Hologram pattern

To understand how a hologram is made and viewed, we need to know about two important properties of light: interference and diffraction. To describe these properties, it is useful to think of light as a wave. All waves can be described as a series of crests (high points) and troughs (low points) where the distance between successive crests is called the wavelength and the number of wavelengths that pass by per second is called the frequency. In the visible spectrum for example, red light has a large wavelength and a low frequency while violet light has a short wavelength and high frequency.

When two waves meet, they can either add to each other (interfere constructively) or subtract from each other (interfere destructively). In holography, two light waves meet and interfere on a piece of film to produce a complicated "picket fence"-like pattern of tiny exposed and unexposed areas. This pattern is characteristic of the object being holographed. In order to make such a detailed pattern, a light source that is monochromatic (has one wavelength) and coherent (all the crests and troughs are in line) must be used. A laser provides such a light source.

Diffraction describes the ability of a wave to spread out when it meets an obstacle. When light waves meet the hologram's interference pattern, they spread out such that a perfect replica of the original object's wavefront is created. It is as if we were looking through a window in which the real object can be seen. The hologram can be thought of as a "window with a memory."

There are two basic kinds of holograms: transmission and reflection. Transmission holograms have light shining through them (the light source is behind the hologram) and reflection holograms have light shining on them (the light source is in front of the hologram). Some can be viewed only with laser light while others, called rainbow holograms, can be viewed with white light. The holograms of objects on exhibit in LightForest are white light copies of master transmission holograms.