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Getting down to basics.

Optical measuring is now a crucial part of virtually all research work. Here, from our experience at Nikon, are some basics of optical measurement that should prove helpful in your work.

Measuring vs. inspection R&D labs usually employ both inspection and measurement in evaluating samples. The two methods give quite different information.

Inspection answers qualitative questions: Is this flat enough? Are there too many pits in this tool? Does the grain of this sample look right?

Measurement answers quantitative questions: How long is this piece? What is the thread pitch of this screw? How deep is this hole?

In your lab, you're probably measuring to get quantitative information that can lead to the development of engineering prototypes or final production-line products, or to evaluate defects in existing materials or products. In the life sciences, measurement can validate the accuracy of diagnostic products or the usability of prostheses.

Optical measuring uses our eyes to compare specific sample features against predetermined standards. In many cases, there's no substitute, for visual information in ascertaining future performance potentials for a product or material.

The right tool for optical measuring must reflect your specific needs--such as the ability to interface with others on the development team--and key factors: sample size, color accuracy, 3-D imaging, the types of samples to be measured, permitted tolerances, and throughput demands.

Your choice of instruments includes optical comparators, video systems, and microscopes.

* Optical comparators, also called profile projectors, use large ground-glass screens for imaging. Compared to microscopes, they offer a much larger field of view and produce less eye fatigue over long usage. In addition, comparison overlays can be used with them to provide go/no-go information for specific samples.

The downside of optical comparators is that they offer somewhat less resolution and are limited in the types of detailed documentation they can provide. They also take up a lot of space.

* Video measuring Video imaging offers on-line documentation and archiving capability, repeatability, and the capacity for image processing and manipulation.

A fully automated video measuring system will find randomly oriented parts with a computerized vision system, adapt to their varying orientations, take a prescribed series of measurements, and compare those measurements to your preset tolerances for evaluation.

Video can be used to measure, inspect, and document. And because you're looking at a monitor, video systems are comfortable to use over long periods of time. Finally, they can offer a graphical part display--a road map--for measuring sequences.

On the negative side, the resolution and color fidelity of video-based microscope measuring systems are subject to the limitations of CCTV cameras and monitors--a situation that is getting better as CCTV technology improves. Video measuring also offers a reduced field of view and sometimes falls victim to electronic "noise."

* Microscopes At low throughput levels, microscopes offer the highest accuracy, best resolution, greatest optical versatility, broadest range of magnifications, and best color fidelity of any optical measuring method.

In addition, microscopes are usually designed in modular form, giving you great flexibility to put together a system optimized for your specific requirements.

Measuring microscopes offer the choice of episcopic (reflected), diascopic (transmitted) or oblique (off-axis) illumination, or a combination of these, depending on need. They also have unparalleled documentation capabilities, both photographic and video.

On the other hand, microscopes are best for small-scale work. They offer a relatively small field of view, are designed for smaller sample sizes, and permit limited stage travel. Moreover, you can get eye fatigue if you don't adjust the eye-pieces for comfort over long periods of use.

How microscopes work In most microscopes, the primary lens (the one closest to the specimen) is called the objective. This is the lens you select from a revolving circular turret (nosepiece) on the instrument. The microscope also has secondary lenses, usually those closest to the eye, which are located in the eyepieces. Many microscopes have condensers and other lenses that serve specific purposes.

Illumination techniques To create a magnified image that is a true representation of the specimen in detail, shape, and color, however, microscopes are also fitted with filters, diaphragms, and special accessories. These enable you to use various imaging techniques, notably the following:

Brightfield is an illumination technique that provides flat, even illumination of the field of view. With brightfield illumination, you can clearly see where things are located, how they are attached to each other, and how the color and grain of the sample appear.

Brightfield is used to see cracks, discoloration, contamination, and dirt, as well as to monitor assembly and orientation of components.

Darkfield lights the specimen surface from an oblique angle, so dirt, contaminants, pits in the finish, deviations in flatness, scratches, and other surface flaws are strikingly clear. Darkfield is used for observing the surface, looking for contamination, and checking for deterioration in a product or part.

In R&D labs, darkfield is used to identify and locate x-y coordinate pairs and to measure the dimensions of defects found in prototypes or production models for reengineering.

Polarized light and differential interference contrast (DIC) are techniques that use polarizing materials or optical prisms (or both) to examine physical and structural characteristics of manufacturing materials.

These techniques are used to examine unprocessed semiconductor wafer substrates, ceramics, crystals and metal alloys. In biomedical labs, DIC is used to optically section individual cells and cell structures.

Fluorescence is a well-known phenomenon whereby a chemical excited at one light wavelength emits light at a longer, usually visible wavelength. It is used, for instance, in semiconductor work to identify and measure the extent of contamination in photoresist chemicals. New antigens and antibodies are often identified by using fluorescence microscopy.

Phase contrast takes advantage of the unequal transmittance of light through a structure based on the different densities of the structure's components. It employs a phase ring in the objective lens and a phase annulus in the condenser assembly of the microscope.

Phase contrast is commonly used in biomedical research to examine unstained materials, either living or dead.

Making measurements There are two basic ways of measuring with a microscope:

Field of view measurement uses small, precise eyepiece reticles that superimpose a pattern or scale over the image. It also provides accurate quantitative measuring because there are no stage movement errors. If the entire region to be measured can be seen at one time in the eyepieces, you get a quick, precise way of measuring.

Stage movement measurement is used when the feature to be measured doesn't entirely fit in the field of view, or when a sample within the field of view must be moved past a fixed point for measurement. Typically, a linear scale or a rotary encoder inside a drum micrometer is used to measure the displacement of the stage as the object on the stage moves past a reference point (usually a crossline) in the eyepiece.

A linear scale registers a direct measurement of how much the plates in the stage move and provides a digital readout that is extraordinarily accurate--down to less than a half micron.

In contrast, a drum micrometer measures the turning of a gear that moves a spindle, which in turn moves the stage. The measurement is read off a graduated drum and a vernier scale. While these measurements are not as accurate as those provided by a linear scale, they are more economical and are sufficient for many users' needs.

A word of advice: Whatever kind of measurement you are doing, use the best optical resolution for feature detection. Also, make sure the microscope stand is massive and stable, to eliminate vibrations that can limit the accuracy of the measurement.

Confocal imaging is based on a completely different kind of theory from conventional optical microscopy. Confocal microscopes use white light or lasers to construct a highly detailed 3-D "map" of a sample. Simply put, they optically section your sample, point by point and layer by layer. They can completely reconstruct the object on a computer screen, letting you rotate the image and see your component or material from any angle.

Confocal systems offer resolution, information, and detail on both moving and stationary specimens that are unparalleled in the world of optical observation.

The system's heart The guts of a measuring system is its optics, and there is never a substitute for optical integrity. True measuring optics are highly corrected for both spherical (shape) and chromatic (color) aberrations and will introduce no errors into a quantitative measuring system. So, buy the best optics you can afford.

RELATED ARTICLE: Shopping Around

There is, of course, a relationship between price and performance in buying a measuring system for R&D work. You can pay as little as $100 or as much as $200,000.

The typical sale for a precision measuring system with a digital readout ranges from $5,000 to $50,000. Video measuring systems range from $15,000 to $100,000.

Before buying, ask yourself:

1. Am I going to use this instrument just to measure, or to inspect as well? If you need to measure small features in the x-y plane, you probably should buy a compound microscope. In most cases a dedicated measuring microscope with a measuring stage is best for this purpose.

2. Do I need 3-D? If you need to see very sophisticated 3-D images at high magnifications, you will want to look into a confocal attachment for your microscope.

3. What size field of view do I need? If you need to measure large components, it helps to have an instrument with a large field of view, such as an optical comparator.

4. How big are my samples? The depth of the specimen determines the working distance you need. The working distance is the amount of space between the surface on which you are focused (the focal plane) and the front element of the objective lens.

Generally speaking, the greater the working distance you require, the less magnification you can achieve, and the lower the overall resolution of your system.

If you have 3-D specimens, look for lenses with the highest numerical aperture. They give the best resolution over long working distances.

Also, consider the x-y size of your samples. When measuring, you will be limited by the distance the stage can travel in each direction. Samples that are too large for measuring microscope stages are best suited for optical comparators.

5. What about future needs? Look for a system that is modular and adaptable. You may only need reflected light right now, but eventually may want to add transmitted light.

6. Is the manufacturer respected? No single company makes the best of everything. Look for a company whose instruments have a reputation for top optical quality and can be retrofitted to meet your changing needs.

7. Is my dealer reliable? Make sure the dealer's salespeople have the service orientation and field knowledge you need.

Jack Isaacson is product manager for measuring instruments and William Chambers is senior technical specialist at the industrial department of Nikon Inc., Melville, N.Y.
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Title Annotation:Optical Measurement; includes related article
Author:Isaacson, Jack; Chambers, William
Publication:R & D
Date:Feb 1, 1995
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