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Yellow's not invisible

Being someone who knows what these "invisible" dots are (in general, not in this specific case), I thought I'd note that they're not "invisible to the naked eye." They're better described as "Little. Yellow. Different." They are dots printed in yellow, when viewed on white paper, are difficult, but not impossible to see. Also, they show up quite well under the right types of light.

If you don't have a color laser handy, you can get a general idea of the concept if you look at recent dollars (i.e. newest $20) and (IIRC) euros. Both have yellow printing on the back that, when scanned in, identifies the image as currency to the scanning program. Each bill has its own unique "constellation." The hiding effect isn't as pronounced on currency, though, as the intent is mainly to make it easy for a copier to pull out the pattern using the appropriate filter.

Government Uses Color Laser Printer Technology to Track Documents

Comments (3)

Ray Chiang:

Funny how this came up the same day that there was another announcement from two of my old advisors at Purdue:

http://news.uns.purdue.edu/UNS/html4ever/2004/041011.Delp.forensics.html

kwc:

All that wasted research -- people just need to use Xerox-brand color lasers! :)

Ray:

Fat chance. It was before your time at PARC, but you can see the fine market share Xerox had before they bought Tektronix's color printer division in 1999:

http://money.cnn.com/1999/09/22/deals/xerox/

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