| Holography & Photography | Characteristics of a Hologram | Off-axis Holography : Leith-Upatnieks Holography : Laser Holograms | Hologram Aberrations | Orthoscopic and Pseudoscopic Images | Classification of Holograms |
Characteristics of a Hologram
Holograms have unique characteristics. These are discussed below:
The light from a reconstructed image from a hologram reaching to the observer's eye is the same as that would come from the original object. One can see in the holographic image the depth, parallax and different perspectives available in the actual object scene. As a matter of
The hologram of a diffuse object can be reconstructed by a small portion of the hologram. If a hologram breaks into pieces, each piece can reproduce the entire image. However, as the hologram size reduces, a loss of image perspective, resolution and brightness result in the constructed image.
A contact print of a hologram will reconstruct a positive image indistinguishable from the image produced by the original.
Two images, usually a real (pseudoscopic) and a virtual (orthoscopic) can be reconstructed from a hologram.
A cylindrical hologram provides a 360 deg view of the object.
More than one independent scenes can be stored in the same photographic plate which can be viewed one at a time, without any cross-talk.
Holograms suffer from aberrations caused by a change in the wavelength from construction to reconstruction and also by a mismatch in the reference and reconstruction beams. Both the chromatic and nonchromatic aberrations are quite important even when only small deviation from the recording geometry are present in the reconstruction geometry. The condition that will eliminate all the aberrations simultaneously is to duplicate exactly one construction beam in the reconstruction process.
Orthoscopic and Pseudoscopic Images
A hologram reconstructs two images, one real and the other virtual which are exact replicas of the object. However, the two images differ in appearance to the observer. The virtual image is produced at the same position as the object and has the same appearance of depth and the parallax as the original three dimensional object. The virtual image appears as if the observer is viewing the original object through a window defined by the size of the hologram. This image is known as orthoscopic image.
The real image is also formed at the same distance from the hologram, but in front of it. In the real image, however the scene depth is inverted. This is due to the fact that the corresponding points on the two images (virtual and real) are located at the same distances from the hologram. The real image is known as pseudoscopic image and does not give a pleasing sensation as we do not come across objects with inverted depths in normal life. Such images were not possible to be formed by other optical techniques till recently. It is now possible to produce such conjugate wavefronts in real-time by optical phase conjugation techniques. Such wavefronts have potential applications in correcting the effects of distorting media on optical imageries.
A hologram recorded by a lens or a concave mirror produces an orthoscopic real image of the object. It is also possible to produce an orthoscopic real image of an object by recording two holograms in succession. In the first step, a primary hologram is recorded by a collimated reference beam. This hologram, when reconstructed by a collimated beam produces an orthoscopic virtual image and a pseudoscopic real image with unit magnification. The final hologram is recorded using the real image of the primary hologram as the object beam. When this hologram is reconstructed it produces a pseudoscopic virtual image and an orthoscopic real image.
Classification of Holograms
Holograms may be classified in a number of different ways depending on their thickness, method of recording, method of reconstruction etc.
Amplitude and Phase Holograms
A hologram may be of an absorption type which produces a change in the amplitude of the reconstruction beam. The phase type hologram produces phase changes in the reconstruction beam due to a variation in the refractive index or thickness of the medium. Phase holograms have the advantage over amplitude holograms of no energy dissipation within the hologram medium and higher diffraction efficiency. Holograms recorded in photographic emulsions change both the amplitude and the phase of the illuminating wave. The shape of the recorded fringe planes depend on the relative phase of the interfering beams. Consequently the reconstructed wave is reflected from the hologram according to the density of the silver deposited with the amplitude variation proportional to the amplitude of the object. Similarly the phase of the reconstruction wave is modulated in proportional to the phase of the object wave. Thus both amplitude and phase of the object wave are reproduced.
Classification based on Hologram Thickness
Thin Holograms or Plane Holograms
Holograms may be thin (plane) or thick (volume). A parameter Q can be defined for distinguishing between a thin and a thick hologram. The hologram may be treated as thin if Q < 1. It has been shown that thick hologram coupled wave theory applies even for Q values of the order of 1. Thus Q criterion is not always sufficient. A hologram may also be regarded as thin if its emulsion thickness is much less than the fringe spacings. Such holograms produce several orders (i) zero order which is the directly transmitted reference beam, (ii) the first order diffraction producing virtual image, (iii) the minus first order diffraction equal in intensity to the first order producing the conjugate image and (iv) higher orders of decreasing intensity.
A volume (thick) hologram may be regarded as a superposition of three dimensional gratings recorded in the depth of the emulsion each satisfying the Bragg law. The grating planes in a volume hologram produce maximum change in refractive index and/or absorption index. A consequence of Bragg condition is that the volume hologram reconstructs the virtual image at the original position of the object if the reconstruction beam exactly coincides with the reference beam. However, the conjugate image and higher order diffractions are absent.
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