When confronted with a hologram for the first time, most people react with disbelief. They look through an almost clear piece of film to see what looks like a solid object floating in space. Sometimes, they even reach out to touch it and find their fingers meet only thin air.

A hologram is a two-dimensional recording but produces a three-dimensional image. In addition, making a hologram does not involve recording an image in the usual sense. To resolve these apparent contradictions and understand how a hologram works, we have to start from first principles.

The concept of holographic imaging

In all conventional imaging techniques, such as photography, a picture of a three-dimensional scene is recorded on a light-sensitive surface by a lens or, more simply, by a pinhole in an opaque screen. What is recorded is merely the intensity distribution in the original scene. As a result, all information on the relative phases of the light waves from different points or, in other words, information about the relative optical paths to different parts of the scene is lost.

The unique characteristic of holography is the idea of recording the complete wave field, that is to say, both the phase and the amplitude of the light waves scattered by an object. Since all recording media respond only to the intensity, it is necessary to convert the phase information into variations of intensity.