There are two basic categories of holograms — transmission and reflection. Transmission holograms create a 3-D image when monochromatic light, or light that is all one wavelength, travels through them. Reflection holograms create a 3-D image when laser light or white light reflects off of their surface. For the sake of simplicity, this article discusses transmission holograms viewed with the help of a laser except where noted.
It doesn’t take very many tools to make a hologram. You can make one with:
A laser: Red lasers, usually helium-neon (HeNe) lasers, are common in holography. Some home holography experiments rely on the diodes from red laser pointers, but the light from a laser pointer tends to be less coherent and less stable, which can make it hard to get a good image. Some types of holograms use lasers that produce different colors of light as well. Depending on the type of laser you’re using, you may also need a shutter to control the exposure.
- Lenses: Holography is often referred to as “lensless photography,” but holography does require lenses. However, a camera’s lens focuses light, while the lenses used in holography cause the beam to spread out.
- A beam splitter: This is a device that uses mirrors and prisms to split one beam of light into two beams.
- Mirrors: These direct the beams of light to the correct locations. Along with the lenses and beam splitter, the mirrors have to be absolutely clean. Dirt and smudges can degrade the final image.
- Holographic film: Holographic film can record light at a very high resolution, which is necessary for creating a hologram. It’s a layer of light-sensitive compounds on a transparent surface, like photographic film. The difference between holographic and photographic film is that holographic film has to be able to record very small changes in light that take place over microscopic distances. In other words, it needs to have a very fine grain. In some cases, holograms that use a red laser rely on emulsions that respond most strongly to red light.
There are lots of different ways to arrange these tools — we’ll stick to a basic transmission hologram setup for now.
- The laser points at the beam splitter, which divides the beam of light into two parts.
- Mirrors direct the paths of these two beams so that they hit their intended targets.
- Each of the two beams passes through a diverging lens and becomes a wide swath of light rather than a narrow beam.
- One beam, the object beam, reflects off of the object and onto the photographic emulsion.
- The other beam, the reference beam, hits the emulsion without reflecting off of anything other than a mirror.