转速测定 meaning in English

tachometry

The special light beam modulator is a disc that rotates at high speed , symmetrically along its diameter is respectively arranged a slit and a small hole , the slit is used to modulate light beam , the small hole is used to the synchrosystem and measure speed of modulator
该调制器为一高速旋转圆盘，其上沿轴向刻制细缝，在圆盘对称位置开小孔；细缝用于调制光束，小孔用于系统同步及调制器转速测定。