Digital holography

from Wikipedia, the free encyclopedia

Digital holography differs from classic " analog " holography in that the wave field created when a hologram is illuminated is not optically reconstructed. Instead, the field diffracted on the hologram is mathematically reconstructed on the computer. The hologram can either be present on holographic film material and then digitized, or it is recorded from the outset with an electronic sensor (e.g. CCD ). For example, when recording a transmission hologram (in which the object wave and the reference wave hit the photo plate from the same side ), the photo plate is replaced by a CCD or CMOS camera, which record the interference phenomena and the intensity distribution in the same way can. The hologram recorded in this way can be used immediately for further digital processing without any detours . In addition, it is now possible to compensate for image errors caused by the lenses used (for example concentric circles when looking at a hologram superficially) with the help of arithmetic operations . The biggest disadvantages only come to the fore with large objects: Films still have a much better resolution than CCD or CMOS cameras. For comparison: film materials used for holography resolve up to 5000 line pairs per millimeter. This corresponds to a "pixel size" of 0.1 micrometers. In contrast, CCD or CMOS cameras “only” have a pixel size of a few micrometers (as of Feb. 2013). Therefore, when using CCD or CMOS sensors, the maximum angle between the object and reference waves is limited to a few degrees. Movies, on the other hand, can shoot large objects without any angle restriction. Must also by using high-resolution CCD cameras, for example, holograms of clouds ( cloud physics are to create), very large amounts of data are processed.

In digital holography can phase of the object wave front be calculated directly from the numerically reconstructed wave field. This eliminates the in the interferometry otherwise necessary to calculate the phase of a plurality of phase-shifted interferograms ( phase shift method ). This direct phase calculation is mainly used in holographic interferometry and microscopy. In holographic interferometry, object states (e.g. different load states) can be compared with one another very easily. In the microscopy Phase objects are directly visible. Digital holographic microscopy is therefore increasingly replacing classic phase contrast microscopy . Another advantage of digital holography is the possibility of calculating numerically different focus levels from a single recording (hologram) (numerical focusing).

Individual evidence

  1. ↑ LP YAROSLAVSKII, NS Merzlyakov: Methods of Digital Holography. Consultants Bureau, New York (1980)
  2. ↑ U. Schnars, C. Falldorf, J. Watson, W. Jüptner: Digital Holography and Wavefront Sensing - Second Edition. Springer (2014)
  3. ↑ Ulf Schnars: Direct phase determination in hologram interferometry with use of digitally recorded holograms. In: Journal of the Optical Society of America A. 11, 1994, S. 2011, doi : 10.1364 / JOSAA.11.002011 .
  4. ↑ Kim, Myung K .: Digital Holographic Microscopy - Principles, Techniques, and Applications. Springer (2011)