Holography

HOLOGRAPHY

Introduction

The word "Holography" is derived from a greek word. In greek 'Holos' means 'whole' and 'graphein' means 'to write', in other way we can say 'graphy' means 'recording'. Therefore Holography means recording the complete information about an object in a holographic plate (similar to a photographic plate).

We know, in Photography if we want to take photograph, we have to illuminate the light over the object. Then the light reflected from the object has to be focussed onto the photographic film with the help of a lens system in the camera. This give rise to a real image of the object in the form of a negative. Thus, after washing and developing the negative, we will get a 2-dimensional image of the object.

Holography

Holography is similar to the photography technique, in which we can construct a 3-dimensional image of an object using a laser beam. That image is called as a Hologram. In this hologram we can see the front, side and top etc., portions of the object, similar to what we see in video games (or) 3D-films.

Types of holographic techniques

There are different holographic techniques by which we can make holograms. They are

(i) Optical holography.

(ii) Acoustical holography.

(iii) X-ray holography.

(iv) Microwave - holography.

(v) Double expanded holography.

(vi) Real time holography.

(vii) Time average holography.

Let us discuss the construction and reconstruction of a hologram by optical holography using laser beam.

Construction of Hologram

Principle

Two beams, viz one from the object (object beam) and the other from the laser source (reference beam) are super imposed on a holographic plate to form an image called hologram.

Working

A monochromatic source of Laser is made to fall on a beam splitter (B) with the help of a condensing lens (C). The beam splitter splits the incident beam into two beams viz,

(i) Reference beam (R) and

(ii) Object beam (O) as shown in Fig. 4.29.

The reference beam will directly fall on a holographic plate and the object beam after getting scattered by the object will fall on the holographic plate.

Therefore the two beams i.e., the reference beam and the object beam will interfere with each other and the interference pattern is recorded on the holographic plate. This on developing and processing we get a hologram.

Precautions

During the construction of a hologram, the following precautionary measures has to be taken.

(i) The distance travelled by the reference beam and the object beam should be almost equal.

(ii) Ratio of light intensity between the reference beam and object be should be 3:1.

(iii) Total darkness should be maintained, while loading the holographic plate.

(iv) The total setup has to be kept in a vibration less table, to avoid vibrations due to the natural seismic vibrations of the Earth.

Reconstruction of a Hologram

Principle

A beam of light (reading beam) having the same wavelength as that of the reference beam [used for construction] is made to fall over the hologram during reconstruction. This beam is diffracted by the hologram, which inturn gives rise to a 3-dimensional image in the field of view.

Working

During the reconstruction process, the object is first removed and the laser beam is switched ON. The laser light (reference beam) will now act as a reading beam (R_B). This reading beam is made to fall on the hologram as shown in Fig. 4.30.

Now, due to diffraction, the observer can clearly see a 3-dimensional image through the hologram. Here, the image appears as if the object is really present. Therefore, the observer can move his head side wise or change the position of the eye ball, so that he can enjoy the fully view of the image. Hence the hologram isreconstructed..

Precautions

The following precautionary measures are followed during the reconstruction of a hologram.

(i) The object should be removed carefully.

(ii) The reading beam should have same wavelength as that of the reference beam. (iii) Hologram should not be disturbed.

Note: While reconstructing the hologram, it is not so that laser light alone should be used. Instead we can use any other monochromatic sources like sodium vapour lamp (or) ordinary beam, with the condition that it should have same wavelength as that of the reference beam used for constructing the hologram.

Applications of holography

1. They are used in the production of holographic stickers.

2. Holography can be used in non-destructive testing.

3. It is used in the identification of finger prints.

4. It is also used in data processing, optical signal processing etc.