Digital Holography: Concepts and Applications (Keynote Lecture)
by
Thomas Kreis
BIAS - Bremer Institute fuer Angewandte Strahltechnik GmbH, Klagenfurter Strasse 2, 28359 Bremen

In digital holography optically generated Fresnel- or Fraunhofer holograms are recorded by a CCD-array and stored in computer memory. The reconstruction of the real or virtual images is not performed optically by illuminating the hologram with the reference wave but numerically by multiplication of the stored hologram with a numerical model of the reference wave. The diffracted field in the image plane is calculated either by the Fresnel-approximation or by a multiplication of its spectrum with the free-space transfer function followed by an inverse Fourier-transform. Both approaches result in a complex field, from which intensity and phase can be determined, while optically only an intensity distribution can be reconstructed. Thus digital holography differs significantly from ESPI/DSPI, where the object is imaged sharply onto the CCD-target and only intensities are reconstructed by a correlation process.

Digital holography offers a wide range of applications: e. g. holographic interferometry for deformation or contour measurement or holographic particle analysis. A subtraction of the reconstructed phase distributions of the wavefields belonging to two loading states of the object yields the interference phase distribution without sign ambiguity and without the necessity of an interferogram evaluation but with a resulting resolution and accuracy comparable to the phase shifting methods. If series of digital holograms with gradually varied load are recorded and stored, one may decide lateron, which holograms should be compared interferometrically. On the other hand motion components leading to high fringe densities can be compensated numerically during evaluation. Effective image enhancement methods exist which employ the numerical availability of the phase distribution.

In digital holography concepts can be realized numerically which are optically impossible: negative intensities to eliminate the zero-order reference wave overlapping the real or virtual image, or the generation of intensity and phase images oriented along the line of sight. The presentation will outline the ways how to record holograms onto CCD-arrays, will show the different approaches to numerical reconstruction, and will present various applications of the method in holographic interferometric deformation measurement, in contour measurement, and in particle analysis.

Date received: February 17, 1999