Application of digital holography to filament size analysis

The potential of in-line digital holography to locate and measure the size and position of filaments, i.e. thin wire-like objects, distributed throughout a thick volume has been investigated. In this paper two approaches are introduced to study filaments of varying diameter. (1) It is shown analytically and experimentally that for a gradual variation of filament diameter, one-dimensional Fraunhofer diffraction theory can be applied with an accuracy of 5% to filaments with a diameter of 60–100 ?m; the x and z positions of filaments in a bundle can be determined with 0.1 mm and 0.3 mm accuracy respectively. (2) The out-of-focus plane approach has been modified and applied to a simple bundle of filaments for the case when their diffraction patterns can no longer be distinguished individually on the hologram. An accuracy of 20% in measuring diameter and 0.3 mm to detect the position is reported. Limitations as well as suggestions to further improve the technique are discussed.