I once imagined a similar idea for building a 3D volumetric display. You need to create a literal shell of LEDs: a spiral of semicircles which grow larger as y

I once imagined a similar idea for building a 3D volumetric display. You need to create a literal shell of LEDs: a spiral of semicircles which grow larger as you move out and around the spiral. So you start with a 1 cm diameter semicircle strip, with each end of the strip mounted on the axis. Then rotate around the axis by 1°, and add a 1.1 cm diameter strip, and repeat. You end up with a surface that looks like a shell you would find on the beach. This shell surface, when rotated on its axis, sweeps the entire volume of a sphere, with each LED strip sweeping a fixed radius from the axis. The strips are offset along the curve of rotation to ensure that the eye can see every layer as it is lit. If there are enough LED strips to create a complete 360° spiral, then a single rotation of the shell sweeps the entire sphere. Thus for a complete shell you only need to spin at 25-60 Hz for a good persistence of vision effect. I never built it but I would like to one day, I'm sure it would be fun, if not very expensive to make.