Synthetic Image Holograms

This chapter is dedicated to synthetic image holograms the elements which can create a reconstruction of a 3D object for observation with the human eye. Holography as a technique of image recording and reconstruction has been extensively developed from sixties of the twentieth century. During this time there have been various attempts to synthesize holograms artificially without the presence of the real object in the classical recording setup. Different approaches have been used, several trying to synthesize the three-dimensional object from two-dimensional views using the classical recording setup, the others trying to calculate the microstructure of the hologram completely in a computer. Today, we can divide synthetic holography into twomajor streams, the first containing the methods for creating the image for observation by human eye and the second consisting of approaches for designing the synthetic diffractive structures for general wavefront generation. The former techniques can exploit various imperfections of human vision and omit several parameters of the optical wave. The latter techniques are usually based on the direct calculation of the microstructure and they try to create the reconstruction in its full complexity. Only the first group of synthetic image holograms will be analyzed in this chapter. The synthetic approach to hologram creation can have several advantages, but also noticeable disadvantages. The most important advantages are connected with flexibility in modifying the recorded object. First, the object need not to exist in reality in a form of a physical model. For most synthetic approaches, it is fully sufficient to have a 3D computer model for preparing the recording data. Also for real physically existing objects it could be tricky to perform the recording process in a classical setup. For example, various outdoor scenes such as buildings and others could not be included in the laboratory setups. Generally, the scaling possibility is very limited in classical holography, so the recorded object (or its model) must be of final size. On the other hand, it is easy to scale the computer model of an object. The next problem is in various corrections of color properties, surface textures, and general fine tuning of the recorded object. While such operations are very simple in the case of computer models, they could bring insoluble problems for real physical models. The stability of the object is also very important. It is crucial to highly stabilize the object for recording in classical holographic recording setup (when exposing with a continuous-wave laser), whereas in a computer stability is not a problem. This can apply also for holograms of living objects or dynamic scenes, where it is easy to take snaphots using photographic techniques, but holographic exposure is almost impossible. Finally, according to the recording technology chosen, other parameters of the synthetic hologram can be highly superior to those of classical holograms (e.g. fidelity of color mixing, contrast of the image, etc.). 10