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