7. Simultaneous multiple laser sheet illumination

The weakness of any sequential analysis is its intrinsic inability to account for events that are strictly simultaneous but distinctly localized in space; a flow that is both three-dimensional and unsteady cannot therefore be accurately characterized by successive displacements of the analysis section. In this case, a satisfactory solution is to use impulse holographic recording, since in principle only holography can fully memorize a three-dimensional phenomenon. However, this solution has a number of practical limitations [cf. , § 5 ], which justifies the use of a tomographic approach: N sections of the flow are spatially sampled using N simultaneous parallel laser beams. The strict simultaneity of the recordings requires the use of N photosensitive receivers, which usually limits...