# How can you make a hologram?

#### How can you make a hologram? — JM, Kettering, OH

The classic technique for making a hologram begins with splitting the light from a laser into two parts. Part of the laser light is used to illuminate a scene while the other part is used to illuminate a piece of film placed in front of the scene. Actually, the film is exposed to light from two sources: (1) the second part of the laser beam and (2) a portion of the first part of the laser beam that the objects reflect toward the film. Lights from these two sources don’t simply add when they reach the film; they interfere with one another. Laser light is unusual in that it is coherent light—a giant wave consisting of numerous identical particles of light. When the wave from the laser and the wave reflected from the objects meet at the film, they interfere. When the crest of one wave joins the crest of the other wave, the two waves form an extra large crest—constructive interference. But when the crest of one wave joins the trough of the other wave, the two waves cancel and produce essentially nothing—destructive interference. Because of this interference, the film ends up recording not only the intensity information that we associate with normal photography; it also records phase information that is an important aspect of waves. This phase information indicates where crests and troughs in the wave occurred. Because the hologram contains both kinds of information, it allows a viewer to see things that they would not see in a simple photograph.

To make a hologram, you should take a laser and split its light into two unequal portions with the help of a laser beam-splitter (or even a glass slide). The laser should operate at only a single wavelength, so that its light is highly coherent, and it should have a coherence length much longer than any distance in the scene—two requirements that are met by most common continuous-wave lasers, including laser pointers and basic helium-neon lasers. Send the stronger portion of the laser beam through a diverging lens and allow it to illuminate a scene that is otherwise in complete darkness. Light reflected from this scene should reach the film holder in which the hologram will be made. Send the weaker portion of the laser beam through another diverging lens and allow it to illuminate the film holder from the scene side. For best results, the light reflected from the scene on the film holder should be about as bright as light from this second beam.

Now place fine-grained black and white film in the film holder. Be sure that the film is sensitive to the laser light—some black and white films aren’t sensitive to red light. Allow light to strike the film for long enough to expose it. Finally, develop the film and observe the developed film while it’s illuminated from behind with laser light that has been spread out by a diverging lens. You should see the original scene as a three-dimensional image.

Unfortunately, there is one detail I’ve omitted until now. To make sure that the phase information is properly recorded, you must be sure that nothing moves by even a fraction of a wavelength of laser light during the entire exposure period. That’s a very demanding requirement. Vibrations are everywhere and they will spoil the hologram. If you want this technique to work, you’ll have to isolate everything—the laser, the optics, the scene, and the film—from vibrations. In a laboratory, this vibration isolation is done by floating a massive optics table on a cushion of air. All of the objects involved in making the hologram are rigidly attached to this table so that they can’t move. As an alternative, you can put all the objects for the hologram on as rigid and massive a surface as you can find and support that surface on a thick layer of foam rubber. Make the holograms at night when there is little traffic of any sort around and be sure that nothing is jiggling about nearby that might shake the floor even a little bit. If you’re careful, you ought to be able to create a hologram with such an arrangement.