I'm just curious if anyone thinks this can be done. I assume if it was possible somebody would have done it by now. From what I understand, Hawkeye is the only device that can measure RPM. At present, sensors all estimate spin, based on racket head speed, contact point and the swingpath. These are all contributing factors but they exclude the incoming ball spin/speed and the impact of the stringbed, which are big contributing factors in the amount of spin imparted on the ball. A proper measurement of spin would be truly wonderful training aid for tennis players but I think it's impossible unfortunately.
I assume Hawkeye automatically counts ball revolutions, rather then having a person in real time counting, by detecting and trace the moving ball? I'm not sure if this is done with a doppler radar, or the camera can distinguish the color of the ball and focus in on it, tracking it's path of flight and rotation. Perhaps a sensor could include a front and rear facing camera that can trace the flight of the ball, then count the RPM.
Actually a front and rear facing wouldn't work, I don't think, as in a typical high to low forehand the racket begins facing the ground and only faces the court for a few milliseconds when the racket strikes the ball