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Microscopes have a long history of use in quality control activities, including observation and documentation. Historically, measuring reticules were placed in a conjugate plane of the optical system and could be calibrated to allow for very coarse measurement. In addition, reticules could be calibrated to a known pattern and used for measuring grain size in metallography.
Modern developments in digital imaging and advanced image analysis techniques can be used to create highly repeatable images, along with a permanent record of the image overlaid with the measuring data. Key things to consider in selecting a microscope are the minimum feature size that must be imaged and measured, as well as the overall part size.
Stereomicroscopes are nearly ubiquitous in modern quality control (QC) and material laboratories. Thanks to their stereo view, they can be a good choice when precise manipulation of a part is required. Higher-performance stereomicroscopes have fixed magnification positions for basic system calibration, ensuring system accuracy and repeatability. When coupled with image analysis software packages, they also provide the advantage of a low-power, high-clarity image; however, they can pose a problem when conducting linear measurements: Two separate optical paths are used to create the stereoscopic view, and neither is orthogonal to the image plane. This can create uncertainty-also called parallax-when the operator erroneously believes the edge of a part has been measured accurately because he or she is viewing it from a slight angle. Some higher quality instruments provide an offset function to remove most of the parallax.
Compound microscopes provide much higher magnification and resolution-down to ~0.35 microns-than stereomicroscopes, and they provide a single optical path that is orthogonal to the part. Using a compound scope provides higher certainty with regard to the edge determination of the measured part and eliminates parallax issues, as displayed in figure 1.
Additionally, compound microscopes provide fixed magnifications, each of which can be certified to provide a superior level of measurement certainty.
With both types of microscopes, image acquisition is achieved via a digital camera coupled with image analysis software. The integration of the camera with the software is key for creating streamlined functionality. The software should offer the ability to calibrate each magnification used. In addition, it should accommodate any intermediate optical components such as supplemental magnifiers or, more commonly, camera adapters that use reduction lenses to provide an optimized field of view.
The Power of Image AnalysisImage analysis is very powerful because it allows for the post-processing of images, using mathematical algorithms or related techniques to enhance the user’s ability to derive needed data from images. Image analysis can begin either by reducing glare and blur or by enhancing the sharpness of a line or edge under examination.
Additionally, the digital environment allows for images to be gathered in rapid succession in the Z-axis for measuring depth. Today’s advanced software aligns multiple images and provides volumetric measuring capability. Most software of this kind can extend the microscope’s depth of field by creating a composite digital image, constructed using the sharpest pixels acquired at various depths.
Multiple image alignment is a software function that tiles or montages a series of images across a part. Each subcomponent of the montage is a single microscope field of view. The software assembles the individual images into a composite single image using pattern recognition algorithms. The larger image then can be used for analysis and measurement.
Pixel counting, the assessment of each individual, tiny data point that comprises the digital image, is the core of any image analysis system. To use pixel counting, the entire microscope system must be calibrated with a known standard, translating each pixel to a size at a given magnification. Once an image is captured, the system uses a variety of techniques to define measurement areas. Whether the feature being measured is a line, circle or other shape, its dimensions can be calculated by counting pixels. However, because the interpretation of where a line starts and stops is subjective, the operator sometimes may introduce error. In some advanced cases, a predefined contrast-based threshold that consistently judges an edge may be implemented. This technique has been used in the semiconductor arena, for instance, for many years.
Selecting and assembling microscope-based systems requires planning and a thorough understanding of the parts to be measured. Size is one consideration, as the operator must fixture the part on the optical system’s frame before measuring it. Both stereomicroscopes and compound microscopes can be configured with fixed stands and stages that allow for easy accommodation of parts up to 4x4 inches. Mounting either type of system on a gantry or boom arm stand provides greater flexibility for larger-sized parts. Boom arm systems sometimes are sensitive to vibration, though, making them difficult to use for higher-magnification observation and measuring.
Users who need flexibility often select compound and stereomicroscopes. Often, they must measure parts of different sizes or use varied image contrast techniques such as darkfield, fluorescence or quantitative polarization for obtaining raw image data of different parts. In addition, users who require the most advanced image processing and databasing capabilities will find the dedicated image analysis software of these conventional microscopes essential. Users in metallurgy, process development and core research typically select this route.
Recently, companies have introduced digital microscopes that incorporate telecentric optics directly coupled to digital cameras. Usually these systems are connected to an illumination system and dedicated software to provide a completely integrated solution. An integrated digital microscope is relatively simple in that there are no eyepieces or other operable optical elements involved. This technology does not provide the same sub-micron resolution and variety of optical contrast techniques-such as fluorescence and quantitative polarization-as conventional microscopes, but it does provide an easy-to-use system that can be ready to measure just moments after being taken out of the box. Digital microscopes have a lot of flexibility in comparison to conventional microscopes; their stands usually allow for angled viewing so that the offset of parts from a mounting plate can be imaged. These systems reduce the time it takes to get started measuring and are designed for use with virtually no training. Such digital solutions are emerging rapidly in all areas of the quality control field.
More advanced integrated digital solutions employ High Dynamic Range (HDR) imaging to optimize contrast. They acquire a series of successive images and then select the image that displays the best contrast. Such digital solutions offer anti-halation imaging to reduce glare; highly reflective areas that would otherwise wash out are now displayed optimally on the final image. Both HDR and anti-halation imaging algorithms provide images of higher quality, contrast and homogeneity, streamlining measurement and enhancing ease of use. In addition, they offer a higher level of accuracy because the measured surface can be seen more clearly, as displayed in figure 2.
Digital microscopes streamline operation and are becoming popular among those who image the same types of parts repeatedly and do not require the ability to implement multiple techniques. They also work well in applications where relatively inexperienced operators must obtain useable data. Integrated digital microscopes have another key benefit: because they come preconfigured with a wide variety of lighting techniques, they provide reliably consistent illumination of complex samples, yielding enhanced repeatability. Integrated digital microscopes have therefore become a tool of choice, both in the laboratory and near the production line.
Both conventional and fully integrated digital microscope solutions share some uncertainty because they are pixel-driven measurements systems. But with careful adjustment and calibration, they allow end users to handle gauging functions with minimal training.
Dedicated Measuring MicroscopesMore advanced than typical compound and digital microscopes are toolmakers’ microscopes, the most sophisticated and traceable measuring microscopes available. They combine compound microscopy with precision stands and measuring stages and are manufactured to provide the utmost in rigidity and temperature tolerance. Furthermore, the microscope frames are manufactured to high tolerances for orthogonality to the stage. The Z-axis of the toolmakers’ microscope is often equipped with linear encoders matched to the stage system so that equal measuring capability is obtained in all three axes.
The measuring microscope stage provides the highest degree of flatness throughout its travel. As the sample moves across the optical axis, linear scales allow for precise, traceable, reproduceable measurements derived from the movement of the stage. Many manufacturers provide edge-sensing systems that can provide for semi-automatic operation, as well. To further enhance repeatability, some add a laser-based autofocus system for precise Z-axis or offset measurement beyond the depth of focus of the lens systems, as displayed in figure 3.
In their basic form, toolmakers’ microscopes are operated manually. But when combined with software and measuring computers, the calculations of angles, areas and other output can be automated to allow for a repeatable and accurate measurement. High quality dedicated toolmakers’ microscopes can be found in quality control labs, shop floors, R&D facilities and anywhere that larger parts are measured or the most advanced contrast techniques are used. Though they are the most robust of the microscope measuring stand choices, they require significant investment both financially and in terms of staff training to get take advantage of their many capabilities.
Today’s microscope-based measuring techniques can be used to control processes or judge the results of an experiment. The precision and repeatability required-along with the need for user-friendly operation and the potential to use the instrument for additional applications-helps users determine the best solution for any individual application.