stroboscope
noun
stro·bo·scope ˈstrō-bə-ˌskōp 
: an instrument for determining the speed of cyclic motion (such as rotation or vibration) that causes the motion to appear slowed or stopped: such as
a
: a revolving disk with holes around the edge through which an object is viewed
b
: a device that uses a flash tube to intermittently illuminate a moving object
c
: a cardboard disk with marks to be viewed under intermittent light
Example Sentences
Recent Examples on the Web Using a stroboscope to generate extremely brief bursts of light, Massachusetts Institute of Technology electrical engineering professor Harold Edgerton photographed this splash made by a drop of milk in 1957. 
Scientific American, 18 Apr. 2018
Scientific American, 18 Apr. 2018 Word History
Etymology
Greek strobos whirling + International Scientific Vocabulary -scope
First Known Use
1896, in the meaning defined above