A surgical instrument and method of controlling the surgical instrument are disclosed. The surgical instrument includes a housing and an elongated shaft that extends distally from the housing and defines a first longitudinal axis. The surgical instrument also includes a firing rod disposed in the elongated shaft and a drive mechanism disposed at least partially within the housing. The drive mechanism mechanically cooperates with the firing rod to move the firing rod. A sensor senses a parameter of light reflected from the surface of the firing rod, which includes markings that change the reflectivity of the firing rod. The measurement unit determines a parameter of the motion of the firing rod, such as the position and speed of the firing rod, based on the sensed parameter of the light reflected from the surface of the firing rod.