STROBOSCOPE

[stro.bo.scope] n [Gk strobos whirling + ISV -scope] (1896): an instrument for determining the speed of cyclic motion (as rotation or vibration) that causes the motion to appear slowed or stopped: as a: a revolving disk with holes around the edge through which an object is viewed b: a device that uses a flashtube to intermittently illuminate a moving object c: a cardboard disk with marks to be viewed under intermittent light