Baseballs have spin that is visible from the side.
I don't know what they are using but probably software that is looking though a fairly high speed camera and counting the seams.
I suppose if you had a mark on the edge of the disc you could see it from the side.
No, they use radar for spin rate, I believe. I suppose it may need to be an overhead shot since the rim is so thin, but I think radar would work.
According a briefing giving to the University of illinois physics department , Trackman's Doppler radar is positioned high behind home plate and tracks the baseball collecting 48,000 measurements per second. It precisely measures the location, spin, angles, velocity and trajectory of a ball in flight.
"The common misconception is that people need to ... get more snap and spin on the disc. Neither Paul [McBeth] nor I nor any good thrower thinks anything about consciously snapping the disc on a drive. The disc rips out of the hand, and all the spin you will ever need comes just from the joints passing each other. So as the elbow passes the shoulder, and the wrist passes the elbow, that's all the spin you're ever going to need. Just from the arm coming around. So you don't need to actually think about spinning it, at all. The ONLY time we think about that stuff is shorter touch shots, where you are not getting a rip out."
-Nate Sexton
This is exactly what I want data on - where is the line between "shorter touch shots" and "ripping out"?
I imagine the line is different for everyone.
I would also imagine that it is a benefit to the player to able throw those "touch" shots, where spin is consciously thought about and controlled, to higher speeds.
This, I think, is really the hypothesis I would like to try to confirm with actual data, but we'd need a huge amount of it.
Do the best players have more control of spin to a higher speed? Does having exceptional control of spin at low speeds actually correlate to a better short game? What about putting, where spin control is most obvious?
And even on the shots where they aren't actively controlling it, there must still be variance. It would be interesting to see who's shots have the widest/tightest ranges of spin rate.
It would also be interesting just to see who has the highest and lowest spin rates.