We have already seen about laser tracking system in my previous post, now let us see the working principle of laser tracking system. Here we are going to track the tip of the finger as shown in the figure below. The laser beam from the source is made to incident over the steering mirror. The steering mirror is tilted in such a way that the reflected beam falls on the finger tip. The laser beam will form a circular saccade on the tip of the finger.
The photodetector will detect the backscattered laser signal from the finger(see the diagram below). The saccade is supposed to remain fully inside the tip of the finger while tracking [fig (a)]. The backscattered signal will be high when the saccade remains fully inside the finger tip. As the object moves, a small portion of the saccade may fall outsid the finger tip, and the backscattered signal will momentarily drop. Due to the synchronous operation of the beam-steering mirrors and the photdetection, an accurate recentering vector(blue arrow) is computed [fig (b)]. The center of the saccade is updated accordingly [fig (c)]. By repeating these three steps continously we can track the movement of the finger tip.
Fig. Updating center of saccade