HOLOGRAMS GO FUTURISTIC.
Scientists now have taken their own bird's-eye view at holograms for a slew of new uses--from images on credit cards and CD-ROMs to thwart counterfeiters, to medical image scans that recreate images of the human brain. A hologram is like a photograph, only infinitely more realistic--it's 3-D, or three-dimensional. For instance, a fiat, two-dimensional photo records light reflecting off an apple from one angle--the front of the fruit. But a hologram captures light bouncing off the apple from every angle and direction, just the way you see the apple in real life (see "Making A Hologram," below).
If you've ever watched 3-D flicks, you may think, "Ha! I've seen lots of holograms." Not so. Three-dimensional movies or photos are actually made with two identical images spaced a few centimeters apart. Special 3-D glasses combine the images to trick your eyes into thinking they're seeing 3-D.
You probably have seen holograms on stickers, watches, key chains, and even swim goggles. But holograms aren't just zany high-tech art. For a close-up look at two futuristic kinds of holograms, read on.
Conventional holograms are made using laser light. But researchers at the Massachusetts Institute of Technology (MIT) have developed a novel approach: They create electroholograms--holograms made with computers, not lasers. "You just compute what the laser would do," says researcher Wendy Plesniak at MIT's Media Lab. Electroholograms can create 3-D images of something that doesn't exist at all--like a design for a 21st-century car.
Car designers often use computer-aided design (CAD) to create a 3-D diagram of a car. But a computer screen can produce only flat images of the CAD Car.
To see the car in 3-D, Honda, a Japanese company, turns to Media Lab to create an electrohologram of its design. This helps car designers view futuristic models from all directions--without ever having to build the car!
Programmers actually use CAD data to create the electrohologram. The computer uses math equations to simulate the action of a laser beam. The imaginary beam hits each point on the two-dimensional car design. Then, the computer calculates the interaction between the waves from the object beam and the waves from the reference beam. Result: a digital representation of the interference pattern.
But this alone won't create a visible holographic image. Light needs to interact with the two-dimensional interference pattern in the computer to display the hologram. Enter: a crystal.
The hologram has to be displayed and illuminated outside the computer so viewers can see it in 3-D. To do this, an electronic device converts the interference pattern created by the computer--currently in digital code--into a radio signal with low and high frequencies. The varying radio frequencies can change a crystal's atomic structure. (Light waves just pass through the crystal.)
The radio signal enters a clear crystal and "carves out" the hologram inside it. Depending on its frequency, the radio signal makes some parts of the crystal more dense and others less dense, Plesniak explains. Shining a real laser beam through the crystal projects a freestanding, holographic image of the car.
Media Lab is now testing a robotic device to let viewers actually interact with the hologram. "Car designers would be able to go in and manipulate the 3-D shape that they're seeing--to make a curve a little more curvy," says Plesniak. The device would let viewers reach into the hologram, touch it, and change its shape!
Suppose doctors want to examine a three-dimensional image of the human brain. Such an image would make it easier to view a tumor and determine the best way to remove it--before cutting open a human skull. One solution: Make a hologram of the brain.
Voxel, a company in Laguna Hills, California, creates holograms out of a patient's CT (computer tomography) or MRI (magnetic resonance imaging) scans. CT scans are high-resolution X rays that "photograph" cross-sectional slices of bones, as well as blood vessels and soft tissue, like the brain. MRI scans are similar but use magnetic fields to peer at soft tissue in the body. Both scans provide very detailed pictures of a person's anatomy, but only as flat images. That's where holograms come in.
Recently hailed by Life magazine as one of the medical breakthroughs for the 21st century, Voxel's medical holograms give doctors a 3-D view of the human body. A hologram of a CT or MRI scan would allow brain surgeons, for example, to measure the exact size, depth, and location of a tumor.
First, a surgeon would send a patient to get an MRI scan of his head. The MRI scanner snaps, say, 100 images of the patient's brain. Each image is a parallel, cross-sectional slice of the brain about a millimeter or less apart.
Then, the surgeon sends the images to Voxel. Stephen Hart, one of the company's founders, explains how he makes a single hologram out of 100 brain scans. Hart uses a machine called a Digital Holography Camera. The camera shines a laser light through the first brain scan, and projects the image on a screen. The laser light reflecting from the screen hits a holographic film, which records the image.
Next, the screen automatically moves back from the holographic film by one millimeter--the same distance between the actual brain scans. The camera superimposes the second scan onto the holographic film, repeating the same procedure for each of the 100 slices of brain. The whole process takes about 10 minutes. Result: a life-size hologram of the person's brain that you can view from any direction.
But that's not all. "The freaky thing is not just looking around the hologram, but sticking things into it," Hart says. A surgeon can bring Voxel's hologram into the operating room, and halfway through extracting a brain tumor, the doctor can take pieces he's already cut out and hold them inside the hologram. This lets him see how much more of the tumor he's got to remove.
How much more futuristic holograms can go is anybody's guess.
RELATED ARTICLE: MAKING A HOLOGRAM
A conventional hologram is made using laser light. Light is basically a wave with a crest (the highest point) and a trough (the lowest point). The distance between one crest and the next is the wavelength.
Conventional light, whether from the sun or a lightbulb, consists of scattered and different wavelengths. Put a prism in front of the waves and you'll see a variety of colors, depending on the wavelength. But, shine conventional light at an apple to try to make a hologram, and it won't work. That's because white light scatters too much.
So holographers have turned to a type of highly focused light that's a cinch to manipulate: the laser beam. Light from a laser is monochromatic. This means light waves emitted by a single laser all have the same wavelength; hence a single color, either blue, red, or green. Laser light is also coherent. Its light waves travel in phase with each other--the crest of one wave aligns with the crest of another.
Since a laser light's wavelengths are identical, the laser is an ideal tool to make holograms. Follow the diagram to understand more:
(1) A type of mirror, called a beam splitter, divides a beam of laser light into two beams.
(2) The first beam, called the object beam, points toward the object to be holographed--in this case, an apple.
(3) The beam of laser light hits a diverging lens, which spreads the light out to capture the entire object.
(4) Light waves bounce off the apple and onto a piece of film that records them. Because of the apple's shape, some light waves arrive at the film sooner than others. The waves of the object beam are now out of phase.
(5) The second beam, called the reference beam, shines directly on the film--all of its light waves arrive at the film at the same time, in phase.
(6) When the waves of the reference beam meet the waves of the object beam on the film, they collide with each other to create an interference pattern.
(7) The film records the interference pattern created by the clash of waves reflected from around the apple and from the reference beam. Holographic depth is created because the film records all the waves emitted from different directions off the apple, not just from a head-on frontal view.
A hologram's interference pattern simulates tiny mirrors facing different angles. When an illuminating light reflects off the recorded interference pattern, you see the three-dimensional image that is the hologram.
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|Title Annotation:||future of animated billboards|
|Author:||CHANG, MARIA L.|
|Date:||Feb 8, 1999|
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