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A version of a scheme of speckle interferometry is considered, which is intended for the analysis of the stationary distribution pattern of steep refractive-index gradients in optically transparent media. The application of this approach to problems of hydrodynamics and gas dynamics allows one to determine the structure of distribution of optical inhomogeneities in liquid and gaseous flows or media. The method is based on the recording of patterns of two speckle structures obtained in the plane of image formation of the object by an optical system when the object is illuminated by radiation having an auxiliary speckle structure. The patterns correspond to the two positions of an object shifted somewhat in the direction of the optical axis of the system. Contrary to the common methods of speckle interferometry, only the state of the object being investigated for two different conditions of illumination by the probe radiation is used for the detection in this case. The difficulties arising in the measurements of the objects with steep refractive-index gradients by the known methods of speckle interferometry are accordingly practically eliminated in the scheme under consideration. In comparison with the schlieren phase methods, this method offers higher possibilities in the variation of sensitivity.

A version of a scheme of speckle interferometry is considered, which is intended for the analysis of the distribution of refractive-index gradients in optically transparent media. The application of this approach to problems of hydrodynamics and gas dynamics makes it possible to determine the structure of the distribution of optical inhomogeneities in liquid and gaseous flows. The method is based on the recording of patterns of two speckle structures obtained in the plane of spatial frequencies by using illumination with an auxiliary speckle structure. One speckle structure is formed in the absence of an object, and the other one is formed by transilluminating an object. In comparison with other known phase methods, the speckle-interferometry methods make it possible to change the measurement sensitivity in a wide range and substantially simplify the interpretation of the results obtained.

The correct interpretation of light scattering data requires knowledge of the extent to which multiple scattering contributes to the measured signal. Generally, this information cannot easily be obtained in conventional light scattering experiments. We demonstrate the difficulties which arise with this problem by experiments with scatterers where the differential scattering cross section does not exhibit scattering minima. We investigate the influence of multiple scattering by performing combined static and dynamic light scattering experiments using the 3D cross correlation technique. We discuss that, despite the increase of the intensity ratio of multiply to singly scattered light with increasing scattering angle, the dynamic properties are more strongly affected for small scattering angles.

Space- and time-dependent stochastic processes can exhibit correlated spatio-temporal fluctuations, which in special cases can be interpreted as the velocity of the process. We show how for a certain class of such processes - time dependent two-dimensional stochastic patterns - this velocity can be determined experimentally. Since the experimental method is based on differential properties of the pattern, we have calculated the joint distribution function of these quantities for a common type of circular Gaussian process.

We measured the distance‐dependent coupling of transverse boundary waves in a system of layers of two immiscible liquids by laser light scattering. The results were explained on the basis of a theoretical treatment of the dynamics of this system. The deviations of the results for very small distances of the two interfaces could be accounted for by the assumption of a small surface viscoelasticity, which is about one order of magnitude smaller than those measured so far with light‐scattering techniques. The technique used in this paper might therefore be well suited to study molecular interactions and relaxation processes at liquid boundaries, which are responsible for this viscoelasticity.

A light scattering technique is described which allows the non-invasive measurement of instantaneous velocity gradient components in fluid flow. Statistical and systematic errors are investigated both experimentally and theoretically. The method represents a basis for the study of temporal evolution of velocity gradients in fluid flow.

The motion of the speckle pattern in the far field limit is uniquely determined by the velocity gradient of the light scattering medium. This is the basic physical effect of a non-invasive method that allows the measurement of velocity gradients with high spatial and temporal resolution down to 75 μm and 1 ms, respectively. We present three different measurement schemes for the analysis of the scattered light. The experimental results refer to mean gradients obtained from measurements on turbulent channel flows.

A laser light scattering technique is described which allows the measurement of velocity gradients and their temporal correlation functions in fluid flow. This non-invasive and gauge-free technique was applied to a turbulent channel flow. Results of the measurements, including the turbulent energy dissipation and the enstrophy are presented.

Random spatio-temporal intensity patterns occur especially in the field of laser light scattering. In many cases the spatial motion of the intensity field of the scattered light contains information on the scatterer (speckle anemometry). We propose a new technique for measuring this velocity, which is especially suited for fast changing patterns (fast speckle boiling), and show experimental results.

An interferometric method in connection with a correlation technique for measuring the velocity of moving parts of plants is described. Velocities down to 5-0 mm d_1 can be measured reliably in a minute. Values of stem elongation rates of etiolated seedlings of a variety of species obtained by this method are very similar to those obtained by the application of linear displacement transducers. Since the optical method requires no mechanical coupling to the plant, velocities of sensitive plant organs can be determined, as shown by measurements of leaf motion.

If a laser beam passes a liquid sample a small fraction of the incident power is always absorbed by the medium. This gives rise to a temperature profile causing gradients of the density and of the index or refraction, and it changes the optical properties of the sample. The theory of this effect (thermal lens) is generalized for thick samples and focussed laser beams. An analytical solution for the beam properties is given as a function of the position in the sample. The theory is applied to light scattering experiments and analytical expressions are given for the time dependence of the intensity of the scattered light, depending on the power of the incident laser beam, experimental layout, absorbance and the thermal conductivity of the sample. The stationary solution for the intensity of the scattered light contains a contribution which is quadratic in the intensity of the incident laser beam. The theory is verified experimentally. Consequence for experiments where the intensity of the incident laser beam varies with time or where the sample itself shows time-dependent properties are discussed.

Optical absorption measurements on TbVO 4 indicate that at 33°K there is a second order crystallographic phase change. Although no magnetic ordering occurs, the distortion direction can be controlled by a magnetic field. This behaviour is compared to DyVO 4 , in which a similar distortion occurs along different crystallographic directions.

Non-homogeneous velocity distributions of light scattering particles generally result in a non-homogeneous motion of the speckle pattern. This has been shown theoretically and experimentally by considering the space-time intensity correlation function of light being scattered by particles within a shear flow.