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Holographic interferometry

If we combine those two optical schemes, it is possible to reach direction of sounding, given beforehand, and convenience of supervision of a picture by not armed eye on a background of diffuser. This optical scheme is given below. Here L1 is lens, L2 - lens with short focus.

Such scheme of holographic interferogram registration allows to project the necessary section of an object on the plane of the diffuser. It improves binding of interferometric strips to the object and makes the picture more convenient for further decoding.

It is possible to not use the lens in this scheme, but the object should placed closely to diffuser.

Figures below show inerferograms of similar objects received at various adjustments of interferometer: a) - adjustment on an infinitely wide strip, b) - adjustment on strips of finite width.

a) b)

There are many other variants of the schemes for holographic interferometry of transparent objects.

The note
In the case of strips of finite width all schemes can sometimes contain additional optical elements regulating adjustment of system of basic strips.

HI of the focused image

Apart from the described schemes exists HI, constructed on the scheme of holograms of the focused image (HFI). They also can be applied both with diffusers and without them. But using the diffuser in this case is not necessary. 

The inerferograms, that were made in the scheme of HFI (see figure: K1, K2 - collimators, L - lens) are convenient to restore in white light. The speckle-structure which depends only on properties of laser beam and lens, becomes averaged. As the result, separate speckles are not visible and the contrast of interferometric strips is slightly increased. Using diffuser allows to cover already existing speckle-field the with additional field, generated by diffuser. The situation is worsened, especially at the small sizes of heterogeneity.

It depends on experiment and available optical elements - what beam (diffused or flat) should be used as reference. HI by the scheme of HFI allows to investigate thin structure of interferometric strips, including photo through microscope. For an example, figures below show interferogram and its enlarged fragment.

a) b)

Unique properties and opportunities of holographic interferometry were already repeatedly mentioned. Holographic interferograms from figures are clearly demonstrate it. All three interferograms, received with the different optical schemes, have recorded perturbations, that initiated by the electrical discharge inside of the solid isolator.

a) b)

Classical interferometry is absolutely powerless in such cases.

The note 1
All optical schemes are over-simplified here. At construction of the real schemes, it is necessary to take into account a type of beamsplitter and sizes of a zone of registration. As result, the arrangement specificity of the object of research also can require complete change of an arrangement of optical elements. And the laser could be placed differently, than on these schemes. 

The note 2
For achievement of the maximal contrast of interference pattern, planes of polarization of both laser beams should be are perpendicular to that plane, at which the beams of light converge or the beams should have circular polarization.

Besides, the perpendicular to a the plane of a plate should be the same as bisector of an angle between the object and reference beams.

The note 3
The modern lasers have large length of coherence. However, differences of a lengths between object and reference beams should be reduced to a minimum.