stroboscopic disk

stroboscopic disk

[¦sträb·ə¦skäp·ik ′disk]
(engineering)
A printed disk having a number of concentric rings each containing a different number of dark and light segments; when the disk is placed on a phonograph turntable or rotating shaft and illuminated at a known frequency by a flashing discharge tube, speed can be determined by noting which pattern appears to stand still or to rotate slowly.