FAQs
Making a simple hologram
In normal light, such as that from a light bulb or the sun, the individual ‘particles’ or waves of light (called photons) move randomly through the air. Coherent light is light in which the photons are moving together in an ordered way.
Imagine that in sunlight the photons move like a crowd of people in a shopping centre, whereas in coherent light the photons move like a marching army. To make a hologram a source of coherent light is needed. The best source is a laser. Laser is an acronym for Light Amplification by Stimulated Emission of Radiation.
A hologram is the recording of two sets of waves. The first wave, called the reference beam, comes directly from the laser. The second wave hits an object, bounces off and interferes with the first wave. The hologram records this very complicated pattern on a high quality photographic plate. The first thing to do is split the laser beam into two parts. Both beams must come from the same laser to make sure all the photons are in step with each other. To split the laser beam in two, a device called a beam splitter is used. A beam splitter can be as simple as a piece of ordinary window glass or a precision coated optical device. Like most things in life, the more you pay, the better the result. The purpose of the beam splitter is to allow some light to go straight through and for some light to reflect off its surface. In this way, the single beam becomes two beams. The next problem is that the laser beam is very small, only 3 or 4 mm in diameter. If we want to make a hologram of something, even as small as a matchbox, the beams must be spread out to cover the object. This is done with a lens. The more powerful the lens, the more the light is spread out. Professional hologram makers use the lenses from a microscope, often called a microscope objective. We now have a laser, a beam splitter and two beams that are spread out. Using some mirrors, the first beam (reference beam) is shone onto the holographic plate. The second beam is directed at an object. The recording plate is placed where these two beams meet. The whole set-up is called a holographic camera: If the reference beam and the beam bouncing off the object both hit the holographic plate from the same side, a transmission hologram is made. If the reference beam hits the plate from one side, while the beam from the object hits the plate from the other side, the result is a reflection hologram.
There are many kinds of holograms, but all holograms fall into two basic categories:
- Transmission holograms
- Reflection holograms
