If you tear a hologram in half, you can still see the whole image in each piece. The same is true with smaller and smaller pieces.

Introduction to How Holograms Work

­If you want to see a hologram, you don't have to look much farther than your wallet. Th­ere are holograms on most driver's licenses, ID cards and credit cards. If you're not old enough to drive or use credit, you can still find holograms around your home. They're part of CD, DVD and software packaging, as well as just about everything sold as "official merchandise."

­Unfortunately, these holograms -- which exist to make forgery more difficult -- aren't very impressive. You can see changes in colors and shapes when you move them back and forth, but they usually just look like sparkly pictures or smears of color. Even the mass-produced holograms that feature movie and comic book heroes can look more like green photographs than amazing 3-D images.

­On the other hand, large-scale holograms, illuminated with lasers or displayed in a darkened room with carefully directed lighting, are incredible. They're two-dimensional surfaces that show absolutely precise, three-dimensional images of real objects. You don't even have to wear special glasses or look through a View-Master to see the images in 3-D.

If you look at these holograms from different angles, you see objects from different perspectives, just like you would if you were looking at a real object. Some holograms even appear to move as you walk past them and look at them from different angles. Others change colors or include views of completely different objects, depending on how you look at them.

Holograms have other surprising traits as well. If you cut one in half, each half contains whole views of the entire holographic image. The same is true if you cut out a small piece -­- even a tiny fragment will still contain the whole picture. On top of that, if you make a hologram of a magnifying glass, the holographic version will magnify the other objects in the hologram, just like a real one.

Once you know the principles behind holograms, understanding how they can do all this is easy. This article will explain how a hologram, light and your brain work together make clear, 3-D images. All of a hologram's properties come directly from the process used to create it, so we'll start with an overview of what it takes to make one.

­

Making a Hologram

It doesn't take very many tools to make a hologram. You can make one with:

  • A laser: Red lasers, usually helium-neon (HeNe) lasers, are common in holography. Some home holography experiments rely on the diodes from red laser pointers, but the light from a laser pointer tends to be less coherent and less stable, which can make it hard to get a good image. Some types of holograms use lasers that produce different colors of light as well. Depending on the type of laser you're using, you may also need a shutter to control the exposure.
  • Lenses: Holography is often referred to as "lensless photography," but holography does require lenses. However, a camera's lens focuses light, while the lenses used in holography cause the beam to spread out.
  • A beam splitter: This is a device that uses mirrors and prisms to split one beam of light into two beams.
  • Mirrors: These direct the beams of light to the correct locations. Along with the lenses and beam splitter, the mirrors have to be absolutely clean. Dirt and smudges can degrade the final image.
  • Holographic film: Holographic film can record light at a very high resolution, which is necessary for creating a hologram. It's a layer of light-sensitive compounds on a transparent surface, like photographic film. The difference between holographic and photographic film is that holographic film has to be able to record very small changes in light that take place over microscopic distances. In other words, it needs to have a very fine grain. In some cases, holograms that use a red laser rely on emulsions that respond most strongly to red light.

There are lots of different ways to arrange these tools -- we'll stick to a basic transmission hologram setup for now.

  1. The laser points at the beam splitter, which divides the beam of light into two parts.
  2. Mirrors direct the paths of these two beams so that they hit their intended targets.
  3. Each of the two beams passes through a diverging lens and becomes a wide swath of light rather than a narrow beam.
  4. One beam, the object beam, reflects off of the object and onto the photographic emulsion.
  5. The other beam, the reference beam, hits the emulsion without reflecting off of anything other than a mirror.

In the next section we'll look at workspace requirements.

You can create your own holography table using inner tubes and sand to dampen vibration.

Workspace Requirements

Getting a good image requires a suitable work space. In some ways, the requirements for this space are more stringent than the requirements for your equipment. The darker the room is, the better. A good option for adding a little light to the room without affecting the finished hologram is a safelight, like the ones used in darkrooms. Since darkroom safelights are often red and holography often uses red light, there are green and blue-green safelights made specifically for holography.

Holography also requires a working surface that can keep the equipment absolutely still -- it can't vibrate when you walk across the room or when cars drive by outside. Holography labs and professional studios often use specially designed tables that have honeycomb-shaped support layers resting on pneumatic legs. These are under the table's top surface, and they dampen vibration. You can make your own holography table by placing inflated inner tubes on a low table, then placing a box full of a thick layer of sand on top of it. The sand and the inner tubes will play the role of the professional table's honeycombs and pneumatic supports. If you don't have enough space for such a large table, you can improvise using cups of sand or sugar to hold each piece of equipment, but these won't be as steady as a larger setup.

To make clear holograms, you need to reduce vibration in the air as well. Heating and air conditioning systems can blow the air around, and so can the movement of your body, your breath and even the dissipation of your body heat. For these reasons, you'll need to turn the heating and cooling system off and wait for a few minutes after setting up your equipment to make the hologram.

These precautions sound a little like photography advice taken to the extreme -- when you take pictures with a camera, you have to keep your lens clean, control light levels and hold the camera absolutely still. This is because making a hologram is a lot like taking a picture with a microscopic level of detail. We'll look at how holograms are like photographs in the next section.

In photography, light passes through a lens and a shutter before hitting a piece of film or a light-sensitive sensor.

Holograms and Photographs

When you take a picture with a film camera, four basic steps happen in an instant:

  1. A shutter opens.
  2. Light passes through a lens and hits the photographic emulsion on a piece of film.
  3. A light-sensitive compound called silver halide reacts with the light, recording its amplitude, or intensity, as it reflects off of the scene in front of you.
  4. The shutter closes.

You can make lots of changes to this process, like how far the shutter opens, how much the lens magnifies the scene and how much extra light you add to the mix. But no matter what changes you make, the four basic steps are still the same. In addition, regardless of changes to the setup, the resulting picture is still simply a recording of the intensity of reflected light. When you develop the film and make a print of the picture, your eyes and brain interpret the light that reflects from the picture as a representation of the original image. You can learn more about the process in How Vision Works, How Cameras Work and How Film Works.

Like photographs, holograms are recordings of reflected light. Making them requires steps that are similar to what it takes to make a photograph:

  1. A shutter opens or moves out of the path of a laser. (In some setups, a pulsed laser fires a single pulse of light, eliminating the need for a shutter.)
  2. The light from the object beam reflects off of an object. The light from the reference beam bypasses the object entirely.
  3. The light from both beams comes into contact with the photographic emulsion, where light-sensitive compounds react to it.
  4. The shutter closes, blocking the light.

In holography, light passes through a shutter and lenses before striking a light-sensitive piece of holographic film.

Just like with a photograph, the result of this process is a piece of film that has recorded the incoming light. However, when you develop the holographic plate and look at it, what you see is a little unusual. Developed film from a camera shows you a negative view of the original scene -- areas that were light are dark, and vice versa. When you look at the negative, you can still get a sense of what the original scene looked like.

But when you look at a developed piece of film used to make a hologram, you don't see anything that looks like the original scene. Instead, you might see a dark frame of film or a random pattern of lines and swirls. Turning this frame of film into an image requires the right illumination. In a transmission hologram, monochromatic light shines through the hologram to make an image. In a reflection hologram, monochromatic or white light reflects off of the surface of the hologram to make an image. Your eyes and brain interpret the light shining through or reflecting off of the hologram as a representation of a three-dimensional object. The holograms you see on credit cards and stickers are reflection holograms.

You need the right light source to see a hologram because it records the light's phase and amplitude like a code. Rather than recording a simple pattern of reflected light from a scene, it records the interference between the reference beam and the object beam. It does this as a pattern of tiny interference fringes. Each fringe can be smaller than one wavelength of the light used to create them. Decoding these interference fringes requires a key -- that key is the right kind of light.

Next, we'll explore exactly how light makes interference fringes.

Light reflection can be specular, mirror-like (left), diffuse or scattered.

Holograms and Light

To understand how interference fringes form on film, you need to know a little bit about light. Light is part of the electromagnetic spectrum -- it's made of high-frequency electrical and magnetic waves. These waves are fairly complex, but you can imagine them as similar to waves on water. They have peaks and troughs, and they travel in a straight line until they encounter an obstacle. Obstacles can absorb or reflect light, and most objects do some of both. Reflections from completely smooth surfaces are specular, or mirror-like, while reflections from rough surfaces are diffuse, or scattered.

The wavelength of light is the distance from one peak of the wave to the next. This relates to the wave's frequency, or the number of waves that pass a point in a given period of time. The frequency of light determines its color and is measured in cycles per second, or Hertz (Hz). Colors at the red end of the spectrum have lower frequencies, while colors at the violet end of the spectrum have higher frequencies. Light's amplitude, or the height of the waves, corresponds to its intensity.

White light, like sunlight, contains all of the different frequencies of light traveling in all directions, including ones that are beyond the visible spectrum. Although this light allows you to see everything around you, it's relatively chaotic. It contains lots of different wavelengths traveling in lots of different directions. Even waves of the same wavelength can be in a different phase, or alignment between the peaks and troughs.

Laser light, on the other hand, is orderly. Lasers produce monochromatic light -- it has one wavelength and one color. The light that emerges from a laser is also coherent. All of the peaks and troughs of the waves are lined up, or in phase. The waves line up spatially, or across the wave of the beam, as well as temporally, or along the length of the beam. You can check out How Lasers Work to see precisely how a laser does this.

In the next section we'll look at light reflection and redundancy.

When light waves reflect, they follow the law of reflection. The angle at which they strike the surface is the same as the angle at which they leave it.

Light Reflection

You can make and view a photograph using unorganized white light, but to make a hologram, you need the organized light of a laser. This is because photographs record only the amplitude of the light that hits the film, while holograms record differences in both amplitude and the phase. In order for the film to record these differences, the light has to start out with one wavelength and one phase across the entire beam. All the waves have to be identical when they leave the laser.

Here's what happens when you turn on a laser to expose a holographic plate:

  1. A column of light leaves the laser and passes through the beam splitter.
  2. The two columns reflect off of their respective mirrors and pass through their respective diverging lenses.
  3. The object reflects off of the object and combines with the reference beam at the holographic film.

There are a couple of things to keep in mind about the object beam. One is that the object is not 100 percent reflective -- it absorbs some of the laser light that reaches it, changing the intensity of the object wave. The darker portions of the object absorb more light, and the lighter portions absorb less light.

On top of that, the surface of the object is rough on a microscopic level, even if it looks smooth to the human eye, so it causes a diffuse reflection. It scatters light in every direction following the law of reflection. In other words, the angle of incidence, or the angle at which the light hits the surface, is the same as its angle of reflection, or the light at which it leaves the surface. This diffuse reflection causes light reflected from every part of the object to reach every part of the holographic plate. This is why a hologram is redundant -- each portion of the plate holds information about each portion of the object.

The holographic plate captures the interaction between the object and reference beams. We'll look at how this happens next.

Capturing the Fringes

The light-sensitive emulsion used to create holograms makes a record of the interference between the light waves in the reference and object beams. When two wave peaks meet, they amplify each other. This is constructive interference. When a peak meets a trough, they cancel one another out. This is destructive interference. You can think of the peak of a wave as a positive number and the trough as a negative number. At every point at which the two beams intersect, these two numbers add up, either flattening or amplifying that portion of the wave.

This a lot like what happens when you transmit information using radio waves. In amplitude modulation (AM) radio transmissions, you combine a sine wave with a wave of varying amplitudes. In frequency modulation (FM) radio transmissions, you combine a sine wave with a wave of varying frequencies. Either way, the sine wave is the carrier wave that is overlaid with a second wave that carries the information.

You can visualize the interaction of light waves[/b] [b]by imagining waves on water.

In a hologram, the two intersecting light wave fronts form a pattern of hyperboloids -- three-dimensional shapes that look like hyperbolas rotated around one or more focal points. You can read more about hyperboloidal shapes at Wolfram MathWorld.

The holographic plate, resting where the two wave fronts collide, captures a cross-section, or a thin slice, of these three-dimensional shapes. If this sounds confusing, just imagine looking through the side of a clear aquarium full of water. If you drop two stones into the water at opposite ends of the aquarium, waves will spread toward the center in concentric rings. When the waves collide, they will constructively and destructively interfere with each other. If you took a picture of this aquarium and covered up all but a thin slice in the middle, what you'd see is a cross-section of the interference between two sets of waves in one specific location.

The light that reaches the holographic emulsion is just like the waves in the aquarium. It has peaks and troughs, and some of the waves are taller while others are shorter. The silver halide in the emulsion responds to these light waves just like it responds to light waves in an ordinary photograph. When you develop the emulsion, parts of the emulsion that receive more intense light get darker, while those that receive less intense light stay a little lighter. These darker and lighter areas become the interference fringes.

In the next section we'll look at the emulsion bleaching process. 

Holographic Magnifying Glass

If you make a hologram of a scene that includes a magnifying glass, the light from the object beam passes through the glass on its way to the emulsion. The magnifying glass spreads out the laser light, just like it would with ordinary light. This spread-out light is what forms part of the interference pattern on the emulsion.

You can also use the holographic process to magnify images by positioning the object farther from the holographic plate. The light waves reflected off of the object can spread out farther before they reach the plate. You can magnify a displayed hologram by using a laser with a longer wavelength to illuminate it.

Bleaching the Emulsion

The amplitude of the waves corresponds to the contrast between the fringes. The wavelength of the waves translates to the shape of each fringe. Both the spatial coherence and the contrast are a direct result of the laser beam's reflection off of the object.

Turning these fringes back into images requires light. The trouble is that all the tiny, overlapping interference fringes can make the hologram so dark that it absorbs most of the light, letting very little pass through for image reconstruction. For this reason, processing holographic emulsion often requires bleaching using a bleach bath. Another alternative is to use a light-sensitive substance other than silver halide, such as dichromated gelatin, to record the interference fringes.

Once a hologram is bleached, it is clear instead of dark. Its interference fringes still exist, but they have a different index of refraction rather than a darker color. The index of refraction is the difference between how fast light travels through a medium and how fast it travels through a vacuum. For example, the speed of a wave of light can change as it travels through air, water, glass, different gasses and different types of film. Sometimes, this produces visible distortions, like the apparent bending of a spoon placed in a half-full glass of water. Differences in the index of refraction also cause rainbows on soap bubbles and on oil stains in parking lots. In a bleached hologram, variations in the index of refraction change how the light waves travel through and reflect off of the interference fringes.

These fringes are like a code. It takes your eyes, your brain and the right kind of light to decode them into an image. We'll look at how this happens in the next section.

In a transmission hologram, the light illuminating the hologram comes from the side opposite the observer.

Decoding the Fringes

The microscopic interference fringes on a hologram don't mean much to the human eye. In fact, since the overlapping fringes are both dark and microscopic, all you're likely to see if you look at the developed film of a transmission hologram is a dark square. But that changes when monochrome light passes through it. Suddenly, you see a 3-D image in the same spot where the object was when the hologram was made.

A lot of events take place at the same time to allow this to happen. First, the light passes through a diverging lens, which causes monochromatic light -- or light that consists of one wavelength color -- to hit every part of the hologram simultaneously. Since the hologram is transparent, it transmits a lot of this light, which passes through unchanged.

Regardless of whether they are dark or clear, the interference fringes reflect some of the light. This is where things get interesting. Each interference fringe is like a curved, microscopic mirror. Light that hits it follows the law of reflection, just like it did when it bounced off the object to create the hologram in the first place. Its angle of incidence equals its angle of reflection, and the light begins to travel in lots of different directions.

The interference fringes in a hologram cause light to scatter in all directions, creating an image in the process. The fringes diffract and reflect some of the light (inset), and some of the light passes through unchanged.

But that's only part of the process. When light passes around an obstacle or through a slit, it undergoes diffraction, or spreads out. The more a beam of light spreads out from its original path, the dimmer it becomes along the edges. You can see what this looks like using an aquarium with a slotted panel placed across its width. If you drop a pebble into one end of the aquarium, waves will spread toward the panel in concentric rings. Only a little piece of each ring will make it through each gap in the panel. Each of those little pieces will go on spreading on the other side.

This process is a direct result of the light traveling as a wave -- when a wave moves past an obstacle or through a slit, its wave front expands on the other side. There are so many slits among the interference fringes of a hologram that it acts like a diffraction grating, causing lots of intersecting wave fronts to appear in a very small space.

Holography and Mathematics

You can describe all of the interactions between the object and reference beams, as well as the shapes of the interference fringes, using mathematical equations. This makes it possible to program a computer to print a pattern onto a holographic plate, creating a hologram of an object that doesn’t actually exist.

Recreating the Object Beam

The diffraction grating and reflective surfaces inside the hologram recreate the original object beam. This beam is absolutely identical to the original object beam before it was combined with the reference wave. This is what happens when you listen to the radio. Your radio receiver removes the sine wave that carried the amplitude- or frequency-modulated information. The wave of information returns to its original state, before it was combined with the sine wave for transmission.

The beam also travels in the same direction as the original object beam, spreading out as it goes. Since the object was on the other side of the holographic plate, the beam travels toward you. Your eyes focus this light, and your brain interprets it as a three-dimensional image located behind the transparent hologram. This may sound far-fetched, but you encounter this phenomenon every day. Every time you look in a mirror, you see yourself and the surroundings behind you as though they were on the other side of the mirror's surface. But the light rays that make this image aren't on the other side of the mirror -- they're the ones that bounce off of the mirror's surface and reach your eyes. Most holograms also act like color filters, so you see the object as the same color as the laser used in its creation rather than its natural color.

This virtual image comes from the light that hits the interference fringes and spreads out on the way to your eyes. However, light that hits the reverse side of each fringe does the opposite. Instead of moving upward and diverging, it moves downward and converges. It turns into a focused reproduction of the object -- a real image that you can see if you put a screen in its path. The real image is pseudoscopic, or flipped back to front -- it's the opposite of the virtual image that you can see without the aid of a screen. With the right illumination, holograms can display both images at the same time. However, in some cases, whether you see the real or the virtual image depends on what side of the hologram is facing you.

Your brain plays a big role in your perception of both of these images. When your eyes detect the light from the virtual image, your brain interprets it as a beam of light reflected from a real object. Your brain uses multiple cues, including, shadows, the relative positions of different objects, distances and parallax, or differences in angles, to interpret this scene correctly. It uses these same cues to interpret the pseudoscopic real image.

This description applies to transmission holograms made with silver halide emulsion. Next, we'll look at some other types of holograms.

The holograms found on credit cards and other everyday objects are mass-produced by stamping the pattern of the hologram onto the foil.

Image courtesy Dreamstime

Other Hologram Types

The holograms you can buy as novelties or see on your driver's license are reflection holograms. These are usually mass-produced using a stamping method. When you develop a holographic emulsion, the surface of the emulsion collapses as the silver halide grains are reduced to pure silver. This changes the texture of the emulsion's surface. One method of mass-producing holograms is coating this surface in metal to strengthen it, then using it to stamp the interference pattern into metallic foil. A lot of the time, you can view these holograms in normal white light. You can also mass-produce holograms by printing them from a master hologram, similar to the way you can create lots of photographic prints from the same negative.

But reflection holograms can also be as elaborate as the transmission holograms we already discussed. There are lots of object and laser setups that can produce these types of holograms. A common one is an inline setup, with the laser, the emulsion and the object all in one line. The beam from the laser starts out as the reference beam. It passes through the emulsion, bounces off the object on the other side, and returns to the emulsion as the object beam, creating an interference pattern. You view this hologram when white or monochrome light reflects off of its surface. You're still seeing a virtual image -- your brain's interpretation of light waves that seem to be coming from a real object on the other side of the hologram.

Reflection holograms are often thicker than transmission holograms. There is more physical space for recording interference fringes. This also means that there are more layers of reflective surfaces for the light to hit. You can think of holograms that are made this way as having multiple layers that are only about half a wavelength deep. When light enters the first layer, some of it reflects back toward the light source, and some continues to the next layer, where the process repeats. The light from each layer interferes with the light in the layers above it. This is known as the Bragg effect, and it's a necessary part of the reconstruction of the object beam in reflection holograms. In addition, holograms with a strong Bragg effect are known as thick holograms, while those with little Bragg effect are thin.

The Bragg effect can also change the way the hologram reflects light, especially in holograms that you can view in white light. At different viewing angles, the Bragg effect can be different for different wavelengths of light. This means that you might see the hologram as one color from one angle and another color from another angle. The Bragg effect is also one of the reasons why most novelty holograms appear green even though they were created with a red laser.

The famous hologram "The Kiss" shows a sequence of similar, stationary images. Your eye sees many frames simultaneously, and your brain interprets them as moving images.

Image © 1996-2007 Holophile, Inc.

Multiple Images

In movies, holograms can appear to move and recreate entire animated scenes in midair, but today's holograms can only mimic movement. You can get the illusion of movement by exposing one holographic emulsion multiple times at different angles using objects in different positions. The hologram only creates each image when light strikes it from the right angle. When you view this hologram from different angles, your brain interprets the differences in the images as movement. It's like you're viewing a holographic flip book. You can also use a pulsed laser that fires for a minute fraction of a second to make still holograms of objects in motion.

Multiple exposures of the same plate can lead to other effects as well. You can expose the plate from two angles using two completely different images, creating one hologram that displays different images depending on viewing angle. Exposing the same plate using the exact same scene and red, green and blue lasers can create a full-color hologram. This process is tricky, though, and it's not usually used for mass-produced holograms. You can also expose the same scene before and after the subject has experienced some kind of stimulus, like a gust of wind or a vibration. This lets researchers see exactly how the stimulus changed the object.

Using lasers to make three-dimensional images of objects may sound like a novelty or a form of art. But holograms have an increasing number of practical uses. Scientists can use holograms to study objects in three dimensions, and they can use acoustical holography to create three-dimensional reconstructions of sound waves. Holographic memory has also become an increasingly common method of storing large amounts of data in a very small space. Some researchers even believe that the human brain stores information in a manner that is much like a hologram. Although holograms don't currently move like they do in the movies, researchers are studying ways to project fully 3-D holograms into visible air. In the future, you may be able to use holograms to do everything from watching TV to deciding which hair style will look best on you.

To learn more about holograms, follow the links on the next page.

Lots More Information

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