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Measuring Feature Size
There are several candidate parameters to describe feature size. The area, determined by counting pixels and often expressed as the “equivalent diameter” (the diameter of a circle with the same area), is widely used. But for some applications other diameters, such as that of the largest inscribed or smallest circumscribed circle, may be more appropriate. Length is usually determined as the greatest distance between any two points in the object, but for a long, curved object like a fiber the length of the skeleton is more meaningful, and there are a great many alternative definitions for breadth. The perimeter of objects is difficult to determine because (except for some smooth objects such as those bound by a membrane or surface tension) the resolved perimeter usually increases in length with magnification.
Whatever measure of size is chosen, determining the values is typically performed on a binary (black and white) image after any necessary processing such as filling of holes, morphological smoothing, rejection of small features due to dirt or noise, watershed separation, and so on. Calibration may be performed by imaging a stage micrometer. Statistical analysis is often performed on the resulting data. This interactive tutorial illustrates typical procedures for measuring the sizes of objects, as well as typical sequences by which data are obtained and presented.
The tutorial initializes with a randomly selected specimen imaged in the microscope appearing in the Specimen Image window. The Choose A Specimen pull-down menu provides a selection of images, in addition to the initial randomly chosen one. The Original button shows the original image. The Calibration button adds a scale bar. The Thresholded button shows the binary image produced by brightness thresholding, while the Processed button shows the result of morphological processing to remove noise, fill holes, separate touching features, etc. as required by each image. The Measured button shows lines showing the measurements performed on the features in each case, and the Report button shows these lines superimposed on the original image for reference. The Results button shows a statistical summary of the measurement results.
Contributing Authors
John C. Russ - Materials Science and Engineering Dept., North Carolina State University, Raleigh, North Carolina, 27695.
Matthew Parry-Hill, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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