so could it easily be tricked by putting a finger over it and quickly removing it in front of a light? or it's too small of a window to trick? a strobe light could fool it?
There's a timing element to it too. Because CRTs display pixels from top to bottom the timing gives information about the exact location of the hit too, not just a boolean hit/miss.
Have to detect a frame of black before detecting a flash of white. And the white has to be detected on the right raster lines. It's not trivial to fake.