Here you can find a simple Asset to track the Z position of a flashlight or a smartphone display light using a monocular RGB cam.
For the moment it's a quick evolution of the first realease that was only using Line MAT and Particle SOP.
I had to go for Grid SOP and Text TOP, because I had some problems with Text SOP and Creep (keep proportions) while the Text scrolls, but I'm looking forwad to solving it so tohave Text SOP points positions.
I know it works also with a normal daylight because the brightness of the display can still be trackable in a normal enviromental light, avoiding direct light to the camera.
Last tip is about the camera lens. I'm pretty sure that fisheye lenses perform better because when you get near to the cam view the white light coming from the display (covering more pixels) gives back a more precise Tz.
I hope that this method could inspire other great people in the community; adding twitter feeds or other online related ideas are more than welcome!