Tapeheads or vinyl records buffs check transport speed for sound quality. Strobe or meter method may be used. Incandescent lights will not work.
The flicker in fluorescent lights have a flicker rate of 120 cycles per second (or 120 Hz) may be used with a disk with black and clear spot pattern on tape of clear and black bars. The flickering 120 hertz lamps makes the moving marks appear motionless at correct transport speed. A steady marker close to lighted tape helps determine slow drift or moving of marks.
Flicker with LED lights may be more noticeable due to the fact that LED lights flicker between less than 10% and 100%, where as fluorescent lights dim to about 35% and back to 100%. Flicker rate at 120 Hz. or proper transport speed for method used.
My first guitar tuner was neon lamp under string with switch to select pitch to tune to. The moving steel string made a shadow and adjust tension to make vibrating string motionless. Frequency of neon light changed for each string in open position.
