chapter 5, Time Background Holography of Moving Objects

Excerpt

5.1 Introduction

Considerable attention is being given to methods of obtaining information about moving objects from statistical analysis of the space and time structures of coherent fields scattered by these objects as well as their coherent images. These methods enable one to solve certain problems that cannot be solved by other methods, especially with the presence of monochromatic background radiation: measuring velocity and acceleration distributions on an object's surface, which yield a deformation map of the object; determining the roughness parameters for moving surfaces; finding the geometric and dynamic parameters of weakly reflecting and transparent moving objects; detecting, in addition to size and velocity measurement, small, spatially unresolvable objects; and finally, imaging objects that are indistinguishable from the surrounding background.

The known methods for obtaining information about moving objects in the presence of monochromatic background radiation, including the situation where the object has the same scattering characteristics as the background, are based on compensating for the object's motion and increasing the contrast of its image with respect to the background image. All of these methods involve complex techniques and complicated algorithms for background suppression that can be realized only with fast and powerful computers. Although several researchers have mentioned the effect of increased contrast between the holographic images of moving and stable parts of the same surface, which manifests itself as dark and bright fringes, they did not pay much attention to this effect and used it only for determining the amplitude of surface vibrations.

It is likely that E. Feleppa was the first to notice that this effect could be used for gaining information about separately moving objects surrounded by a stable background. While studying the motion of erythrocytes and leukocytes in blood by means of holographic methods, he recorded with a He-Ne laser beam a long-exposure hologram of a thin blood sample placed between two glass plates.