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MACHINE VISION

VideoMeasurement

by Fred Mason Optical Gaging Products Inc.Rochester, NY

An Introduction To Video Measuring Systems - 7160 V

ideo measurement is anaccepted way of monitor-ing critical dimensions ofmanufactured parts. It letsmanufacturers ensure that processes areunder control and that parts are withinspecification. Video measurement systemsanalyze an image of the part, so the pro-cess is non-contact. The image is usuallymagnified many times, so the system islike a microscope with the eyepiece re-placed by a camera. This alone does notmake a video measurement system. Manysystems magnify the image of a part andpresent it on a video monitor, but do notmeasure. For example, an inspector might make decisions by visual analysis of amagnified image. So how is a video mea-surement system different?Video systems measure, relying onhigh resolution images for the neces-sary accuracy. Vision systems typicallyanalyze lower resolution images tocompare a part to a master, count partsin the image, or do simple measure-ments. Video systems are usuallystandalone — you take the part to thesystem. Vision can be standalone, or itcan be done on-line, viewing parts asthey pass by on a belt, for example.The key difference is that vision sys-tems usually “look” at parts that may be in motion, while video systemsmeasure parts that are on the machinefor the specific purpose of being mea-sured.

Image is everything

Video measurement relies on imageprocessing. This requires the image to beconverted to electrical signals. This isdone by “reading” the signal levels fromeach of the pixels in a digital sensor(camera). The image is magnified untilthe feature of interest covers many pixelsso individual details can be determinedunambiguously. Good video measure-ment systems use sub-pixeling algo-