The difference between holographic and conventional optics is similar to the difference between thinking about light as a wave and as a particle. Traditional glass lenses work by refraction, treating light as a particle: ``Whenever light moves from one medium, such as air, and strikes another medium, such as glass or plastic,'' it is bent at an angle, says scientist Allen L. Cohen of Allegran Hydron in Woodbury, N.Y. It is as if the light was composed of rays of particles, entering the lens at one side, being bent by the glass, and leaving the lens on the other side, he explains.
Holography is based on diffraction. Think of light not as a particle, Dr. Cohen says, ``but as a wave. When you imagine waves striking objects, all sorts of wave patterns and interference patterns can be set up.'' When light enters a holographic lens, which is nothing more than a block of plastic covered with microscopic valleys and ridges, the light forms an interference pattern and is sent off in one or more directions.
By using computers to calculate the pattern for the lens's surface features, it is possible to make one that will manipulate light in any way that can be imagined, Cohen says. Once the pattern is decided upon, a variety of techniques can be used to put the holographic pattern onto the plastic. The most common is to etch the block lithographically, using the same techniques that are used to create electronic microchips. The pattern can also be directly cut into the block ``with a million-dollar lathe that's mounted on tons of stabilizing foundation,'' Cohen says. Such a lathe has to be able to cut a pattern accurately ``to within a fraction of a micron,'' he adds.