, that can be seen at any one time. The detail, or resolution, increases as magnification increases.

Optical microscopes illuminate the subject with artificial light, either a light bulb or light-emitting diode. Bulbs must be replaced relatively often and can frequently heat the part being measured. LEDs solve these problems.

“Next to optics, the most important thing for highly precise viewing is illumination,” said Craig Wollschlager, marketing manager of the Industrial Division of Leica Microsystems Inc., Bannockburn, Ill. “Everyone is switching to LED. It requires less power, is more eco-friendly and provides cost savings.” Halogen bulbs have to be changed every 2,000 to 3,000 hours, he said. LEDs last 20,000 to 50,000 hours.

With an LED, added the product manager of New Milford, Conn.-based Vision Engineering Inc., Joe Michalek, “the color of the lighting is very natural.”

Microscopes can illuminate the subject from above, below, or above and below. For opaque objects, illumination from the top (incident illumination) is necessary. For translucent objects, illumination from the bottom (transmitted illumination) works best.

Optical microscopes are often used in manufacturing applications, primarily in assembly work or quality control. Two common types are stereomicroscopes and measuring, or toolmakers’, microscopes.

In stereo

Stereomicroscopes feature two separate optical paths and incorporate two objectives and two eyepieces that deliver a slightly different view to each eye. This provides the user a 3-D view of the subject.

“You get depth to the item instead of a flat field,” said Ken Kreiman, product manager at Cole-Parmer Instrument Co., a Vernon Hills, Ill., distributor of laboratory/industrial instruments and equipment. “The stereomicroscope works just like your eyes. Your eyes see two images and your brain makes it one 3-D image.”

Stereomicroscopes are available in two basic configurations. The Greenough design has two converging optical lens systems for 3-D viewing. The Common Main Objective (CMO) design has two parallel optical lens systems for 3-D viewing. Because the two image paths are parallel rather than converging, imaging equipment can be added.

“You can easily integrate a camera onto a Common Main Objective microscope so you can document what you are doing,” said Michael W. Metzger, general manager of sales and marketing at Nikon Instruments Inc., Industrial Systems Group, Melville, N.Y. “You really should not put a camera on the Greenough-style scope, because it would have to be placed normal to the converging angle of the optical path in order for it to properly capture the image.”

Stereomicroscopes generally are used for visual inspection, not measurement. “With a stereomicroscope, you can do visual checks but there is very little dimensioning,” said Nikon Technical Sales Specialist Dennis Blahunka. “They are mainly used for visual inspection of defects, imperfections and impurities.”

Vision Engineering’s Michalek noted, though: “You can use [stereomicroscopes] for basic measurement by [installing] a retical in the eyepiece. (A small glass circle etched with fine measurements that fits within the eyepiece.) This superimposes a measurement scale, or ruler if you will, along with the subject image. But the accuracy of those types of measurements is going to be far less than what you get with a measurement microscope.”

Leica Microsystems’ high-end stereomicroscope is the fully automated M205A. (There is also a manual version.) The M205A features FusionOptics. “Without expanding the body of the scope or changing optical interfaces, we’ve been able to increase magnification without losing resolution and depth of focus,” said Wollschlager.

The M205A offers a 20.5:1 zoom and the highest magnification is 320× with a 2× objective. “But the resolution is what is really important,” said Wollschlager. “You have the 320× magnification with resolution of less than half a micron. With FusionOptics, we have increased the resolution to 1,050 lp/mm (lines per millimeter),” allowing the user to view a structure as small as 476nm.

Other high-end stereomicroscopes include the Lynx from Vision Engineering and the Stereo Discovery from Carl Zeiss MicroImaging Inc., Thornwood, N.Y.

The Lynx line features a 7× through 40× stereo zoom-magnification range. “You can go up to 120× or 160×, depending on the configuration, but in the medical device world, we see a lot of 7× through 40× or 9× through 60× magnification,” said Michalek. Besides the medical device market, Lynx is used in general manufacturing and the electronics, precision engineering and plastics industries.

Zeiss’ Stereo Discovery V20 features a zoom factor of 20. Resolution is 1,000 lp/mm, with a maximum magnification of 345× (eyepieces, 10×). It also is flexible in that it can be easily switched from “stereo” mode to “macro on-axis” mode, which means a single optical path that’s perpendicular to the sample. The main industrial applications for the V20 are research and development and failure analysis, as well as quality assurance and high-end production applications.

Measuring down

A measuring microscope utilizes a monoscopic optical lens system wherein one optical path looks straight down at the object. It produces a flat image, allowing very accurate placement of cross lines or edge-detection tools for taking measurements. But 3-D imaging isn’t possible.

“You focus in precisely on the plane that you are interested in to pick up an edge or a feature,” explained Michalek. “You focus in one area, set your readout to zero on that Z-axis and, using the focus knob on the machine, plunge down or come up in height and focus on another plane. [The mic shows the] relationship between those two spots very precisely.”

He said measuring mics provide readings on parts “that are microns in size. You could actually take a reading on a part that would fit on the head of a pin.”

Vision Engineering’s high-end measuring microscope is the Hawk 5000. It provides 3-axis measurements, and its PC-based software integrates familiar interface conventions with data processing and analysis tools, including statistical process control. The video-based Hawk 5000 VED offers advanced edge-detection-measurement tools.

Nikon Instruments’ NEXIV automated video systems handle a range of measurements for mechanical, molded and stamped parts, as well as other types of workpieces. “If someone wants to measure small devices, they can use one of the NEXIV vision systems that would allow them to measure X, Y and Z dimensions,” said Blahunka.

NEXIV systems are available with various zoom systems featuring magnification ranges of 0.5× to 120×. They have no eyepieces. The camera takes images of the part and the software projects them on a screen. The operator looks at the screen and identifies the edge of the part from which to take the dimension and clicks on that edge or feature. “You’re actually measuring the image of the part, not the part itself,” Blahunka said.

“These vision systems are fully automated,” he continued. “If someone needs to measure 50 parts at a time, they can write a program and the program will automatically measure all 50 parts, or measure the key features or measure to the dimensions of the features of the parts. The automation removes human subjectivity. The computer makes the decision whether a dimension passes or fails.”

The NEXIV is a turnkey package that comes complete with hardware, staging, camera, optics, computer and workstation. The system can measure down to the micron level and is recommended for aerospace, automotive, medical and stamped parts.

Carl Zeiss MicroImaging offers another high-end approach: the Axio CSM 700 true-color, confocal microscope.

“The CSM system’s [operation is based on] the confocal principle, which means it only allows the camera to see the image that is in the shallow depth of focus,” said Zeiss Product Manager Tom Calahan. “It takes multiple images while moving the part in the Z-direction and builds a stack of images. From that stack, it reconstructs a 3-D image.”

The Axio CSM 700 microscope is for taking topographic measurements, height measurements and surface measurements. It provides height information in steps from about 20nm up to several millimeters. To complete a measurement, only the highest and lowest surface points need to be set, then the scan is performed automatically. Image color information is also provided.

The system is used for inspecting precision-machined metal surfaces in the machine and toolmaking industry and for R&D, quality control and failure analysis.

“The CSM is not a traditional microscope,” said Calahan. “It doesn’t have eyepieces; it is totally digital. It is designed for routine topographic 3-D micromeasurements. The basic resolution is 20nm and you can scan from submicron resolution up to 15mm, depending on the objective [lens] you are using.”

Costs to consider

When choosing a microscope, cost is, of course, a major consideration. “Optical quality is a major part of a microscope’s price,” said Kreiman. The higher price means you get a better lens. Things will look clearer and you won’t get as much distortion on the edges of the view.”

Stereomicroscopes can range from $1,000 to $15,000. Measurement microscopes can be anywhere from $15,000 to $60,000. Nikon Instruments’ measurement microscopes start in the $20,000 to $25,000 range, and the NEXIV systems start at about $40,000. The Carl Zeiss confocal system runs about $100,000, depending on the configuration.

So how does one choose the best microscope? “You have to figure out the right thing for you, and that is not easy,” said Kreiman. “Especially if you are going to use the scope for multiple purposes.”

Factors to think about include the accuracy requirements of the parts and their size. Another consideration is what the purchaser plans to do with the information the mic provides.

“Are they doing anything with the data once they get a measurement, such as trying to export it to an SPC system?” asked Michalik. “Or are they just visually looking at the result on the display and then making an adjustment to a machine?”

Because there are so many styles and features to consider, Nikon Instruments’ Metzger recommends that those in the market for a microscope seek professional advice. “So many people do not understand optical systems and end up using them incorrectly,” he said.

About the author: Susan Woods is a regular contributor to MICROmanufacturing and Cutting Tool Engineering magazines. E-mail: susanw@jwr.com.

The stereomicroscope works just like your eyes. Your eyes see two images and your brain makes it one 3-D image. The Greenough-style stereomicroscope consists of two converging optical lens systems, each of which incorporates a separate eyepiece and objective. The CMO-style stereomicroscope has a single objective lens through which left and right channels are used to view the object. Each channel operates parallel to the other. The vision system is fully automated. The computer makes the decision whether a dimension passes or fails. Illustration courtesy Florida State University.