Answer Geek: How Holograms Work

-- Q U E S T I O N: How does a hologram work? I have a hologram on my desk at work. No matter which direction I look from, the holographic car will only show the side view. Can you explain this, so I can finally get some work done instead of dedicating hours staring at something that just seems to mock me?

— Dwight J.

A N S W E R: But sitting all day staring at a hologram? I don't know, Dwight. That's pretty pathetic. So I'd like to make a deal with you: How about if I answer your questions and then you go back to work?

Holograms at Work

While most of us haven't spent as much time staring at a hologram as you have, Dwight, we all know what one is. In recent years they've become ubiquitous as additions to credit cards and software packaging, where they are used as an anti-counterfeiting device, rather like a watermark on a check.

But this type of application isn't as interesting as other useful examples. Holograms of the same object taken at different times, for example, can be used to provide very precise measurements of changes in size and shape, making holography a good technique for industrial stress analysis and quality control. Using CAT scans to produce holographic images is an increasingly common diagnostic technique for doctors and dentists. There is even a new generation of optical devices that use holographic projections as lenses. And holographic data storage may turn out to be one of the next big advancements in the field of computer science.

Shedding Some Light

So what is holography, anyway? In simplest terms, it is a photographic technique for producing three-dimensional images. In conventional photography, a picture is created by allowing light to strike a chemical (typically, silver bromide) embedded in a layer (or layers, if it's a color image) of gelatin on a strip of film. The result is a two-dimensional record of the way the objects in front of the camera lens reflected the available light when the shutter snapped open.

A hologram records not just reflected light, but light interference patterns. That leads to a three-dimensional image that reproduces not only darkness, light, and color, but depth, texture, shape, and the relative position of objects.

To create those light interference patterns, a laser is necessary. The reason? The light emitted by a laser is coherent, which means that all of the light waves emitted are at exactly the same wavelength, with all of the peaks and troughs in phase with each other. In contrast, sunlight or the light from a flashbulb — typical light sources for conventional photographs — are a combination of light waves of many different wavelengths.

When you make a holographic image, you start with a laser light beam and split it into two separate beams by passing it through a half-silvered mirror. The two beams then pass through lenses that make them expand. One of the two beams is directed onto a very high-contrast, finely-grained photographic film. This is called the reference beam.

The second beam is aimed at the object being photographed so that some of the light is reflected onto the photographic film. This second stream of light is called the object beam. Because the object beam has been scattered en route to the photographic film, the two beams are no longer perfectly in phase. The now out-of-step waves interfere with each other, creating very, very fine lines called fringes.

On the Fringes of Technology

It is the fringes that give a hologram its dimensionality. That's because the degree to which the two beams are out of phase is due to the way light is scattered by the object, which results in part from the relative distance that various parts of the surface of the object (or objects) are from each other.

That brings us to the second part of Dwight's question. "No matter which direction I look from," he notes, "the holographic car will only show the side view." The answer is pretty straightforward, I think. Holograms are true three-dimensional images and not some sort of cheap optical illusion, a fact that becomes apparent when you look at one from slightly different angles and see slightly different aspects of the image. But there is a limit. Because the object beam comes from a single direction, the final image can only show the perspective that the object beam "saw" it from in the first place.

So, Dwight. I hope that answers your questions. So please, get back to work. And the next time you need to kill some time, why don't you try something a little more creative? Personally, I've always been fond of scheduling unnecessary meetings as a good way to avoid work because it has the twin virtues of reducing the productivity of fellow employees and providing good cover for your schemes.

Todd Campbell is a writer and Internet consultant living in Seattle. The Answer Geek appears weekly, usually on Thursdays.