In this first installment of Reader Mailbag, we answer a question from (ahem) Crispy HERB Polenta, who writes:
Do you take requests for blog postings? I was looking at the cover of the new Rolling Stone 40th anniversary issue and wondering how they made it all shiny and stuff, and then I thought, "Why, if anyone knows how this works, it's probably Gregg Favalora, Mister Fun With Optics hisself!" And thus, Mister Fun With Optics, this question. The cover has a silver background that shimmers like a CD -- I'm not sure of my terminology here, whether this is refraction or (my guess) diffraction -- and the interior of the giant "Ro" is made up of lots of tiny red fragments that sparkle in different directions.
I don't think these are new tricks, but I'm really curious how it gets done and embedded in a piece of paper only slightly thicker than normal. How about it, Mister Fun With Optics?
My ego more than adequately stoked, I set out to answer his question.
Hey, Mr. Polenta -
The magazine cover with eye-catching rainbow streaks and colorful sparkles is similar to the packaging on quite a few products these days, such as Edge shaving cream and Listerine.
I suspected that you're quite right; the magazine cover art is indeed diffractive, our rainbowy adjectival friend. When light hits a material with a very fine pattern, such as many hundreds or thousands of grooves or lines or squiggles per millimeter, various colors of the light may change direction. (Red changes direction more than blue.) That's why CDs (and people's hair in sunlight) have rainbow patterns. They're... groovy!
But how is it made? What company manufactures it?
Enter the sleuthy holography folks over at www.HolographyForum.org. According to the crack squad of readers, several companies sell a collection of fairly standard glinty/rainbowy paper. The holo-experts explained that the rainbow stuff is probably made with a very high-resolution version of a dot-matrix printer that can output so many dots per inch that room light breaks up appreciably into its component colors. I don't have the specifics on just what that pattern is, but it gets into Fourier transforms and stuff like that. Probably bunches of superimposed gratings at various angles.
If you want to buy some, maybe the nice folks at Unifoil can help you out; the cover might be from their Unilustre line.
The red sparkles are no less high-tech; Ed Wesley suggests that they're created by WaveFront Technology, Inc. They have all sorts of neat stuff in addition to the White Sparkle Film that composes the Rolling Stone cover.
I hope this answers your question! If you want to learn more, feel free to jump right in to that holographyforum.org link and ask away.