Yellow Dots of Mystery: Is Your Printer Spying on You?

Step 3: Scanner method

Scan your printed page on a color flatbed scanner at 600 dpi; this makes the dots visible in the scanned image. Modifying the scanner isn't necessary, since it already has a blue light.

The dots can be seen easily by enlarging the scanned image, or by performing a color separation in software and examining the blue channel. A wide variety of image-processing software can perform color separations.

Here are two ways of examining the blue channel with free/open source software:

1. In GIMP, go to Layers, then Channels, then Paths. In the channels tab, deselect the Red and Green channels.

2. With ImageMagick, run the following command at a command line (represented here by the $ prompt):

  $ convert -channel RG -fx 0 scan.tiff blue.png

This creates a new image blue.png containing only the blue channel.

You can also try converting to greyscale, by mapping the blue channel's value to intensity:
with ImageMagick, run the following command:

  $ convert --fx b scan.tiff grey.png

You can also do this interactively in Python if you have the Python Imagining Library (PIL) installed. From a Python prompt, run

  >>> import Image  >>> Image.open(scan.tiff).split()[2].show()

to see a greyscale image formed from the blue channel's intensity.

PIL can also help enhance the contrast. For instance, you can try the following to sharpen the color contrast between the dots and the page:

  >>> import Image  >>> blue = Image.open(scan.tiff).split()[2]  >>> blue.point(lambda x: (256-x)**2).show()