Scratch Holograms

cults3d

https://youtu.be/Pw8y0gSaMNw Recently I visited a museum in Los Angeles where they had these metal discs that were roughly the size of vinyl records, etched with numerous scratches and illuminated from above. When you looked at these objects, the scratches collaborated to create an image of a polyhedron floating in space! And what's more, when you walked around, you could see this virtual polyhedron from different angles. After some research on Google, I discovered that these things have been rediscovered at various times and studied in detail relatively recently (1990s) by William Beaty. The fundamental concept is this: when you scratch metal, the scratch acts somewhat like a very thin reflective tube. If you illuminate this with a point source of light, each eye sees the point source as a bright spot somewhere along the scratch. If the scratch is curved, then each eye sees the bright spot at a slightly different angle. Your brain's depth-perception algorithms interpret what you see as a point of light floating in space. It turns out that the radius of the scratch curve corresponds to how deep you see the point of light. This means you can take a polyhedron and place virtual points of light along each edge, and then put numerous scratches on metal (or plastic), and when illuminated from above, the light scatters off the collection of scratches in just such a way that you see the polyhedron, and what's more, it's like a hologram because you can move left and right and see the polyhedron rotating. Anyway, I was curious to know if these things could be created using 3D printing. The result is this, and while the effect isn't as good as what I saw in the museum, it's not bad for a few grams of plastic! I included the Python script used to make these, as well as two designs: a tetrahedron and a cube.