Recording and Reconstruction of the Image
2.5 Recording and Reconstruction of the Image
It is not the image of the object itself that is recorded on a hologram, but the structure of the light wave reflected and dispersed on the object. To obtain a hologram two light beams must be incident on the photographic emulsion at the same time: the beam that is dispersed on the object (or that travelled through the transparent object) and carries information about the object – object beam and the beam coming from the same source – reference beam that does not have any barrier on its path. These two beams must interfere with each other. In addition, we need the recording material sensitive to the particular wavelength of the applied laser radiation and having sufficient resolution (minimally 1500 lines/mm). The interference image is recorded on a holographic plate. This first step is called the record on the holographic plate. The exposed and photochemically processed holographic plate is called hologram.
A more simple description of the principle of holography is shown in Fig. 2–5 (Miler, 1974), (Vest, 1979).
1– laser, 2 – beam splitter, 31, 32 – mirrors, 4 – micro objective, 5 – lens,
6 – object, 7 – holographic plate, ABC – length of the reference branch,
ADEC – length of the object branch
The narrow beam coming from the laser (1) is split up on the beam splitter (2) into two beams. The beam that passes through the beam splitter (2) is reflected from the mirror (31) and directed to the diverging lens (5) where it is expanded and falls on the object (6) from which it is diffusively reflected towards the holographic plate (7). This beam is called the object beam. The second one, the reference beam, is created by reflection from the beam splitter (2) and is reflected from the mirror (32) and further directed to the micro objective (4) where it is expanded and the waves fall on the holographic plate (7). Any chosen point of the object (6) is the source of a light wave that interferes with the reference wave in the crosshatched part of the holographic plate. In this manner, all light beams reflected from the individual points of the illuminated part of the object (7) interfere with the reference wave. The resultant interference field can be recorded on the holographic plate or film.
After photochemical processing the hologram is put into its original position as during the recording and is illuminated only by the reference beam (by closing of the object beam with the shutter (6)) (Fig. 2–6), while the original object is set aside the optical set-up. In consequence of diffraction of the illuminant reference beam the observer sees the object (7) through the hologram in its original position as three-dimensional. This second step is called image reconstrution (Fig. 2–6 a).
If we use a camera or videocamera instead of the eye, the object can be documented by photography or another way.
If the hologram’s photographic emulsion is oriented to the observer, then the real image appears on the same side of the hologram as is the observer. Such display is called pseudoscopic. This image is real, therefore we can record it without lenses – by location of the holographic plate in the plane where the image is formed (Fig. 2–6 b).
a) Reconstruction of holographic image
b) Reconstruction of real image
1 – laser, 2 – beam splitter, 3 – mirror, 4 – micro objective, 5 – shutter,
6 – hologram, 7a – virtual image, 7b – real image,
8a – direction of observation, 8b – display plane
For technical realisation of the experiment the difference between the optical propagation paths of the reference and object waves must be smaller than the coherent length of the laser. That is to say that in the chart displayed in Fig. 2–5 the ABC distance must be equal to the ADEC distance. The visibility of the interference fringes creating the hologram decreases if the difference between the optical propagation paths of the beams increases, which leads to degradation of the reconstructed holographic image quality.
