How Things Work: Holograms

From security emblems to magazine ads, holograms produce vibrant, 3-D images through the diffraction and reflection of light.

Depending on the viewer’s line of sight, holograms display different angles or colors of the same image, or different images altogether.

There are two types of holograms: reflection and transmission holograms. A reflection hologram produces an image that can be observed in normal light, whereas a transmission hologram produces an image that is observable only with laser light.

At the bare minimum, creating a hologram requires a laser, an object, and holographic film.

To create a transmission hologram, laser light is shined through a beam splitter — a series of mirrors and prisms. The beam splitter creates two beams of light — the object and reference beams.

The object and reference laser beams then each pass through a diverging lens, causing each beam to widen in diameter.

After passing through the diverging lens, the object beam reflects off a mirror and onto the object. Once the light meets the object, the intensity (amplitude) of the object beam changes because some of the light is absorbed by the object.

The object beam’s angle of reflection off the object is the same as the angle at which it first encountered the object. Because the object’s surface is not perfectly smooth, the light is reflected in many different directions.

Unlike the object beam, the reference beam does not reflect off the object. Rather, it travels directly from the diverging lens to the holographic film.

Once the object and reference beams reach the film, they interfere with one another. Interference is the combination of laser beams that results in a new light pattern. Depending on how closely their wave structures overlap, the light can be amplified (constructive interference) or de-amplified (destructive interference).

Holographic film is coated with a mixture of silver halide. Once the object and reference beams reach the film, this mixture preserves the phase and amplitude differences between the reference and object light beams.

The final step in the creation process is development. During development, higher-intensity regions of light show up darker in the hologram, while lower-intensity regions of light appear lighter. These differences in light value are called interference fringes. Different amplitudes of light create different levels of contrast between these regions.

After development, the hologram is usually bleached to make it transparent, and then it is ready to be viewed. When viewed correctly, the hologram produces a virtual image of the object.

To view a transmission hologram, a laser beam is first shined through the front of the hologram, where the interference fringes are located. Some of this light simply passes through the hologram unaltered, while other light reflects off the film or diffracts through slits.

This process of reflection and diffraction produces a light pattern with the same properties as the object beam (before it merged with the reference beam). This light beam forms a virtual image of the object that is located in back of the hologram but in front of the viewer.

The hologram can then be embossed by transferring the interference pattern onto plastic and shiny foil. Embossed holograms are commonly seen in ads or on credit cards.

Besides being used as eye-catching decorations, holograms have a number of practical applications.
In the medical field, experts have combined holography with various medical instruments to produce high-resolution pictures of the body.

X-rays, for instance, can be used to take pictures of body parts. An X-ray laser is shot at an X-ray mirror, which is specially made with silicon and molybdenum to reflect the laser beam. The sample is then hit by the laser beam, thereby creating the object beam.

Endoscopic holography is used for a similar purpose: to take pictures of cavities within body organs. An endoscope is a device that uses light to see the inside of body organs. The pictures are high-resolution and require no contact with the body organs.

Holograms are also used as security emblems. In particular, they are placed on CDs, software, and expensive clothing to validate their legitimacy.

If you want to experience holography first-hand, you can purchase a Litiholo Hologram kit from Liti Holographics and try it yourself (www.litiholo.com). The kit includes a laser, an object, and film, and the creation process takes only minutes.