Watermark-type photocopying question

I just photocopied an Illinois Certificate of Title of a Vehicle.
The original is printed on paper that appears to be grey and tan, with a pattern like a 3-dimensional cube terreration (correct word?), where it can look like the cubes are angling one way or another.
When photocopied, three columns of the word “VOID” appear.
The Secretary of State’s site notes this as a security feature. But I was wondering if anyone could explain how this effect is achieved?
When held up to a light, you can see the watermark of the State of Illinois seal, but not the words “VOID”.

It’s called a void pantograph.

That page also gives a description of several other anti-copy techniques.

Thanks for the terminology and the link, but can anyone explain exactly how this is done? The page states only: This is a special pantograph screen that has the word “VOID” hidden in the pantograph by the use of special screens and background designs. When photocopied by a color copier, the word “VOID” appears on the copied document.

What do they mean by “special screens and background designs”? How does the design show up on the photocopy, when it is absolutely invisible under normal light? Is it printed with a special kind of ink or something? Does a copier use a specific spectrum of light that causes this effect?

(Oh - and the term I was seeking in the OP was tesselation.)

Photocopiers generally use a way brighter light then you’d use to look at it. Try looking at it with a really bright light source. Like holding a flash light behind it in a dark room or in front of a car headlight at night.

My guess is either it’s embed in the paper or it uses special low reflective ink.

Don’t have it before me, but yesterday I held it up to a sunny window and saw nothing other than the watermark. I’ll try when I get home.

It is not special inks. It is more due to the fact that the patterns are made up of very fine lines. The lines are well below the resolution of the copier or scanner. Scanners and copiers are designed to reproduce normal sized text. You could make much higher resolution scanners that would copy these things correctly but they would be more expensive and probably slower than what we have now.

That was my understanding about the fancy border, that it was so intricate that it was hard to copy clearly. But would you elaborate please upon how the “fine lines” that are “below the resolution of the copier” make the words “VOID” appear on the copy.

I don’t have your specific document in from of me. But other documents that I have seen have the void obscured by the fine lines to the point that people cannot make out the void. The scanners lower resolution and lack of spatially frequency band limiting optics will remove the fine lines. It also helps that copiers intended mainly for text documents employ a threshold below which they decide an area is white. This tends to make the fine line completely disappear instead of making the region a light gray. I have played around with copiers and changing the contrast you can make those sorts of things appear and disappear. When you muck around with contrast the rest of the document sometimes starts to look crappy because you get the text on a somewhat muddy looking gray.

Copiers and scanners could have lenses put on them that would blur the fine lines such that when they copied these sorts of things you would just get sort of a light gray where these complicated patterns are on the paper but that would add expense and make the edges of the text somewhat blurry.

This is similar to micro printing showing up as black lines or fine lines of the fancy borders showing up as gray. They are exploiting artifacts to make it apparent to the casual observer that this is a copy.

Of course! It’s the lack of spatially frequency band limiting optics. Why didn’t I think of that! :smack:

(Man, it’s confusing being an 18th century man in a 21st century world!)

when you ask a question about technology they tend to have technical answers.

The second paragraph was trying to explain in simpler terms what I meant by band limiting optics.

It really means that your eyes are slightly blurry as compared with the rods and cones inside your eyes. So fine lines close together tend to be seen as a level of gray or general brightness of color.

Scanners without the lenses that the eye has tend to pick certain spots and assume that the color of that specific spot represents the color or brightness of the area around the spot. This is called sampling. There is a ton of theory and interesting findings about how often you need to sample to reproduce images accurately. So it is a crap shoot if the copier will see a fine line in a specific area or just the white background.

Basically the guys designing documents are hacking the scanners and copiers to make weird images appear.

As and interesting side note The US government and EU governmets have convinced printer and copier companies to detect that they have currency and prevent people from printing money.