I know that a number of people are interested in arduino controlled milling machines, so I thought I would post this.
It appears that milling machines can be used to carve perspex blocks that display images when light is shone through them.
The light used is just ordinary sunlight or torch light, no lasers needed. The strange thing is that the images displayed are of high resolution yet the milling is quite crude. Furthermore the milling on the block can hardly be seen and certainly does not resemble the displayed image.
There are some clever mathematics involved in calculating how to mill the blocks to display the images, but the people involved don't seem to fully understand how such high resolution images can result.
A long lost Chinese technique that does something similar. Shining light on a polished metal coin displays the image carved on the reverse of the coin.
Very interesting. I am smelling Fourier optics. If you treat light propagation as Fourier transform, then with an assumption you have point-like incident light, and image (that photo) at a fixed distance, you can derive the intensity distribution needed on the glass and mill accordingly. I don't know about roughness of milling but this is very cool. I'll show this to students taking Fourier optics if I get to teach it next time.
It is very clever. They also seem to have figured out how to get the same perspex block to display several different images depending on the angle the light hits it from.
One application seems to be for use in buildings. The milling really is very light so apparently a block would look like a window, with little obvious distortion, but it can project images.
I think they were going to mill glass - I did not know you could do that.
Also interesting that the ancient Chinese had managed something similar without the befit of computers.
Main info at http://lgg.epfl.ch/caustics
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...)
They were careful to not get much of a picture of the light reflected off the surface of their piece of glass/plastic, but what it behaves like is a complex lens. You can see the image go in and out of focus as it is moved back and forth in front of the paper.