Monday, April 5, 2010

Hello Holography

Are you tired of your boring old two-dimensional photographs? Ever gone through your old family photo albums and wished that you could relive some moments? Don't despair; holograms are here!

Holograms are commonly described as "3D photographs," though the processes involved in making the two different types of photographs are quite different. Both conventional photographs and holograms are made on a flat piece of photographic film that reacts to different intensities of light, but holograms render information about the depth of the object: the object appears to literally pop out of the page. How do holograms manage to do that?

When you take a conventional photograph, your camera opens the shutter to let light through to hit the film. The light that enters your camera has already hit and reflected off the object that you're capturing. The object reflects light with different intensities (brightness) depending on the physical characteristics of the object. The chemicals on the film (usually a light-sensitive compound called silver halide) react with the light. How much the film reacts depends on how intense the light is. The regions that react more are darker in the resulting photograph. So, a photograph is merely a record of the intensity distribution of the object. However, it does not record any information about the phase of the light waves (see Figure 1), which we need if we want to know anything about the depth and dimensions of the object (a point that is further from the camera will have a phase different from the phase of a point closer to the camera).

Figure 1

How, then, do holographers capture—and then render—information about depth? They do it by making use of a standard or reference. This is similar to when you measure something with a ruler. You could just lay your ruler down on the surface that you're measuring and record a number, but that number is useless if you don't know what that number is relative to. You need to designate a specific point as zero (i.e. the reference). In holography, this reference is called the reference beam. The reference beam will combine with the light from the object, creating an interference pattern (see Figure 2). The film records the interference pattern. Since the intensity at any point in the interference pattern also depends on the phase of the light from the object, the hologram contains information about the phase as well as the intensity of the light waves.

Figure 2

Figure 3

In order to get a useful interference pattern, a laser must be used for the light source because lasers emit light of a single wavelength (color). Light from the sun or light bulbs contain multiple wavelengths. It is crucial for the reference beam and the light from the object to have the same wavelength because we want an interference pattern that looks like the wave in Figure 2. If the two light waves have different wavelengths, they would not combine to form a wave that we want (Figure 3 is an example of this).

Now, let's look at the set-up for doing holography (Figure 4). A laser is pointed at a beam-splitter (e.g. a prism), which splits the beam into the reference beam and the object beam. These two beams will continue to have the same wavelength throughout the whole process. The object beam only reflects off the object, and the reflected light contains waves with different phases depending on the physical characteristics of the object. The reference beam and the object beam are then recombined, and interference patterns are created between the waves at each point. These interference patterns are then recorded on the film.

Figure 4 (

When you look at the finished product, it might look like a sheet of random squiggles, but not like the image of the original object. Remember that the film only records the interference pattern between the object beam and the reference beam. You “bring it to life” by illuminating it with the reference beam. Going back to our distance measurement analogy, if you measure only the distance between point A and point B, you cannot go back to that space and pinpoint where point B was if you didn’t record where point A was. In the case of holography, point A is the reference beam and the distance measurement is what the film captures. Bringing the image to life is figuring out where point B is. When you shine the reference beam on it, the film will reflect that light, thus recreating the object beam. When that light hits your eyes, you don’t see a flat 2D image; you see the 3D representation of the object. Seeing that light is the same as simply looking at the object directly (light has to bounce off of an object and into your eyes in order for you to see that object). Your brain will interpret the light from the film the same way it will interpret the light from the object, so the image will appear as it would appear if you were simply looking at the object directly.

If I’ve piqued your interest in holography and you’d like to see some holograms, you should go to the “Explore Holography” event on April 27th from 3-4PM at the MIT Museum, where Seth Riskin, the Museum’s Manager of the Holography and Spatial Imaging Initiative, will take you on a behind the scenes tour to look at these marvels of science and art. Enrollment is limited and pre-registration is required. If event fills up before you get a chance to register, you can still visit the Museum’s holography collection (the largest and most comprehensive collection of holograms in the world) on MIT Museum Free Day (April 24th, 10AM-5PM) or on your own time (the Museum is open daily from 10AM-5PM).

Next time, I’ll talk about the different applications of holography.

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