Recording and Readout

Holography is a two step imaging process. The first step is the recording of the holographic image and involves the illumination of the object with a coherent beam of light. The resulting scattered wave emanating from the object is allowed to interfere with a reference wave, and the resulting interference pattern is recorded on a photosensitive medium (a photoemulsion requiring subsequent development or a real time recording material). The procedure involved in recording the scattered and reference waves is depicted in Fig.6a: a spherical wave from a pinhole $S$ (the object or subject light beam, $I_{S}$) and a plane wave (the reference beam, $I_{R}$). The interference pattern is illustrated in Fig. 6b; the concentric interference patterns are similar to that seen on a Fresnel zone plate, shown in Fig. 6d. Holograms produced from the interference of a spherical and plane light wave are called Fresnel holograms [7] . For a more complex object, for example a sculpture, the interference patterns in the hologram are a superposition of the scattered light from different parts of the object, i.e. from every "point" with an order of magnitude of the wavelength of light.

Figure 6: Hologram recording and reconstruction [2]

The second step of holographic imaging is the reconstruction or readout of the hologram. For this, the hologram is illuminated with the reference beam under the same angle of incidence as that used in the recording step (Fig. 6a, c). The scattered light from the hologram produces two images of the object ($S_{1}, S_{2}$) on either side of the hologram. The image $S_{1}$ coincides with the object (in our case the point light source $S$, Fig. 6a) and is called the virtual image, the second image $S_{2}$ on the opposite side is called the real image ⁴ Holographic images are optical "copies" of the object. Holographic imaging is a wave front reconstruction process, which allows one to obtain a real three dimensional image of volume objects without lenses or objectives. For information processing, a multichannel holographic recording and readout is possible which widely increases the field of application.