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.