Many measurements assume that a measuring device performs in the same way throughout its measuring range. In other words, a 1 mm distance measured near the center of a machine will measure 1 mm at its periphery. But will it? This column discusses some of the factors that can cause nonlinear performance of various measuring devices (and what can be done about them).

Is linear the right word?
What I mean by the linear performance of a measuring device is that a given measurement should repeat no matter where it’s performed within the measuring range of the device. For example, a 0.125” pin should measure the same no matter where it’s positioned within the volume of a coordinate measuring machine (CMM), or within the beam of a laser micrometer. A distance of 0.125” should repeat if that change in distance occurs at one extreme of the device’s range or travel as well as within the middle of its range or travel. A measurement of 0.125” within the field of view of an optics-based system should measure the same if in the center of the image or at the edge. By “linear” I mean uniform performance throughout the entire range of the measuring device.

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