Test Drive a Microscope

The tests I’ll describe won’t quantitatively evaluate a microscope the way a quality-control lab would, but they will help you get a “feel” for the quality of a microscope. If you require accurate, quantitative tests, you can use tests described elsewhere.¹

The qualitative tests fall into three categories: ergonomic, mechanical, and optical. But no sharp boundaries exist between them. For example, a microscope that suffers from so much field curvature that it can’t focus an entire image at once is difficult and tiring to use. You can think of “difficult” and “tiring” as ergonomic faults, but they stem from optical causes.

Ergonomics is the science of how easy things are to use. In the case of an optical microscope, ergonomic quality largely results from how well the instrument fits you and the way you work. Since everyone is different, ergonomic quality ends up being a matter of how well a microscope adjusts to individual users.

The oculars should feel like they are the same distance from your eyes, rather than one being closer than the other. The oculars should be close enough to your eyes to completely fill your field of vision, but far enough away so that they aren’t tickling your eyelashes. You shouldn’t feel as though they poke into your eyes.

For proper viewing, look directly into the ocular tubes. If you don’t, the flat image plane will appear tilted. You may see this effect without a sample in the microscope, but you’ll notice it more when using a flat, well-focused sample.

Binocular microscopes typically have an extra focus adjustment on one of the oculars to compensate for differences between a user’s eyes. To adjust the ocular focus, look through both oculars normally, and then close the eye on the side with the extra ocular-focus control. Use the microscope’s main focus controls to bring the image into sharp focus for the open eye.

Then, open your eye and close the other one. Use the extra ocular focus control to bring the sample into sharp focus for the open eye. When you open both eyes, you should see a single, clear, sharply focused image. If your sample has texture, you should see it in three dimensions as a solid object. The microscope controls should make it easy to perform this procedure.

Pay careful attention to eye comfort. The neuromuscular systems that focus your eyes are interconnected. So when microscope oculars aren’t properly focused, your eyes tend to compensate for the “misfocus,” which leads to headaches and eye discomfort.

Also pay attention to how comfortable the controls are to use. A control can be easy to find and easy to move in the proper direction, but if you have to place your hand at an uncomfortable angle to reach it, you won’t like using the microscope.

An inverted microscope places samples upside down on a flat stage behind the microscope’s optics. This type of microscope suits many failure-analysis applications in which users examine mounted or plastic-encapsulated samples. Note the microscope controls on both sides of the housing. (Courtesy of Olympus.)

An upright microscope used to examine semiconductor wafers provides a large stage and space for stage movement, thus making it easy to examine any part of a wafer. The manufacturer has placed controls on the microscope below the stage and on the main microscope housing to the right and behind the oculars. These controls may position filters or other elements in the optical path. (Courtesy of Nikon.)

Use a Real Sample

If you don’t have a sample mounted in the microscope, place a sample on the stage or wherever it mounts in your instrument. Note how easy or difficult it is to get the sample in place. If you plan to use the microscope often, easy sample placement is a must.

Now focus on the sample. How difficult is it to find the focus knob without taking your eyes from the oculars? How hard is it to tell the coarse-focus from the fine-focus control? Do you confuse the focus controls with the controls used to position the sample?

Next, find out how easy or difficult it is to change objectives. The objective-changing mechanism should let you switch lenses without disturbing the sample and without taking your eyes away from the oculars.

Now, pay attention to the sample stage. The stage supports the sample and moves it under (or over) the microscope’s objective. Manufacturers offer stages that fit the many jobs microscopes must do in the electronics industry. You should ensure that a supplier offers the stage or stages you need to move a sample, whether it’s an IC or a PCB.

Test the Actions of the Controls

As you test a microscope’s mechanisms, keep in mind the following points for the various types of controls:

• Position and direction. Can you find the controls without taking your eyes from the image? Can you distinguish the control you want just by feel? When you move a control, does the image move the way you expect it to? Are the controls smooth, or do they make optics, images, and the sample “jump” or jerk from setting to setting?

As you move the focus control, for example, it should move the microscope’s optical assembly parallel to the optical axis and hold it steady. After you let go of the focus knob, the optical assembly should not slide under its own weight.

When you adjust the focus, an image should remain steady. It should not shift in the field as you move the controls. When you change objectives, the image should remain centered on the same point. When you change objectives, you should not have to adjust the stage to recenter a sample’s image.

• Smoothness and resistance. Jerky image movements make it difficult to bring what you want to see precisely to where you want to see it. Uneven movements arise from three sources in a microscope’s mechanisms: uneven sensitivity, too much backlash, and stickiness. Controls should operate smoothly with a uniform slight resistance. They should not stick and release.

• Sensitivity. How much does the image move when you move a control? If a coarse control moves an image too quickly, you’ll lose your point of reference in an image. But if a fine control produces too little image movement, you’ll lose patience long before the image moves to where you want to observe it.

• Stability. How much vibration do you see? If you bang the workbench, does the image jump or vibrate? Any vibration that does get through to the image should damp out in a fraction of a second. Certainly, the image should be unaffected by vibrations transmitted through the air.

The whole instrument should be stable and solid enough that it doesn’t slide or tip during normal use. In other words, you shouldn’t feel the microscope is going to fall over every time you grab the focus knob.

Test Optics, Too

Be sure to test a microscope’s optics as part of your qualitative evaluation. A few tests ensure the optics produce useful images of the parts or components you need to inspect.

Figure 1. A typical microscope target used to help you evaluate optics provides light and dark patterns and characters. Patterns of line pairs, circles, dots, and radial lines are also available. (Courtesy of Edmund Industrial Optics.)

Because the optical system creates an image of a real object, you can’t evaluate optical quality without using a target object. Targets can be either highly stylized or realistic. Stylized targets make it easy to see particular effects. Patterns of equally spaced black and white lines help you find the microscope’s limit of resolution. Finely ruled rectangular grids bring out distortion effects. You can obtain test patterns (Fig. 1) from companies such as Edmund Industrial Optics (Barrington, NJ; www.edmundscientific.com).

Although stylized targets help ferret out optical flaws or determine the degree of imaging errors, they don’t give you a good idea of what real samples will look like. For that type of test, you need products, components, or assemblies of the type you want to inspect.

Here’s how to tell if a microscope produces quality images:

• Images should look real. If you view a 3-mm-long surface-mount resistor through a 100X microscope, it should look like a 30-cm-long component. If you are using a stereo microscope, the image should look lifelike.

• Edges should be sharp. Unless your microscope is designed to resolve features much smaller than the wavelength of the light you use to view them, you shouldn’t see any interference fringes (see “Observing Interference Fringes,” below). If you do, the microscope’s numerical aperture is too small for the magnification you are using. If the edges appear as a soft blur without fringes, you are dealing with inadequately corrected Seidel aberrations.

• Colors should be true at the edges. Chromatic aberration, traditionally called “residual color,” produces a rainbow effect at the edges of an image. Professional quality instruments should have no residual color.

• The whole field should come into focus simultaneously. A microscope can fail this test in two ways. First, features in the center might come into focus before or after features at the edges. Field curvature in the objective lens causes this problem. Second, the top of tall features could come into focus first, followed by areas lower down. In extreme cases, the top of a component goes out of focus before the bottom comes into clear focus. An inadequate depth of field causes this problem. If you need to observe “top” and “bottom” features simultaneously, be sure the microscope you’re testing provides sufficient depth of field.

• There should be no visible distortion. Straight lines should appear straight no matter where they exist in an image. To detect distortions, move a straight edge across the sample area. First, move a horizontal edge up or down, and then move a vertical edge left or right. Distortion will appear as ripples in the straight edge as it moves across the field of view.

• You should see what you’re looking for. It seems obvious, but be sure a microscope lets you clearly see the features you want to examine.

If the instrument you are testing passes all these tests made with real samples, the instrument will perform well for you. “Failing” a test doesn’t mean the instrument won’t perform well in another application. A microscope that works well for inspecting bare boards will likely lack the depth of field needed to inspect populated PCBs.

Some differences boil down to aesthetics. You may simply like the positions and angles of knobs on one instrument more than those on another, and you may not be able to explain why. There’s nothing wrong with choosing a microscope that meets your needs and that you like. T&MW

FOONOTE

1. Malacara, Daniel, ed.,  Optical Shop Testing, John Wiley & Sons, New York, 1992.

C.G. Masi works as a freelance technical journalist. He is the former chief editor of Test & Measurement World. E-mail: tmw@cahners.com.