How the light brightness stays constant when we increase the frequency of stroboscope
To keep the same brightness, you have to keep the same duty cycle (percent of on time over off time).  So to change the rate without changing the brightness, you have to increase
delayMicroseconds(onTime) by the same percent you increase delay(strobeDelay).  But note that as you go slower, the on time will be too long to freeze the motion you are trying to study.  (If it's just for a party effect this may not matter.)