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Holographic interferometry

Something from the history

The first holographic interferograms have appeared because of mistake or casual circumstances. Moreover, it has not attracted attention. Not all experimenters for the first years of holography clearly understood its theoretical bases. The first steps and all efforts were directed on making exactly the holograms. It's known, certainly, from interferometry, that the shift of beams during an exposure already on half of wave length makes impossible to record microinterference pattern, which represents the hologram. Therefore, possible measures to maintain mechanical stability of the optical scheme were accepted. Massive granite and marble plates were used, also massive holders of optical elements. But the first lasers had very low power, and photoplates - similar sensitivity. Thus, it was very difficult to achieve stability of half of wave length during the process of hologram recording. As result, the restored image sometimes covered with strips, which distorted a general picture. Those holograms were rejected as defective, and the experimenters with new efforts fought with vibrations.

For sure, such situation could not proceed any longer. Critical mass enlarged. Someone should reflect. And such person was found. And he was not the lonely genius, but some independent groups of the experimenters. It is interesting, that the first message (1965) concerned holographic interferometry with time averaging. The methods of two exposures and real time are younger than the first message approximately on two-three months. So, in the middle of 60's holographic interferometry has appeared and begun to develop.

Methods of holographic interferometry

It is possible to write down several wave fronts on the same plate and obtain several holograms. If these waves are coherent (and often it happens), they interfere at joint restoration, and there will be only differences between these waves on resulting interferogram. 

This method holographic interferometry has received the name a method of two exposures.

It is possible to use another method. Hologram of the object, after processing, is established with high accuracy on a former place - often plate is processed directly on a place of shooting. At the subsequent hologram exposure in the scheme, that's used at its recording, the object wave restored from the hologram by a reference beam, will interfere with a new wave going from object. As result, it is possible in a real time to trace changes of processes occurring in object of research. 

Such method holographic interferometry is called a method of real time.

For more complete clearness it is possible to underline, that: 

  • In classical interferometry 
    waves interfere, which
    at one moment of time had passed different trajectories.

  • In holographic interferometry
    waves interfere, which
    at the different moments of time had passed identical trajectories.

Partially, this fact allows to use in experiments optical elements of usual quality without worsening of an interferometric pattern all of heterogeneity of an optical path, that is constant in both exposures, will be compensated.

The unique feature holographic interferometry allows to study processes occurring inside optically nonuniform media and with diffusely reflecting objects. Such was absolutely impossible in classical interferometry!

Classical interferometry dealt only with processes and objects varied in a real time. In holographic interferometry the real-time information can be recorded and compared with information of other moments of time. Thus, one more factor - time has appeared. The examples are researches of vibrating objects.-->