Does your printer hide tracking dots on every page?

If it is a colour laser printer or a colour photocopier, almost certainly yes. Since the 1990s most makers have printed a grid of yellow dots, each about a tenth of a millimetre across, over the whole page. The pattern repeats every centimetre or two and carries the printer's serial number and, on most machines, the date and time. Inkjet printers do not do it, and a black and white laser printer cannot, because it has no yellow. To see the dots on a page of your own, scan it in colour at 600 dpi and drop the scan on DOTS, which finds them, shows the pattern, reads it where the layout is known, and writes a copy of the scan with the dots painted out. Nothing is uploaded.

What the dots are

The pattern is called a machine identification code, and the trade name is forensic tracking or printer steganography. It is not a defect and it is not a fault of one brand. It was added by agreement between printer makers and governments so that a counterfeit banknote or a forged document could be traced to the machine that printed it. That was the stated purpose. The marks go on every page, so they also trace the flyer, the letter, the lease and the leaked memo.

Yellow was chosen because it is the colour the eye separates worst from white paper. Each dot is a single spot of yellow toner, and a page carries hundreds of them in a repeating block. Under a magnifier they look like dust. Under a blue light they turn dark, because blue paper reflects blue and yellow toner absorbs it, which is why the classic way to see them is a blue LED torch in a dark room.

Which printers do it

The Electronic Frontier Foundation kept a list of printers that did and did not, built from pages readers sent in, and stopped maintaining it years ago because the answer had become "assume yours does". Xerox, Canon, HP, Brother, Konica Minolta, Ricoh, Kyocera, Epson, Lexmark and Dell colour laser machines have all been found printing the dots. Some newer machines have moved the code somewhere less obvious than a grid of yellow, which is one more reason to check a page rather than a list.

The check is short. Print a page with a little text on it, scan it in colour at 600 dpi, and save the scan as PNG rather than JPEG, because JPEG compression smears exactly the faint, small, isolated spots you are looking for. Then open the scan in DOTS. If the page shows a lattice, the printer does it. If it shows nothing, try a page from a different tray or a different colour mode; a page printed in "black only" mode on a colour printer sometimes carries no dots and sometimes still does.

What can be read

Only one layout has a published decoding. In 2005 the Electronic Frontier Foundation worked out the block the Xerox DocuColor series prints: fifteen columns by eight rows, with the top row and the leftmost column as parity, and the other columns holding the minute, the hour, the day, the month, the year and the serial number. DOTS reads that block and checks the parity, so a reading that does not add up is reported as a bad read rather than a wrong date.

Every other maker uses a layout that nobody has published. For those, the page finds the dots, works out the pitch and the angle of the grid, measures how big the repeating block is, and shows it folded onto one tile so the code can be seen; it does not pretend to read it. Two pages from the same printer show the same tile, which is enough to say they came from one machine even without knowing which one.

What removing them does

DOTS writes a copy of the scan with every dot painted over with the paper beside it, so the copy carries no code. That protects the copy. It does nothing for the sheet of paper, which still has the dots on it, and it does nothing for the other ways a page can be traced: the wording, the fonts, a crease, a mark on the drum that the printer leaves on every page, the metadata in the file the page was printed from. In 2017 a document published by a news site was traced, according to the reporting at the time, in part by the dots on the scan it published. The lesson usually drawn is "remove the dots"; the fuller lesson is that a scan of a page carries the page's marks, and only some of them are yellow.

If you are going to share a scan, the cleaned copy is the one to share. If you are going to hand over the paper, nothing on this page helps; print it on a machine that does not mark, or on a black and white one.

What the scan has to be

• Colour. A greyscale or black and white scan has already thrown the yellow away.
• 600 dpi. At 300 dpi a dot is one pixel wide and the finder cannot tell it from noise. At 600 it is two or three, which is enough.
• PNG or TIFF. JPEG at the scanner's default quality blurs the dots into the paper.
• Not a phone photo. A phone camera is too soft at the scale of a tenth of a millimetre, and its own processing removes small flecks of colour on purpose.

DOTS says so when a scan is too soft or too small, rather than reporting a clean page that was never checked.

What this does not cover

The dots are one identification mark, and the only one that has been studied in public. Some machines are believed to mark pages in ways that are not yellow and not dots, and nothing here finds those. A page printed from a file also carries whatever the file carried: a PDF's author field, a Word document's revision history. SCRUB and FLATTEN deal with the file before it is printed; DOTS deals with the page after.