Amazing Laser Application 8: Holography!

What’s the application? Holograms are images of objects that appear three-dimensional– if you move your head as you look at a hologram, you will see the usual parallax effects, unlike a normal photograph, which is fixed. So, if your hologram includes one object that is partly behind another object, you can see around the obstruction by moving a bit to the side, just as you would if the original objects were in front of you.

What problem(s) is it the solution to? 1) “How can we jazz up flat images and make them look more lifelike?” 2) “How can we make credit cards harder to copy?”

How does it work? The key to holography is the interference of light. The simplest sort of hologram to make is a transmission hologram, which works very nicely to illustrate the key ideas, as in this image from HyperPhysics:

To make a transmission hologram, you split the beam from your laser, and use half of it to illuminate your object, while the other half falls directly on the film. The two different beams will interfere with each other, and what you record on the film will be the interference pattern, rather than a normal image of the scene. If you look directly at the film, you will just see a pattern of stripes and whorls that doesn’t look like much of anything.

If, rather than looking directly at the developed film, you shine a laser onto the film from the same angle as the beam that directly illuminated it in the original set-up, though, the light from the laser will diffract off the interference pattern, and what you end up with on the far side of the film will be identical to the light from the other beam, the one that was illuminating the object. Which means that somebody on that side of the film looking back through it will see an image of the object that was originally present, as if it were still there, and illuminated by the laser being used to project the image.

This diffracted light has all of the characteristics of the original light, including the phases and everything. Which means that it recreates the full pattern of the original light in the cone of the beam, including the parallax effects and all that. So you get a complete three-dimensional image.

With a slightly different arrangement of components, you can make a reflection hologram, and you can convert one of those into a rainbow hologram that doesn’t require coherent light to make it work. Which leads to all those shiny rainbow hologram stickers and holograms on credit cards and currency.

Why are lasers essential? In order to get a clean interference pattern, you need a coherent source of light. That is, the waves that are directly illuminating the film need to have basically the same phase as the waves that are bouncing off the object– in the absence of the object, the question of whether the peaks line up to give you constructive interference should only depend on the lengths of the two different paths they followed to the film.

In an incoherent source like a lamp, the phase of the light fluctuates rapidly and randomly. That means that the interference pattern you get changes from one nanosecond to the next, and unless you’re absurdly careful about setting things up, you’ll just get a big smear on the film. A laser has vastly better coherence properties, and will give you a steady pattern, provided your paths are reasonably similar in length.

You could make a hologram using heavily filtered light from a lamp, and extraordinary care, but you really don’t want to do it that way. A laser turns it into something that can be done as a lab for non-science majors.

Why is it cool? Dude, it’s a three-dimensional image!

Also, because it is an interference phenomenon, the pattern at any given point on the film depends on light from all parts of the illuminated object. Which means that any piece of the image contains the whole picture. This makes holography a very robust image-storing method, and also provides the hook for a really good William Gibson story.

(The “every part contains the whole” aspect is also what inspired yesterday’s optics quiz. It’s not exactly the same, but the end effect is somewhat similar, and for a similar reason– just as the light that makes up an image from a lens passes through all parts of the lens, the light that produces the interference pattern at the hologram film comes from all parts of the illuminated object.)

You don’t have to take my word for it, though. Holograms are sufficiently cool that Dennis Gabor won the 1971 Nobel Prize in Physics for inventing the technique.

Why isn’t it cool enough? It’s still a bit of a hassle to make holograms, and their use is somewhat limited. It hasn’t yet led to the three-dimensional home viewing systems that I was promised by science fiction stories when I was a kid. No matter what cheesy effects CNN may use.