Holographic_Interferometry_of_Diffusively_Reflecting_Objects

3. Holographic Interferometry

The interference phenomena are given by the wave character of light and occur if there is an obstacle in light impulse propagation. They are based on the principle of light beam interference (see chapter 1). As a consequence of phase distribution of the object wave the interferential comparison of the object and its holographic image is possible. If we illuminate both the object and its hologram with coherent waves, we can observe both the object and its holographic image at the same time through semi-permeable mirrors. It sometimes happens that in some places the object and reference waves are in phase while in other places they are in phase opposition. It means that we see different points of the object with different intensities – as if the object was covered with interference fringes. If the object is identical to itself (i.e. if it is the same as at the time of recording) the whole of its image, (in the right setting of its position) must be in phase with the image reconstructed. But if the object is deformed in some place – a quarter of the wavelength is enough – the phase of thus created wave is different in this place, which is manifested by a significant amplitude change in the interference field of both waves and, as a final result, by interference fringes if we observe both the object and its holographic image at the same time. The same effect appears if we record the object before and after deformation on the same hologram. During the reconstruction, the displacement of the object surface is also manifested by the presence of interference fringes. These techniques are elaborate to such an extent that we speak about a new discipline with a new name: holographic interferometry (Trolinger, 1996), (Dado et al, 1998).

The application of interference methods is possible only if the phase differences of the compared waves on small distances differ only by several multiples of 2λ. If the displacements are much bigger than the wavelength interference fringes are created as well, but their density is so high that they can be identified only with difficulties.

When utilising holographic methods the basic characteristics of the investigated object such as: the degree of transparency, reflectivity, surface micro relief, external dimensions, shape and time stability must be taken into consideration.

The research subject of the holographic interferometry applications can be diffusively reflecting objects (non-transparent) – particularly real objects, from which the light wave is reflected. To improve its reflection characteristics, the surface is sometimes covered with diffusively reflecting coatings, for example with aluminium or water-emulsion paint. If the studied object is a wood-based product or article then any surface treatment is groundless.

Another subject of study can be also the objects through which the light wave passes, called phase objects (transparent objects, optical inhomogenities) as they change only the phase of the light wave.

During the interaction of the light wave with the object changes of the wave amplitude and phase characteristics occur. They are manifested in the interference pattern. The amplitude changes influence the pattern contrast and are usually detected by photometric methods.

The phase changes are detected by interferometric methods. They are manifested in the arangement of interference lines and correspond to information about shape changes of the investigated non-transparent body surface or about the changes of the optical paths of beams passing through transparent objects. The phase changes can occur when the light wave is reflected from the investigated object or when it passes through the transparent object. In the first case the light wave carries information about the relief of the reflecting surface while in the second case it carries information about spatial distribution of the refractive index (Urgela, Osvald, 1993).

The methods of holographic interferometry are utilised to study object deformations, vibrations (Indisov et al, 1983), or small displacements. They are also applied in the field of fluid mechanics, heat transmission, mass transfer, environmental technology and mainly as visualisation methods of investigation of inhomogenities within transparent objects, for three-dimensional recording of the elements in fluids and in the investigation of physical fields (Longauer, 1994), (Urgela, 1999), (Marko, 1999), (Šályová, 1999), (Pavelek et al, 2001).

The optical methods obtain information about the state within the investigated object through light radiation, which does not influence the process on the surface of the given object or within the given phase object.

With respect to the requests put on the experimental equipment and its operation the optical visualisation methods are mostly applied only for laboratory measurements, namely on specially modified test specimens (models) to make the investigated area optically accessible.

3.1 Holographic Interferometry of Diffusively Reflecting Objects

Practical importance of the methods of holographic interferometry for the research of diffusively reflecting objects is connected with the solution of many tasks. In some cases the character itself of an interference image enables us to draw serious conclusions about defects in material structure (Keprt et al, 1995), resonance frequencies, and forms of objects vibrations (Agren, Stetson, 1992), (Marčok, Černecký, 1997), to compare the studied objects with the reference ones and to perform other qualitative and quantitative tasks. In many experimental studies it is necessary to obtain quantitative information about the object or the process, which requires the interferogram analyses.

Holographic interferometry allows us to measure displacements of the body surface points in the interval from 0.1 up to 100 µm and, when using moiré effect the upper limit can be extended up to the millimetre scale. The measurement accuracy represents one fifth of the wavelength of the used light, which is not possible to reach by other methods.

The importance of the experiments increases in the new and non-traditional fields such as refraction mechanics, optimisation of designing, dynamical and random processes, rheologic characteristics of materials, influence of inhomogenities within materials, diagnostic and defectoscopic quality control of materials, investigation of resonance frequencies of saw blades, penetration of the cutting wedge into wood, formation of particles during woodworking operations, applications at the construction of musical instruments – violins, investigation of tensions in the gullet of saw tools.