Holograms can be easy to make. Given an optical table, a holographic setup with high-quality optical mounts, and suitable developing chemicals, your average optical researcher can turn out pretty holograms in droves. For the rest of the world, however, holography is a difficult endeavor.
Limited by the requirement that many components in a holographic setup must be stable to subwavelength accuracy over the exposure time, serious home holographers have come up with the concept of “sandbox holography,” in which hundreds of pounds of sand in a box serves as a stabilizer and vibration damper for optical mounts half-buried in the sand. But how many parents, for example, are eager to build a basement holographic sandbox and film-development station for their tentatively interested child?
A kit developed by Liti Holographics (Newport News, VA) simplifies holography to the point where holograms of small objects can be created with a simple setup on a desk. The holographic film develops automatically, eliminating chemicals.
Finding the medium
For several years, Liti Holographics has produced commercial and consumer holograms-for example, 14 × 14-in. full-color holograms that have a few seconds of animation. In mid-2004, the company began to make its own holographic film material, says Paul Christie, the company’s president and chief technical officer.
The resulting medium was self-developing, but did not provide sufficient film quality to be used for high-quality display holograms. But the film was well suited for no-fuss home holography, says Christie. Further research has resulted in an improved medium that does not fade over time.
The traditional photoactive silver-halide-based holographic film is slightly altered when exposed to light, but is light sensitive even after exposure, explains Christie. As a result, one of the first steps in the developing process is to chemically “stop” or “fix” the film so that it no longer continues to accept exposure; then the rest of the chemical processing can take place. “The Litiholo film is a self-developing material, with the active components being ‘used up’ as the exposure takes place,” says Christie. “At the end of the exposure, all active components have been depleted and the film is not only developed, but is also no longer sensitive to light. The hologram is immediately ready to be viewed after exposure.”
Occasionally, areas of the film are not completely exposed and remain colored; this residual coloration can be removed by holding the finished hologram plate up to an ordinary light bulb for a brief period, rendering the areas clear. This final step is primarily cosmetic, however, and is not required, says Christie.
The mechanical setup consists of laser-cut sheet-acrylic parts that interlock to hold the holographic plate, optics, and 633-nm laser-diode source. The film, which comes in the form of a glass plate, is sensitive to red but not blue light, allowing use of a blue light-emitting diode for seeing in the darkness that is needed until exposure is complete. A single diverging laser beam illuminates both the film and the object (which is placed close to the film), reducing problems with relative motion.
Reasonable care must be taken with the setup. The laser diode must be run for five minutes before use to stabilize it; before exposure, the components must be left alone for a few minutes to mechanically stabilize. Holograms can be made of small, stable objects such as model cars (see figure). Plants make poor subjects: not only do they wave in air currents but they can grow enough during exposure to ruin the hologram.
“Probably 90% of the Litiholo hologram kits so far have been sold to individuals who have never made a hologram before, or have had very limited experience with holography,” says Christie. “The kit is also a fun and educational project for kids, although we joke that our largest market is people who want the kit for themselves, but justify the purchase by getting it for a son or daughter.” The kit sells for $139.