Measuring Microscopes: a View of the Field

Microscopes optically enlarge objects, which means items in the circular area that one sees through the microscope’s eyepiece, known as the “field of view,” appear larger than they actually are.

Most viewing microscopes offer a choice of lenses in increasing magnifications. To make them useful for measurement, you can place a scale in the field of view and measure its diameter through different lenses. Once you know the diameter of the field of view, you can estimate the size of any object you put in it by comparing its width to the object’s width. This is a pretty coarse way to make a measurement, however.

Although many manufactured parts will not fit into the field of view of a standard viewing microscope, external tools can be added to expand its measuring capability. These external tools include an X-Y stage with built-in scales that precisely track the position of the part under the microscope. This turns 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 in this way with relative ease.

Most of today’s measuring microscopes 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 are small enough that multiple ones can fit within the limited field of view, there is no ability to measure multiple parts at once. What is needed is an expanded field of view, so that the microscope can view many parts at once and still offer high viewing magnification.

A “flat field” lens system addresses the problem of the small field of view, providing a much larger field of view without the typical distortion in focus between its middle and its edges. The obvious benefit of this type of system is a larger viewing area in which the part (or parts) can be placed anywhere, not just in the center. Once the lens magnification is known and the working area is calibrated, measurements can be made anywhere in the larger field of view. And the kicker is that most flat-focus lenses have a large depth of focus area as well, so parts of different heights can be placed in the field of view without the need to refocus the system.

Besides an expanded and accurate working area, there are other subtle but key additional benefits of flat-field lenses:

• Time savings, because the operator can place multiple parts in the field of view and slide positioning is not required to bring the parts into an area where they can be measured.

• No slides or scales to add mechanical positioning errors to the measurement.

• Improved repeatability, because the influence of different operator positioning skills is eliminated. 

• Faster measurement, because all parts are 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 microscope systems also include computers that not only show the image from the video camera, but also provide powerful software that can measure lengths, diameters, angles and more using canned measuring routines. The software also has the ability to string these routines together to create a measuring program for the part, enabling an operator to simply follow the guided sequence for a particular part, move it around to various locations and complete the part cycle.

“Field of view” measuring microscopes do not require part positioning, however, and things get better in terms of speed. A similar part program is created, but instead of moving the part around to various locations, the software simply recognizes the part, 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.

Field of view systems are still measuring instruments, however, and therefore are still subject to the common barriers to accuracy. Many of these barriers are eliminated by the absence of manual positioning or operator involvement, but there are still several to consider. Remember to:

• Check the calibration of the optics often.

• Ensure the glass surface on which the part sits is perfectly clean and has no scratches. (Scratches can confuse the measuring program, causing it to treat these as features on the part.)

• Clean off dirt and fingerprints from the upper lens, as they can cause the image to be out of focus and potentially influence the results.

Then you can simply place the parts on the measuring surface and start measuring.