The french inventor Michel David implemented a volumetric display prototype. The 3D image is created by a projecting light on a spinning helical vane. He is also citing a previous work on helical displays: FELIX 3D display, and this 2002 paper from the FELIX 3D folks traces the technology as far back as 1958. Michel puts the cost of his machine at €40, and though his results are definitely proof-of-concept quality (beside i find it really funny that he uses a Dremel-Tool to rotate the axis, i owe one myself 😉 ). Some comments on the project state that the prototype itself ‘looks like a DaVinci sketch.’, which i would also agree on.