HEAD MOUNTED DEVICE AND TRACKING METHOD

A system includes a tracking device and a light source. The tracking device has a camera and a first controller. The camera captures a first image and a second image. The first image is captured during a first period of time (t0-t1) and the second image is captured during a second period of time (t2-t3). The first controller is coupled to the camera and configured to obtain first timing information and second timing information; form timing instructions; and communicate timing instructions to light source over a communication interface. The light source is configured to use timing instructions to illuminate first amount of light and a second amount of light. The first controller is further configured to calculate image intensity difference between the first and second image to identify from first image pixel(s) illuminated by the light source.