Holography
Holography
Holography is a method of producing three dimensional image of an object due to the interference of coherent light waves. The word holography is derived from two Greek words 'holos' (meaning complete) and 'graphos' (meaning writing). The principles of holography were formulated by the British scientist Dennis Gabor in 1948. The practical development of holography became possible only after the discovery of laser. In 1971 Gabor was awarded the Nobel prize in Physics for this invention.Principle
Holography operates in two stages namely recording and reconstruction. Recording is the process of making the hologram and reconstruction is the process of reading the hologram. The recording is based on interference of light waves while the reconstruction is based on diffraction of light waves.In conventional photography, a negative image is recorded first and using it a positive print is produced. The positive print is only a two dimensional record of light intensity received from a three dimensional object. It contains information about the square of the amplitude of light wave that produced the image. Information about the phase of the light wave is not recorded. In holography both the phase and amplitude of the light waves reflected or scattered by the object are recorded on a photographic plate. Another coherent wave of the same wavelength as the scattered wave (called the reference wave) is superimposed on the scattered wave. Interference occurs and the interference pattern is recorded. It contains information about the amplitude and the phase of the scattered wave. The recorded pattern is known as hologram. In the reconstruction process, the hologram is illuminated by a wave of the same wavelength as the reference wave. A virtual image and a real image of the object are produced.
Recording of the hologram
Recording on a photographic plate depends on intensity variation. To record the phase, the phase variations have to be converted into intensity variations. For that, the waves scattered by the object are superimposed by a reference wave of the same wavelength. The resulting interference pattern is recorded on the photographic plate. The negative obtained by processing the photographic plate is the hologram. A coherent beam of light from a laser is split into two parts. One part illuminates the object and the other part falls directly on a photographic plate placed in front of the object. That part of the beam which falls directly on the plate constitutes the reference wave. The beam scattered from the object and proceeding towards the photographic plate is called the object wave. The hologram has no resemblance with the object.
Reconstruction
The hologram is illuminated by a coherent beam identical to the reference wave used in the recording process. Two diffracted beams are produced. One of them is a divergent beam. This produces a virtual image in the same relative position as the original object. The other beam is convergent. It produces a real image symmetrically placed with respect to the virtual image. On the illumination by the reconstruction wave, the hologram becomes transparent. For an observer looking against the direction of the divergent beam, it acts like a window, through which he sees the virtual image. This is an exact three dimensional replica of the original object. Changing the angle of viewing, the observer can see around the object. The convergent beam produces a real image which can be photographed.
No comments:
Post a Comment