Sometimes when I leave a disk unlabeled, I have to look at the underside to see. You can tell because the CD has a darker inner ring where the data is burned.
While this data isn't boldly darker, it's visible. The color you see is the composite of all the 1s and 0s in the files you've burned. The 0s are the natural state of the disc; you add 1s as you go to make an actual file. Since data is usually only about 20% 1s, it stands to reason the darkest possible shade of data on a CD is 5x darker than what you normally see.
By perfectly syncing the linear stream of data to the spiral of the disc, it is theoretically possible to line it up so that you could create a coherent image, and possibly even a photograph.
While not particularly useful, I propose a software that creates ISO files that, when burned to a standard CD, would show a line drawing, block drawing, or even a photograph.-- neo_, Sep 10 2014 DiscT@2 http://en.wikipedia.org/wiki/DiscT@2disc tattoo [xaviergisz, Sep 10 2014] LabelFlash http://en.wikipedia.org/wiki/LabelFlash [xaviergisz, Sep 10 2014] [marked-for-deletion] baked. Was going to link to DiscT@2 but [xav] beat me to it. But it was pretty much exactly this concept.-- ytk, Sep 10 2014 random, halfbakery