How do holographic images work?
Visa cards are printed with little holographic doves as forgery protection, and I've seen similar holographic images printed on things no thicker than a piece of construction paper. Soon there will be chocolate bars with holographic decorations etched on the surface (this according to Scientific American). How are these little holographic pictures made and how do they fool the eye into seeing depth where there really is none?
As is often the case with technical subjects, Susannah, we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong. Being a journalist and therefore shameless, we naturally opt for the latter. What follows is the Ollie North explanation of holography — it might get you past a congressional committee, but don't try it on your Ph.D. board.
A reflection hologram, the kind found on a credit card, is a high tech version of those plastic novelty pictures we used to buy at the dimestore — the kind where the image changes when you tilt it. The hologram's surface is an emulsion that can be thought of as consisting of many tiny facets, each containing a fraction of a larger image. As you look at the hologram you see a set of facets that together constitutes one perspective of the holographed scene. As you tilt the hologram, a different set of facets comes into view showing the scene from a slightly different perspective. The changing perspective creates the illusion of three dimensions.
Simple, no? OK, now for a Jack Anderson-like expose of the many lies and omissions in the preceding.
L&O #1. There aren't really any tiny facets. Actually what you've got is a set of quasihyperboloidal interference fringes. Interference fringes reflect a percentage of the light that strikes them. Amounts to the same thing as tiny facets, but they look a lot different and from the standpoint of conceptual grabbiness they're strictly from hunger.
L&O #2. The change of perspective isn't the only thing that creates the 3-D effect. There's also parallax shift. Your eyes, being two inches apart, look at the scene from slightly different angles, and thus see two different sets of "facets." Your brain combines the two images to create one scene with the illusion of depth, just as with a stereoscopic viewer.
L&O #3. I didn't tell you anything about lasers, wavefronts, or coherent light. Do I hear anybody complaining? I didn't think so. However, for those who must know, lasers are essential to creating holograms because they're the only known way to create the requisite interference fringes. Memorize the preceding sentence so you can mutter it next time some would-be expert (e.g., your precocious eight-year-old) starts quizzing you on the subject. We may not explain everything in this column, but we give you enough to get by.