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Holographic interferometry

Interferometry of phase objects

The real objects, that are used in experiments, change both phase and amplitude and direction of propagation of radiation. It results that the basic criterion of applicability of each method is the size of an error which is brought by them to final result.

It is considered to count, that it is allowed to use interferometric methods of diagnostics for transparent objects, if maximal refraction on an object does not exceed 0.1 mrad, and the influence of absorption can be neglected. Such transparent objects also have received the name phase, that are varying only phase of sounding radiation.

It is always necessary to remember, as classical and holographic interferometry give correct results only for phase objects.

Types of interferometer adjustment

Shape and the ways of decoding interferometric pattern, that is given by any interferogram (holographic or classical) depend on adjustment of interferometer.

1) Infinitely wide strip

If after registration of the first basic hologram, neither object nor reference beam are not changed, all changes in resulting interference pattern will be caused only by phase shift in each point of wave front, caused by the processes which have occurred in object between exposures (in a method of two expositions) or during shooting (in a method of real time).

In this case we can say, that interferometer is adjusted on an infinitely wide strip.

This term has appeared because of fact, that at interference of identical wave fronts propagating in the same directions and having constant phase shift in all points, resulting interference pattern represents a unique strip dark or light, depending on a relation of phases of interfering waves.

At such way of adjustment, changes in object between exposures will be shown in occurrence of closed strips, which contour heterogeneity, and take place inside it. However, it is possible to decode such interference pattern, only with known sign of change of the phase shift at transition from one strip to another. In a received picture this information is absent. 

2) Strips of finite width

If an object or reference beam or photoplate has turned on some angle during the interval between exposures, resulting interference pattern will be observed on a background of regular system of parallel strips. The step of these strips is defined by the size of an angle of turn, and the orientation of strips depends on methods of their creation.

Such interferometer adjustment has named as adjustment of strips of finite width.

In this case, heterogeneity in the object is shown as a curvature of basic system of strips.

Preliminary information is not necessary for decoding such interference pattern. The sign of change phase shift, or direction of the strips counting, is set by a direction of shift of optical elements of the scheme at creation of system of these strips.

Preparation and treatment of silver-halide emulsion at holographic interferometry

It is a half of problem to write down and to restore holographic interferogram. It is required to decode the received picture. To obtain correct results from decoding, it is necessary to take into account a number of features, which extremely influence a reliability of the information, received with holographic interforogram.

The technology of information production, using the holographic interferograms, is very sensitive to slight emulsion shrinkage, that distort recorded interference pattern and, as result, its incorrect decoding. Therefore it is necessary to accept special measures to complete shrinkage elimination, or to achieve its minimum.-->