Main info at Geometric Computing Laboratory ‐ EPFL
It appears that milling machines can be used to carve perspex blocks that display images when light is shone through them. The strange thing is that the images displayed are of high resolution yet the milling is quite crude.
There is text that says that images can be produced with standard milling techniques, but I'm pretty sure that the block they were using in the demo is produced in some other way; it certainly doesn't look like any milled plastic that I've ever seen (unless you want to assume a pretty massive amount of post-milling polishing...)