3.5 Holography
Holography is the method of obtaining three-dimensional photographic images. These images are obtained without a lens, so the method is also called lensless photography. The theoretical principles of holography were developed by the British physicist Dennis Gabor in 1947. The first actual production of holograms took place in the early 1960s, when the laser became available. By the late 1980s the production of true-color holograms was possible, as well as holograms ranging from the microwave to the X-ray region of the spectrum. Ultrasonic holograms were also being made, using sound waves.
A hologram differs essentially from an ordinary photograph in that it records not only the intensity distribution of reflected light but also the phase distribution. That is, the film distinguishes between waves that reach the light-sensitive surface while they are at a maximum wave amplitude, and those that reach the surface at a minimum wave amplitude. This ability to discriminate between waves with different phases is obtained by having a so-called reference beam interfere with the reflected waves.
Thus, in one method of obtaining a hologram, the object is illuminated by a beam of coherent light—n beam in which all the waves are traveling in phase with one another. Such a beam is produced by a laser. Essentially, the shape of the object determines llie form of the wave fronts—that is, the phase at which the reflected light arrives on each point of the photographic plate. Simultaneously, a portion of the same laser beam is reflected by a mirror or prism and directed toward the photographic plate; this beam is called the reference beam. The wave fronts of this latter beam, not having been reflected from the object, remain plane-parallel and produce an interference pattern with the wave fronts of the light reflected by the object. If the object is a point, for example, the wave fronts of the reflected beam will be spherical; the interference pattern produced on the film will then consist of concentric circles, the space between circles decreasing with increasing radius.
The interference pattern produced by a more complicated object will be much more complicated, so mere inspection of the resulting hologram will reveal only an intricate pattern of dark and light structures thai bear no apparent relationship to the original object. When the hologram is viewed in coherent light, however, the recorded object becomes visible; and when the hologram is viewed Iron) different angles, the object is also seen from different angles. The three-dimensional effect is obtained because the hologram reconstructs in space the wave fronts that originally were produced by the object.
How this happens can be understood by again using the example of the hologram of the point. Coherent light arriving at the concentric circles on the hologram is diffracted on a diffraction grating. The diffraction angle of the beam increases with the distance from the ccnlcr of the concentric rings, thus reconstructing the spherical wave fronts, and the viewer sees the point at the same relative place where the real point was when the hologram was made. The wave fronts of more complicated objects are reconstructed in the same way. The intensity distribution of the reflected light is recorded in the degree of blackening of the interference patterns on the film.
To a certain extent, holography can be applied in optical microscopy, especially for the study of living organisms. The most successful application of holography, however, is in interferometry. If two holograms of the same object are recorded on the same plate, then upon reconstruction the two holographic images will interfere. If the object has undergone a deformation between the two recordings, phase differences in certain parts of the two images will result, creating an interference pattern that clearly shows the deformation. Because wave front differences of a fraction of a wavelength of light thus become visible, this method is extremely sensitive for deformation studies.
Another important application is the storage of digital data, which can be recorded as bright and dark spots in holographic images. A hologram can contain a large number of "pages" that are recorded at different angles relative to the plate, thus allowing the storage of a very large amount of data on one hologram. By illuminating the hologram with a laser beam at different angles, the pages can be read out one by one.