Figures 1 thru 3 demonstrate both the process of construction and the nature of a hologram. It can be seen from these figures that a hologram is essentially the record of the interference pattern of two light waves. The reference wave is usually emitted from a source such as a laser (as designated in Figure 1) which provides our very “coherent” form of light. Coherence refers to the purity of frequency. A light wave containing only a single frequency of light is perfectly coherent. The object wave arises from light reflected off the object of which we intend to make a hologram. In the simple case of Figure 2, we are considering an object wave arising from a single point source, while Figure 1 entails a more complex object wave. The "++" in the diagram (Figure 2) indicates a point on the recording plate where the reference wave and object wave constructively interfere, i.e., where the crest coincides with crest, trough with trough, and thus "wave addition" occurs. The "+-" indicates a locus of destructive interference where there is not such a coincidence of crests and troughs, actually where crest meets trough.

When a reconstructive wave - a wave of the same frequency as the reference wave - is beamed on the photographic emulsion containing the interference pattern of the two waves, the original wave front is reconstructed (Figure 3). The reconstructive wave is diffracted as it passes through the plate, analogous to that which happens to water waves as they flow through a line of barriers and bend around them, though in this case the "barriers" are the loci of constructive (++) and destructive (+-) interference. Figure 3 shows that one set of waves travels downwards while yet another set passes through undeviated. A third set travels upwards in the same direction that the simple one-point source object wave in Figure 2 (wave set B) was moving, or similarly, in the same direction the complex object wave in Figure 1 would have been moving. A viewer in the path of these upward waves then believes himself to see the source which generated the wave-set located in depth behind the hologram and in three dimensions. We might say then that the upward traveling waves "specify" the nature of their source of origination, namely a pyramid form with a globe in front of it, or, as in Figure 2, the waves specify a luminous point located behind and below the plate.

Though the waves that would deviate upwards from the "zone plate" or hologram of Figure 2 appear as if they were circular wavefronts originating from the point P, in the reconstruction process no point is truly there. These waves therefore specify what is termed the virtual image. The downward traveling waves however converge at a point exactly opposite the virtual image. This is termed the real image since a card placed there would reveal the presence of an actual concentration of light.

1) The distributive property. Firstly, we can consider each point of an illuminated object as giving rise to a spherical wave which spreads over the entire hologram plate. Thus we can consider the information for each point to be spread over the entire hologram. This implies, conversely, that the information for the entire object is found at any point in the hologram. In fact, we can take a small corner or "window" of the hologram of the pyramid scene in Figure 1 and reconstruct the image (wavefront) of the entire scene with a reconstructive wave. The size reduction lowers the intensity of both the real and virtual images because less light falls on the reduced area. The realism of the virtual image is also reduced. In principle, any point of the hologram carries sufficient information to reconstruct the whole scene.

2) Context Sensitivity - Modulating the Reconstructive Wave. It is possible to record a multiplicity of wavefronts on the same holographic plate. We can do this by changing the frequency value of the reference wave. Thus we could make a hologram of a pyramid and ball, a chalice, a toy truck, and a candle successively, using reference waves of frequency f1, f2, f3, and f4 respectively. By modulating the reconstructive wave, i.e., changing its frequency appropriately from frequency f1 through f4, we could reconstruct the successive wavefronts which originated from each object (Figure 4).