Lenticular printing

Last updated: September 8, 2008.

Every day, there are hundreds—perhaps even thousands—of advertising messages knocking on your head trying to gain access to the part of your brain that decides to buy things. With so much money at stake, it's hardly surprising that advertisers go to such extraordinary lengths to catch out attention. The only trouble is, our brains habituate: they quickly get used to seeing the same thing (psychologists call it a "visual stimulus") over and over again. So the advertisers have to keep thinking of new tricks to stay one step ahead. One of their latest ideas is to print posters, magazines, and book covers with "lenticulars"—images that seem to change as you move your head. Let's take a closer look at how they work!

Photo: The LEGO® robot image on the cover of my book Cool Stuff Exploded changes as you tilt it back and forth. A plastic lenticular insert shows you one of two different images depending on which side you look from.

What have lentils got to do with it?

Nothing! Lentils are tiny orange, green, or brown pulses popular with vegetarians and—no—they have nothing to do with how book covers work. The connection between "lentil" and "lenticular" is simply a matter of words. Lenticulars are so-called because they use lenses, which are pieces of plastic or glass that bend (or "refract") light to make things look bigger or smaller. Lenses got their name because some of them just happen to look a bit like lentils! You can find out how lenses work in our article on binoculars (we even tell you how to make a lens of your own, in about 5 seconds flat, from a drop of water).

Photo: Lentils like this one gave lenses their name. Convex lenses bulge out in the middle like lentils, while concave lenses "cave in" in the middle and bulge out at the edges.

How do you make a lenticular?

How do you make something like our book cover up above? You take your two different images and load them into a computer graphics program. The program cuts each image into dozens of thin strips and weaves them together so the strips from the first image alternate with the strips from the second. This process is called interlacing. If you look at the doubled-up image printed this way, it's just a horribly confusing mess, but not for long! Next, you place a transparent plastic layer on top of the doubled-up image. This is made of dozens of separate ridges called lenticles. Half of them lean to the left and half of them lean to the right, but they're arranged so they alternate: left, right, left, right. That means, when you look at the image from the left, the left-leaning lenticles show you only half the printed strips—the ones directly underneath them—so you see only the first of the two images. When you look from the right, you can't see through these left-leaning lenticles at all. Instead, you see only the strips under the right-leaning lenticles—and the second image. Move your head back and forth and the image flips back and forth too like a kind of "visual see-saw".

Photo: Here's the cover of my book in close-up. Now you can see the individual lenticles—the tiny plastic ridges that send images one way or the other, depending on where you're eyes are in relation to the book cover.

For all this to work properly, everything has to be printed with incredible precision. The lenticles have to be exactly the same size as the printed strips underneath them and lined up with them exactly. In theory, you can show many different images with a lenticular: you could have half a dozen different images and as many different kinds of lenticles, all pointing in slightly different directions, so an advertising poster slowly and subtly changes its message as you walk past!