Quality 101: Up Close With Dimensional Measurement Microscopes

August 2, 2010
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Today's microscope has become a robust measuring instrument often found near-line, on the production floor.

Although the measuring microscope was once found solely in the R&D lab, today’s microscope-with advanced optics, staging and lighting systems, available digital imaging devices and even connectivity to quality information systems-has become a robust measuring instrument often found near-line, on the production floor. Source: Mitutoyo America Corp.


The use of microscopy for industrial measurement has its origins in the pure sciences, particularly geology. As geologists became familiar with optical magnification to study the structure and grain of minerals, it was only a short step to the first important application of microscopy to manufacturing-metallurgy.

In the nineteenth century, metallurgists found that magnification allowed them to examine grain, inclusions and crystalline microstructure of metal specimens. By the turn of the twentieth century, the tool-and-die industry was following metallurgy in routine application of microscopes to inspection of finished die edges and surfaces, as well as of the surfaces of cutting tools used to machine them. As a result, even today, industrial microscopes are often called toolmakers microscopes no matter what their application.



The reticle is a round glass disk on which a scale has been etched. A typical eyepiece reticle would be a 5 millimeter or 10 millimeter linear scale presenting 50 or 100 divisions. Before using the eyepiece reticle, it is necessary to calibrate it using a stage micrometer. Source: Mitutoyo America Corp.

Manufacturing Micro-World

In today’s manufacturing micro-world, quality assurance routines often require dimensional measurement of features so small as to be invisible to the naked eye-that is, at scales of no more than a few microns or millionths of a meter (micron = 0.000001 m). To put these dimensions into perspective, a red blood cell has a diameter of 7 microns and a human hair is about 40 to 120 microns thick.

Some manufacturing techniques such as 3-D stereolithography build up features at the molecular level, too small for measurement via conventional techniques since contacting anvils would displace them, preventing accurate results. In other cases, parts may be too soft or fragile for contact measurement, or hole diameters may be too small to allow introduction of contact probes, or the part may be large but with numerous features at the micron level.

Applications in which these kinds of characteristics are commonly found include the semiconductor, electronic and electrical industries, as well as in precision automotive and aerospace parts, resin molding, and tooling and medical manufacturing.

In many such cases, optical microscopy offers a practical measurement/inspection solution. Therefore, although the measuring microscope was once found solely in the R&D lab, today’s microscope-with advanced optics, staging and lighting systems, available digital imaging devices and even connectivity to quality information systems-has become a robust measuring instrument often found near-line, on the production floor.



Bright Field

A View to the Basics

The fundamentals of optical design are common to all microscope systems and include:

  • Magnification: the degree to which observed features appear larger.

  • Aperture: the opening through which light travels and which determines how tightly the light rays come to a focus on the image plane.

  • Resolution: the ability to distinguish discrete artifacts from one another.

  • Depth-of-field: alternately referred to as depth-of-focus, the distance along the optical axis in which the image appears in acceptable focus.

  • Field-of-view: the portion of the original image that is visible at a given magnification; the greater the magnification used, the smaller the field of view.





  • Dark Field

    Taking measurements with a basic direct view microscope is accomplished using one of two techniques. First is by use of a scale (or reticle) built into one of the eyepieces. The second way is to measure movement of the stage.

    The reticle is a round glass disk on which a scale has been etched. A typical eyepiece reticle would be a 5 millimeter or 10 millimeter linear scale presenting 50 or 100 divisions. Before using the eyepiece reticle, it is necessary to calibrate it using a stage micrometer.

    A stage micrometer is simply a microscope slide with a pattern of known dimensions etched on its surface. The stage micrometer is placed directly on the stage of the microscope and brought into focus.

    By rotating the eyepiece, the reticle and stage scales can be positioned parallel to each other. Calibration then involves determining the number of intervals on the eyepiece reticle that correspond to a certain distance on the stage micrometer; the value of one interval of the reticle is calculated as a result. The reticle can be used to measure any planar dimension in a microscope field since the eyepiece can be turned in any direction while the object of interest can be repositioned with the stage manipulators.



    Polarization

    Measuring the length of a feature using stage movement makes use of linear scales mounted to the stage on which the subject of measurement is fixed. The stage is mounted to a frame-the operator then controls the motion of the X and Y axes-precisely tracking the linear displacement of the item under measurement.

    Effective lighting of the stage-and of the item being measured-is critical for accurate observation. There are several types of stage illumination. The most basic set-ups use tungsten, hotter and less bright than other options. Fluorescent illumination provides cooler and brighter light than tungsten-beneficial when operating over long periods of time. High-end measurement microscopes commonly offer selectable halogen or LED lighting.



    High-end microscopes may offer illumination functions such as dark-field mode to observe surface scratches and small stepped features; polarization to observe coloration or contrast through polarization analysis; and differential interference to observe small surface steps and other elements in color contrast using polarization in combination with a differential interference prism. Source: Mitutoyo America Corp.

    Advanced, High-Performance Microscopes

    Advanced measurement microscopes overlay electronic control and image processing capabilities onto direct view microscopes to increase accuracy and enhance throughput for improved overall measurement and inspection productivity.

    Linear encoding. High value-add microscopes may incorporate digital linear encoders to sense stage displacement on both X and Y axes, eliminating error associated with visual interpretation of stage movement.

    Advanced illumination. High-end microscopes may offer illumination functions such as dark-field mode to observe surface scratches and small stepped features; polarization to observe coloration or contrast through polarization analysis; and differential interference to observe small surface steps and other elements in color contrast using polarization in combination with a differential interference prism.

    3-D measurement. Advanced microscopes can be optioned to obtain measurement results in the vertical axis by imaging different planes of the feature at discrete distances from the lens. The auto-focus systems used in these systems improve repeatability by reducing human error.

    Digital imaging and analysis. Typical high-end measurement microscopes have adapters enabling attachment of digital cameras and/or vision systems. This enables image display on a monitor for simultaneous viewing by groups, for example in training scenarios.

    Additionally, options are generally available enabling high-end microscopes to be used as manual vision measuring systems. Accessories typically support export of image data to PC-based software to run routines such as automatic edge-detection, image analysis and data manipulation in 2-D and synthesized 3-D.

    As manufacturing processes become more dependent on inspection and measurement at the micron level, the role of ever evolving, ever more capable microscopy systems is assured.



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