The principle

The principle of holography is illustrated in Figure 9.3 by considering the simplest case of a single point object. Let us assume that the object is at the point O. We illuminate the object with light from a reference source S (preferably a laser). The diagram shows light waves from the reference source spreading directly towards the right. Waves from S also illuminate the object at O, are reflected from O, and then overlap the waves coming directly from the reference source. The two waves combine and interfere constructively and destructively at stationary points in space.

reconstructed wave

Figure 9.3 The principle of holography.

reconstructed wave

Figure 9.3 The principle of holography.

The hologram is made by placing photographic film in the area of overlap. The brightest light will be in places where the phases of the two waves are identical and there is constructive interference. When the film is developed we can make a hard positive record, so that it transmits only at these maxima. This is now our hologram. Let us now put the hologram back into the place where the film was originally.

We can now use Huygens' construction for the waves leaving the hologram. The wavelets originating at the points of transmission all start out in phase independent of their previous 'history' before they reached the hologram. We can take away the object, and illuminate it with the reference source alone. To quote once more from Gabor's speech: 'They (the phases) are right for the reference source S, but as at the slits the phases are identical, they must be right also for O, therefore the wave from O must appear reconstructed.'

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