Rotation velosity sensor for a tape reel disk in which light is reflected off of the reel disk

A rotation velocity sensor comprising:

• a reel disk;
• a light source which emits light onto the bottom of said reel disk;
• a light sensor which detects the rotation velocity of the reel disk by detecting the light reflected from the bottom of the reel disk;
• said reel disk having a plurality of flat portions and a plurality of grooves alternately formed on the bottom of said reel disk, whereby said plurality of flat portions reflect light evenly and said plurality of grooves reflect light irregularly, each of said grooves having a sloped surface formed therein, wherein said sloped surface comprises two portions, each of said portions slopes outward toward an adjacent one of said flat portions from a central portion between two adjacent flat portions.