Holography allows various interferometric methods for measuring processes of heat and mass transfer to be used. Holographic Interferometry has displaced the Mach-Zehnder-lnterferometry completely, because not only is it much cheaper to use, but is also much easier and convenient to handle. With Holographic Interferometry there is no need to machine or manufacture windows for test sections, mirrors and lenses of the optical components with special precision or accuracy, because imperfections are automatically balanced by the holographic two-step procedure.
Gabor (1948) invented a new method for recording and storing optical information which was called holography (see Holograms). Unlike photography which can only record the two-dimensional distribution of the radiation emitted by an object, holography can store and reconstruct three-dimensional pictures. The name holography comes from the ability of the method to record the totality (holos) of the information related to the wavefront of the light namely the amplitude, the wavelength and the phase position. It could only be used when the laser had been developed, which was 10 years later. A detailed description can be found in the literature: Ostrowsky (1980), Vest (1979).
Holographic Interferometry is a combination of interferometry and holography. Figure 1 shows a simple holographic arrangement for the examination of transparent media. The main difference between classical and Holographic Interferometry is that the object beam is compared with itself.
Therefore, holographic and Mach-Zehnder-Interferometry differ from each other in such a way that the first is a two-step method (see Figure 2) and the second is a two-way method. In the former the undistorted object wave, called the comparison wave, is stored on a photographic plate and can be reconstructed after its development (then it is called hologram) by an illumination with the reference wave. The heat or mass transfer is then introduced, so that the momentary object wave, which is called measuring wave, experiences an additional phase shift. Now the measuring and the comparison wave interfere behind the hologram so describing the physical phenomenon which has been introduced and is to be investigated. (It must be pointed out, that macroscopic interference caused by the superposition of two object waves has to be clearly distinguished from the microscopic interference which takes place when the object and the reference wave are superimposed during the recording of the hologram.)
Figure 3. Growth and condensation of a steam bubble (p = 1 bar, water temperature 8 K below saturation flow, velocity w = 0.25 m/s, heat flux = 9 W/cm2). (From Mayinger (1994) Optical Measuremens-Techniques and Applications, Springer Verlag, Berlin, with permission).
Several object waves—one after the other—can be recorded on one and the same hologram. When illuminating the hologram with the reference wave all are reconstructed simultaneously. This principle is used for the double exposure technique, see Mayinger (1994): in a first exposure the comparison and in a second exposure the measuring wave is recorded. From the interference pattern one can determine the differences between the comparison and measuring conditions caused, for example, by heat transfer.
Disadvantages of this method are that transient processes cannot be continuously observed and that the most favourable moment for the exposure with the measuring wave cannot be preselected, because the interference pattern only appears after the chemical process.
This method allows a continuous observation of the process under investigation like Mach-Zehnder-Interferometry. After the first illumination in which the comparison wave is recorded on the photographic plate, it is developed and fixed The chemical process can proceed in situ in a special housing made of glass, or the holographic plate can be removed, chemically treated and accurately repositioned afterwards.
Now the object wave of the unheated test section is reconstructed by illuminating the hologram with the reference wave. Simultaneously the test section is then irradiated by the measuring wave, however now with the heat or mass transfer process switched on. Both object waves interfere behind the hologram. The continuously observable interference pattern is a result of the temperature or concentration field produced by the heat or mass transfer (see Figure 3).
If the refractive index is simultaneously influenced by more than one parameter, for example by temperature and concentration, the interferogram cannot be evaluated directly. Attempts have been made to use the dependency of the refractive index on the wave length of the light. This can be done by recording two interferograms originating from the light of two different wavelengths and from that to evaluate the temperature and the concentration field separately (Panknin, 1977).
See Mayinger (1994) and Interferometry
Gabor, D. (1948) A New Microscopial Principle, Nature, 161, 1948 ; 1949, Microscopy by Reconstructed Wavefronts, Proc. Roy. Soc., A 197, 1949; 1951 Microscopy by Reconstructed Wavefronts II, Proc. Roy. Soc., A 197. 1951.
Mayinger, F. (1994) Optical Measurements—Techniques and Applications, Springer Verlag, Berlin, Heidelberg, New York.
Ostrowsky, Y. I. (1980) Interferometry by Holography, Springer Verlag, Berlin, Heidelberg, New York.
Panknin, W. (1977) Eine hol. Zweiwellenlängen Interferometrie zur Mes- sung überlagerter Temperatur- und Konzentrations-grenzschichten, Ph.D. Thesis, Tech. Univ. Hannover.
Vest, C. M. (1979) Holographic Interferometry, John Wiley, New York.