Posts: 3

1. Interference Can Stop Things

   30.Mar.07, 14:15 EDT
   
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   Interference Can Stop Things
   By Jason Sapan
   Holograms are photographs of three dimensional impressions on the surface of light waves. Therefore, in order to make a hologram you need to photograph light waves. This presents something of a dilema.
   
   As we all know, it can be problematic to take a photograph of a quickly moving object. If you've ever had a picture come back blurred from the film lab, you know all too well. When a person moves too quickly in a photograph, their image blurs. And they are only moving at about 20 miles an hour. Try to imagine the problems associated with trying to photograph a photon. To start, a light wave moves at the speed of light. Thats about 186,000 miles per second. Thats more than half way to the moon in a second. Considerably faster than someone's hand waving. In fact, its so fast that the very idea of even capturing it on film would appear impossible. What we need is a way to stop the photon so it can be photographed. And this technique is called INTERFERENCE.
   
   Imagine yourself standing on a small bridge over a pond of still water. Lets further imagine that you were to drop a pebble into the pond. As it hits the water it creates a circular wave. This wave radiates outwards in an ever growing circular path. We've all done this.
   
   Now, if you drop two pebbles in the water, you would create two circular waves, each of which would grow in size and eventually cross the path of the other wave and then continue on its individual expanding path. Where the two circular waves cross each other, you might say that they interfere with each other. And the pattern that they make is called an interference pattern. Not too difficult to envision. This is what interference is. Two waves interfering with each other as they cross paths. No permanent impact is left on either wave once it leaves the area of overlap. Each wave looks exactly the same as it did before it crossed the other waves path. Well, maybe its grown a little bit bigger, but that's about it. So, what's the big deal about interference in that case?
   
   Here it is. As waves cross paths and interfere, the pattern they make is called a standing wave. It is called a standing wave because it stands still. And since it stands still, it can be photographed.
   
   This solves the problem of how we can photograph something moving at the speed of light. But it doesn't answer the bigger question. Why does it stand still?
   
   To understand that, lets envision a photon. Remember? It looks like a corkscrew. And if we view it from the side it looks like a sine wave. Now, try to imagine a river whose streambed lies on a wavy rock formation that looks like a sine wave. This river would be full of rapids. In fact, it would be great for white water rafting. Although the water in the river is flowing furiously downstream, the pattern of water above the rapids is stationary. You might think of it as a standing wave. The wave energy is flowing through this standing wave without altering it and vice versa. It is just a momentary pattern that the water takes as it passes over a bump.
   
   When two light waves pass through each other each wave acts like a bump to the other. Their respective corkscrew shapes interact. And the result is like rapids of light. The standing wave patterns are stationary even though the light waves energy continues to move.
   
   When waves meet they perform addition and subtraction. When two waves of equal size meet at their high points (called crests), they add together to make a wave twice as high at that point. Conversely, where two waves of equal size meet at their low points (call troughs) they add together to become twice as low. And when one wave at its high point meets another wave at its low point they subtract and cancel out. But it isn't really cancelled out in the sense of being destroyed. Its more a case of there being no light at that spot. If you follow the wave down its path just a drop further it will be meeting the other wave at a different relationship and once again be visible. Its a situation of infinite possibilities. Just like the patterns possible as the waves of two pebbles meet in a pond. At any point you may notice that the standing wave pattern has produced a place where the waves have added together to get higher or subtracted to become lower or even just gone flat. There's a few terms that are used to describe the possible encounters. If the waves add and get higher its called CONSRTRUCTIVE interference. If the waves subtract or cancel altogether its called DESTRUCTIVE interference. I like to think of the interference pattern as a fingerprint of the encounter of two individual waves. Each object you make a hologram of creates its own interference pattern that identifies it. In holography, there are two basic waves that come together to create the interference pattern. First and foremost is the wave that bounces off the object we are making a hologram of. Since it bounces off the object, thereby taking its shape, it is called the OBJECT wave. You can't have interference without something to interfere with. So a second wave of light that has not bounced off an object is used to perform this function. It is called the REFERENCE wave. When an object wave meets a reference wave creating a standing wave pattern of interference, it is photographed and called a hologram.

2. IMPRESSED WITH LIGHT

   30.Mar.07, 14:13 EDT
   
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   IMPRESSED WITH LIGHT
   By Jason Sapan / Holographic Studios
   So what are holograms? The best way to think of a hologram is to envision them as impressions on light waves.
   
   Light is a wave. All waves behave more or less the same. For one thing they tend to echo. They reflect off of many surfaces. Sort of like sound waves echoing to make SONAR or microwaves in RADAR. The wave is sent out; it hits an object; it bounces back. Pretty simple idea. But what you don't think about is that when a wave bounces up against an object it takes its shape. Like pressing a piece of clay up against a key. The key leaves a three dimensional impression in the clay. Well if you imagine the clay as a light wave, basically the idea of holography is throwing the clay up against the key, having the key make an impression on the front of the clay, letting it bounce off, and finally storing the shape of the clay permanently.
   
   Now with SONAR or RADAR we are dealing with waves that are not visible. You can't see sound waves or microwaves. However with light waves we are working in the visible spectrum and consequently things that are visible are things that tend to record on photographic film. So in effect, a hologram is a photograph of the impression left on the surface of a light wave after it has bounced off an object.
   
   Film Emulsion
   Lets look at a hologram recorded on silver halide film. What is film? Well, first there is a base material of clear plastic or glass. Then there is a very important layer that contains the photoreactive chemistry. They call it the emulsion. Its a very special composition. And there's always room for it. Its jello. Plain old gelatin, without any flavor or color of course. Inside the gelatin there are two chemicals joined in a molecule. The are suspended like fruit in jello. In an emulsion, each chemical retains its own identity. Just like each piece of fruit floating in the jello does. First, there's silver. As we all know silver has a very unique property. It tarnishes when it combines with oxygen. And it turns black. Next, there's iodine. The stuff you use to kill the germs on a cut. Its a very reactive chemical. So reactive that when it mixes with the silver to make silver iodide, it results in a silver that will tarnish very quickly.
   
   When a light wave goes into this layer of jello its energy is transferred to the silver iodide molecule. Remember how a light wave looks like a corkscrew? Well, try to imagine this corkscrew winding up its energy into the silver iodide molecule just like a wind-up toy. You give it a good twist and the energy goes into making the toy run. In the case of the silver iodide molecule you give it a good wind up of light energy. Its sort of like setting a bear trap. Your are putting your energy into pulling the trap open. Now its set to snap shut. The same thing is happening in the silver iodide molecule. Light gives it energy to be ready to snap onto another atom. When you put it in a bath of photographic developer, it grabs hold of the oxygen in the bath, and tarnishes. That's why black and white negatives are black. And so are holograms before they are bleached. So, you can think of photography or by extension holography as the art of selectively tarnishing silver in jello where light has energized it.
   
   Now in the case of a hologram, the patterns of light wave impressions are what is being photographed in the layer of emulsion. Generally film emulsion in holography runs about 10 microns thick. A micron being a millionth of a meter (a meter is about a yard) in size. That's pretty small, but a photon measures about a half of a single micron in size. That's smaller than an ants asshole. So the emulsion seems pretty large to a photon. That's how we are able to photograph this microscopic wave impression in film and make holograms. Holograms are photographs of the three dimensional impressions stored on light waves. Sort of like fossils.
   
   When you pour plaster into a fossil, let it harden, and then remove it, you have a three dimensional sculpture of the impression that was left in the stone. Similarly, when you pour jello into a jello mold, let it set, and then remove it, you have a three dimensional sculpture of the shape of the jello mold. And when you put light into a hologram, you get a three dimensional sculpture in light of the object that left its impressions on a photon and was captured within the thickness of a photographic emulsion.
   

3. "Good Vibes"

   30.Mar.07, 14:11 EDT
   
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   Vibration is in itself NOT a problem in holography. A holographic camera could be shaking like mad without in any way disturbing the hologram. You see, vibration is not the problem. To understand the problem you have to think about how holograms are made. Basically, two waves of light meet and interfere on a film plane. The interference of the light waves form a pattern. If one wave moves it smears this pattern. By smearing the pattern, its like taking your hands and smearing a fingerprint. It loses its information as a blur. The same happens with the hologram. The more one wave moves, the more the overall wave interference pattern is destroyed, until you dim totally out. But if both light waves vibrate in unison no smearing occurs.
   
   Now, if your table is stiff, there is much less chance of one wave moving. The stiffness prevents the table from bending. And it is the bending that allows one of the light paths to change thereby moving that light wave in the interference pattern. So, stiffness is the real consideration in building an isolation table. The stiffer the table, the brighter the hologram.
   
   RESONANCE
   Another interesting related topic is resonance. Too many people build isolation tables based on some drawing they saw in a badly written book. Usually, they end up making layers of crap to absorb vibration while totally failing to deal with stiffness! In so doing they create a whole new problem. Resonance. Do you remember seeing an old film clip about this bridge in the Seattle Tacoma region called "Galloping Gertie"? The winds in the canyon hit the bridge at a resonant frequency and caused it to wave like a flag before collapsing.
   
   Or, when you sing in the shower and hit a note that makes the whole room seem to amplify that note. That's resonance. It happens when waves combine VERY constructively, adding as they match in size with the container they are placed in.
   
   In table construction, too many layers is like putting a spring on a spring. You can bet it will resonate. And instead of canceling the harmful vibrations, you are now amplifying them. Not a good idea.
   
   So be very careful not to build a system that only makes your work harder. Keep it simple. Make it a stiff one!
   
   LIGHT
   There seems to be great confusion about the nature of light. Actually, it is pretty simple. Think of a basic atom. You have a nucleus and electrons spinning around it. Electrons are the stuff that makes electricity. They have an electrical charge. Normally, the atom is a fairly balanced system. But, if you put energy into this system, you can pump it up. For instance, if you send electricity in, the electrons of the electricity bump up against the electrons of the atom. As they collide its like a billiard ball hitting another billiard ball. The first ball (electron) transfers some of its energy into the second ball (electron) and sends it off. In the case of an atom, the electron is sent up temporarily into a higher orbit. Its something like blowing air into a balloon. The sphere the balloon occupies gets larger. Now, if you don't tie off the end of the balloon the air you just blew into it will come back out. The same thing happens in the atom. And as the electron returns to a lower orbit it releases the energy that originally sent it flying up into the higher orbit.
   
   A good way to envision the release of energy is to think of yourself in a pool of water chest high. If you sweep your arm just below the surface of the water you make little whirlpools. This is the energy transferring from the movement of your arm into the water. The energy swirls like a vortex. I like to think of it as a "corkscrew". A wave of light is a corkscrew or whirlpool of electromagnetic energy released as an electron returns from an excited orbit. The sweeping motion of the electron back to its normal or ground state is like the sweeping motion of your arm in the water. The electrical charge of the electron is transferred into a whirlpool of electromagnetic energy spinning off the atom. And it looks like a corkscrew. Think of how a typical corkscrew that you use to open a wine bottle looks. Now, turn it on its side and look at it. Its a sine wave. But that's only if you look at it sideways. Most drawings in books show light waves in this way. However, light waves are three dimensional. Hence the corkscrew model. You can think of the electron as being sheathed in an electromagnetic field. When it is energized by a collision it gains a bit too much of this electromagnetic jacket. As the electron returns a little bit of it twists free and tears off, becoming a free electromagnetic field twisted like a corkscrew. It twists because of the spinning motion of the electron. This a photon -a single wave of light.