Metrology
Metrology
Measuring Microscopes
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Measuring Microscopes: A View of the Field
George Schuetz, Mahr Federal Inc.


When you look into the eyepiece of a microscope you typically see a circular area, known as the "field of view." And because microscopes optically enlarge objects, the items in the field of view appear larger than they are. Most viewing microscopes offer a choice of lenses to use, and objects appear larger as the magnification increases. One interesting learning tool is to measure the field of view by placing a scale in the microscope's field of view and measuring the diameter with different lenses.
 
Now if you put something small in a field of view with a known diameter you can estimate the size of the object by comparing its width to the field of view. This is a pretty coarse way to make a measurement, but in my high school biology lab this was big-time.

In the world of manufacturing many small parts will not fit into the field of view on a standard viewing microscope, so external tools are added to expand measuring capability and turn it into a measuring microscope. These include an X-Y stage with scales built in that precisely track the position of the part under the microscope. What this does is turn the viewing microscope into an edge locating device, and the scales are used to keep track of the part as it moves from one point to another as seen through the microscope's field of view. This is the basis for most measuring microscopes, and parts big and small can be measured with relative ease.

Most measuring microscopes today are pretty advanced in that they combine excellent lenses, a high-resolution video camera, precision X-Y tables, and powerful software that make small part measuring easy and accurate.

The problem with this type of measuring microscope is that the measurement sequence can be time-consuming, and unless the parts being measured are small enough to fit multiple parts within the limited field of view, there is no ability to measure multiple parts. What was needed was an expanded field of view so that the microscope could view many parts, and still offer high viewing magnification.

To address the problem of the small field of view of most microscopes, a different type of lens system is used. The "flat field" lens provides a much larger field of view without the normal distortion in focus between the middle of the field of view and the edges. The obvious benefit is that now there is a large viewing area and the part (or parts) can be placed anywhere in the field of view. Once you know the lens magnification, and the working area is calibrated, measurements can be made anywhere in the large field of view. And the kicker is that most flat focus lenses have a large depth of focus area so that parts of different heights can be placed in the field of view without needing to refocus the system.

Besides an expanded and accurate working area, there are some subtle but key additional benefits:
• Time is saved because the operator can place multiple parts in the field of few, and there is no slide positioning to bring the parts into an area where they can be measured
• There are no slides or scales to add mechanical positioning errors to the measurement
• Repeatability is better because the influence of different operator positioning skills is eliminated
• Speed of measurement is much faster because the parts are all there, in view and ready to be measured; and since the field of view is large and has a long depth of focus, there is no need to refocus with each part change.


Most higher-end measuring microscopes have computers as part of the system, not only to show the image from the video camera, but to also provide measuring software. These powerful programs can measure dimensions such as lengths, diameters, angles, and more using canned measuring routines. And they have the ability to string these routines together to create a measuring program for the part. Now an operator can simply follow the guided sequence for his particular part, move it around to the various locations and complete the part cycle.

However, with "field of view" measuring microscopes, no part positioning is required and things get better in terms of speed. A similar part program is created, but instead of moving the part around to the various locations, the software simply recognizes the part, even orients it to the pre-assigned position, and makes the measurement automatically, with no operator influence. In fact many of the programs can have multiple parts laid out in the field of view, at any orientation, and can recognize and measure them all. This is much faster than manually moving parts around, and entails much less operator influence.

Despite all the capabilities found in measuring microscopes and field of view systems, they are still measuring instruments. They are all subject to the common pitfalls that steal measurement accuracy. For a field of view system, many of the enemies of precision have been eliminated since there is no manual positioning or operator involvement, but there are still several to consider. Always remember to:
• Check the calibration of the optics often
• Ensure the glass surface the part is sitting on is perfectly clean and has no scratches (scratches can confuse the measuring program because it may treat these as a feature on the part)
• Clean off dirt and fingerprints from the upper lens as it causes the image to be out of focus and potentially influences the results.

Other than these basic measuring practices and a little preparation, the operator can simply place the parts on the measuring surface and start measuring.

                                                          

Most measuring microscopes today are pretty advanced in that they combine excellent lenses, a high-resolution video camera, precision X-Y tables, and powerful software that makes small part measuring easy and accurate. Pictured is the MarVision MM 320 measuring microscope from Mahr.