The Rev Counter
This technology is used to show the rotation speed of the ball. It is used when spinners are bowling, to show viewers the idea how much each ball is spinning. The technology is also able to show the RPM or revolution per minute through a counter, demonstrating how fast the ball is spinning after release.
Here are two theories:
- A high speed camera focused on the ball, possibly using the same images that are captured for the Hawkeye system. As a starting point, an upper limit of 3000 rpm gives 50 rps. At normal (at least, as normal as cricket-playing countries get) PAL video rates of 50 fps that's a maximum of 1 revolution per frame, so high speed should easily be able to resolve the rotation. And, of course, there's no guarantee that the figures presented in the image are precise. I suspect the last two significant digits are bogus. Designing such a system, I expect, would be quite challenging, but the problem would be in designating the period and exact area of interest, and doing it in a few seconds.
- The Doppler Radar that anon mentioned in the other answer. Check out this device: X2 Elite. It is a 3D Doppler tracking radar used in tennis and golf, and possibly in cricket.