ok...here we go again...what about the dual image numbers ...This is known as ‘dual-image numbering’ (in Canada, it’s called ‘halo numbering’) because you can see the number on both sides of the check, and it makes alteration nearly impossible.” Dual-image numbers are control numbers that use ink containing two different components: black and red. The black component is printed on the front of the document and is surrounded by a red halo. The red component penetrates the paper fibers and is usually visible on the back of the check. As the red ink soaks into the paper, it becomes very hard to remove the number. This makes tampering with or changing the control number virtually impossible. Furthermore, color xerox copiers can’t reproduce the effect, they just pick up whatever color is on the surface of the paper! (And reprinting such a check is no small feat either: the image scanner again doesn’t look beyond the surface and you still need to print the control number and nothing else with ink that soaks into the paper...) In his white paper “Check Fraud and Identify Theft — Volume III” (2004) (page 7), Abagnale mentions that this red halo is sometimes applied to the entire codeline. Definitely a good idea, because that protects the check efficiently against alterations! For example, the scam called “the float” is rendered impossible because you need to alter the routing code on the codeline to pull it off!

http://www.spielberg-ocr.com/printing.html#DualImageNumbers