Scratch holograms

From RetroTech, the online museum of 20th century technology.

Scratch holograms are holograms that are readily drawn by hand using a technique discovered by Bill Beaty. The 20th Century Technology Museum is demonstrating a couple of different ways to make scratch holograms at our booth at the Maker Faire in Austin on October 18-19, 2008. More detailed information about scratch holograms is available at Bill Beaty's website.

Viewing tips

Scratch holograms are best viewed using a "point" light source; that is, a small, bright light that is far enough away from the surface, such as the sun. It is often easiest to place the hologram on a flat surface (such as the ground) and move around the hologram to find the correct angle for the light. The light reaching the surface of the hologram should reflect off of the scratches on the surface and appear as a single, bright point of light on each scratch. The hologram appears as a pattern of bright points of light which move as you change your position (or that of the hologram or light source). An LED flashlight or halogen spotlight trained on the hologram in a darkened room also generally provides good viewing conditions. Note that the angle of illumination and viewing are very important, so you may need to move things around until you get a good image. More viewing tips can be found on Bill Beaty's website.

Fabrication

We are making our scratch holograms for Maker Faire via two hand-drawn methods. The first is very much analogous to the technique described here. Instead of manually placing a compass tip on a pattern, we have pre-dilled patterns used to position a scribing arm at each point in sequence. This method is very time-consuming, since a good hologram can require hundreds of arcs to be drawn individually. The second method we use is to draw a pattern with glue and adhere fine sand to the glue. This pattern is then drawn directly over the plastic surface in one arcing motion, leaving a less precise but often quite good scratch hologram. Unfortunately, the sand wears off after a several uses, requiring a new template to be made.

We experimented with using a vinyl cutting machine (formerly known as a pen plotter) to draw the arcs directly under the direction of a computer program, but we were not able to achieve satisfactory results. This is most likely due to the fact that the vinyl cutter traces arcs that have tiny (less than one millimeter) wiggles in them because of the way the motors in the cutter work. These wiggles effectively broaden the area on each scratch where the light reflects, creating a blurry hologram that is not nearly as pleasing as the hand-drawn ones. Research is ongoing toward automating the process of fabricating scratch holograms with less manual effort.