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Microscope Examines Raw Materials

Photonics Spectra
May 2005
Anne L. Fischer

In the constant battle to stay ahead of the competition, a recognized leader in the development of consumer goods, foods, health care items and other products must analyze complex 3-D materials with extreme precision. For example, examining raw materials -- including liquids, and metallic and nonmetallic solids -- under a microscope requires the highest possible depth of field.

A 13-mm depth of field at 25× magnification allows the manufacturer to see deeper into the structure of a sample. This increases the ability to capture a quality image and to understand the structure of the product. To capture that image, however, requires excellent resolution, which is especially important when analyzing materials and processes on a scale of nanometers to millimeters.

In the past, the instruments that the manufacturer used had not met its needs completely. That equipment included a common benchtop zoom stereomicroscope (5× to 400×) with a custom digital camera setup and a computer to perform image capture and analysis. The microscopes are typically set up with multiple fiber optic illuminators.

Lighting with these microscopes is an issue: After setup, lighting can vary during use because the angle of light on the projections and depressions in a sample can be inadvertently changed. Also, it is nearly impossible to replicate the lighting from setup to setup because conventional microscopes require external light sources to observe the target. Without having preset positions, it is much more difficult to duplicate the external light source setup each time because the environment may be different.

This is frustrating for the manufacturer when using image analysis software that relies on a gray-scale threshold for finding the edges of physical features: The threshold location moves, depending on the ever-varying light setup. In the past, optical microscopes' lighting and depth of field were so poor that the company would skip optical microscopy and go directly to scanning electron microscopy (SEM) for imaging. But SEM imaging too has drawbacks, including cost and lack of portability.

To perform higher-quality analysis of complex structures, the manufacturer turned to a digital microscope from Keyence Corp. of Rolling Meadows, Ill., that measures 400 × 130 × 385 mm and weighs about 28 pounds. The instrument includes several zoom lenses that range from 0.5x to 5000× magnification and that have external illumination sources composed of fiber optics.

It also includes a 15-megapixel color CCD camera that replaces the eyepiece and that can be positioned on a five-axis microscope stand or operated by hand (see figure). Zoom lenses allow the magnification to be changed from low to high while keeping the image in focus. Images can be fed directly to a 15-in. liquid crystal display screen, which allows a group of people to view the object under analysis. The manufacturer found this feature to be advantageous.

Microscope Examines Raw Materials
The digital microscope allows the nondestructive analysis of raw materials by means of a handheld CCD-based imaging system. Courtesy of Keyence Corp.

One of the most important features -- from the manufacturer's point of view -- is built-in lighting. The light, which originates from a basic halogen lamp, is supplied through a fiber optic ring around the lens. This allows easy control of reproducible oblique lighting and makes it possible to enhance the projections and depressions of raw material samples at a constant degree.

Different attachments help the manufacturer overcome specific challenges. For example, a polarizing filter suppresses glare during observation through transparent films or coatings, and an adjustable adapter offers vertical and lateral illumination, which is especially useful in observing porous or granular surfaces that may otherwise create obstacles for consistent lighting. In addition, when looking at the edges of raw materials or reaching into hard-to-view areas, the manufacturer can switch to a fiber optic view by pressing a button. The microscope also has a lighting-shift function that allows the user to switch from full-illumination mode to one-quarter illumination, which helps to enhance surface details.

In sample inspection applications, what would take five to 10 minutes with SEM now takes one to two minutes with the digital process. Image-processing software assists in viewing difficult features such as foreign objects, fine particles or liquids. An image-enhancement feature helps adjust white balance to achieve true color very quickly, and an adjustment changes the camera so that it sees only certain colors. For the manufacturer, the presence of additional colored materials can interfere with sample evaluation and interpretation; therefore, color suppression is useful.

One of the most important benefits of using the new setup is speed and the ability to communicate by taking pictures and placing comments directly on the images. However, the manufacturer found that, for its applications' requirements, the feature-measuring tools that come with the digital microscope are not as sophisticated as those offered in third-party image analysis software.

Although the digital microscope's included software converts images into numbers and statistics in the form of distance, angle, radius and more, the manufacturer needs more detailed analysis of the raw materials under observation. It must save images and transfer them to another computer system for advanced functions.

Although the system is at least twice as expensive as a midrange stereomicroscope with a third-party software package added, it is about one-quarter the cost of a not-so-portable SEM system and does not have an SEM's annual maintenance contract.


GLOSSARY
microscope
An instrument consisting essentially of a tube 160 mm long, with an objective lens at the distant end and an eyepiece at the near end. The objective forms a real aerial image of the object in the focal plane of the eyepiece where it is observed by the eye. The overall magnifying power is equal to the linear magnification of the objective multiplied by the magnifying power of the eyepiece. The eyepiece can be replaced by a film to photograph the primary image, or a positive or negative relay...
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