Abstract

The study of a large number of small objects distributed over an extended 3-D field in space often calls for holographic recording. Examples are: bubble dynamics in cavitation research or particle image velocimetry (PIV) in flow studies. When instantaneous recording of the complete scene is required, ordinary photography fails since there is no time for subsequent focusing on separate planes in depth. The holographic image, however, can be investigated in detail later on. Sometimes, the evaluation of the holographic image encounters difficulties, since information from a certain region in depth is disturbed by signals from other regions. In PIV of 3-D flow fields, for example, photographs of different planes in depth must be made for further Young's fringes analysis. Noise from the many out-of-focus particles obscures these fringes. A solution to this problem makes use of the limited coherence of the light. Illumination is made by folding a light sheet back and forth through object space. Light scattered by the objects is recorded on a holographic plate, each light sheet being matched in optical path length to a separate reference beam. Careful adjustment of path differences between the light sheets eliminates holographic cross-talk. Upon reconstruction, each reference beam reproduces the objects within one light sheet only, thus reducing disturbances by light from other sheets. Application of the technique to particle image velocimetry of windtunnel flows is demonstrated.

© 1990 Optical Society of America