Detect Printer Steganography

click on the image to enlarge

Recently I learned that many color laser printers print an (almost) invisible pattern of tiny yellow dots on every page. This is called printer steganography and was put in place to prevent forgery. However it is also a privacy issue as outlined here, because every pages contains some additional information, that most people are not aware of.

I was curious if it was possible to visualize the pattern. In the article mentioned above, a test setup with a microscope and blue LED is described. Alternatively I found that it was sufficient to use a simple scanner and do some post processing with GIMP. Many scanners allow to scan with equal or higher resolutions and color depths than the printer can print.

The test page was printed with 600 dpi and also scanned with this resolution. The color depth was set to 16 bit. To make the little dots visible, an edge detection filter (in Gimp 2.6 under Filters, Edge-Detect, Edge...) was used. In the resulting image, the dots were visible best in the blue channel. The other two channels were removed as described here by adding an entirely green and an entirely red layer and setting the blending mode to subtract.

An excerpt of the empty part of the page can be seen here:

click on the image to enlarge

The pattern is repeated over the entire page. Interpreting the pattern is not so easy. Maybe it contains information like printer serial id, time, etc., but more effort would be required to figure that out exactly.