| This has been known for over a decade. http://seeingyellow.com/ https://en.wikipedia.org/wiki/Printer_steganography Anything printed on most (but not all) color printer can be traced back to the printer on which it was printed (serial number) and often date stamp as well. https://www.eff.org/pages/list-printers-which-do-or-do-not-d... Furthermore, most image editing programs and many scanners and copiers often refuse to capture currency because of microprinted circles. https://freedom-to-tinker.com/blog/felten/photoshop-and-curr... http://www.cl.cam.ac.uk/~mgk25/eurion.pdf http://www.rulesforuse.org/pub/index.php?lang=en (note: official US currency images have link-rotted away) |
Edit: as I noted on the list of printers page, we think that newer printers are also doing something that we can't see, possibly based on perturbing dithering algorithms so that the dithering is different from printer to printer in a distinctive way. So when we didn't see yellow dots from newer printers, that doesn't necessarily mean that they aren't printing tracking codes. The reasons for thinking that tracking codes became more pervasive in newer printer models rather than being phased out are suggestions in documents obtained via FOIA, and rumors from people who worked in the industry.