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Holography

Holography. Traditional imaging. Image, record intensity distribution in 2-d plane. Holography. From Gr, “ holos ”: Record the interference pattern (fringes) between coherent object beam and reference beam . Then we get phase and amplitude information. Why does 3D look 3-D?.

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Holography

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  1. Holography Traditional imaging Image, record intensity distribution in 2-d plane Holography From Gr, “holos”: Record the interference pattern (fringes) between coherent object beam and reference beam. Then we get phase and amplitude information.

  2. Why does 3D look 3-D?

  3. Holography Need good spatial and longitudinal coherence lengths

  4. Hologram recording Simple example Object wave can be made up by summing plane waves. Imagine one of these coming at angle qobvs the z axis. Put film at z = 0. Illuminate also with reference beam along z. Spacing h of fringes on screen: when changes by 2p

  5. Hologram recording Three different angles of plane waves give three interference patterns, but same angle to reference beam

  6. I(y) depends on phase and amplitude of Eob. Expose a film Hologram reconstruction The film becomes a grating of spacing . Send a reconstruction beam along z: What diffraction angles do we get out? For the first order, we get out exactly !

  7. I(y) for two object waves (and reference) • Imagine infinite number of gratings superposed from all object angles...The reconstruction diffraction creates all the object angles again.

  8. A zone plate is a crude hologram of a point source of light!

  9. Hologram reconstruction When we shine the reference beam on the film, we get 1) undeflected part of the reconstruction beam, with no image information (diffraction order m = 0) 2) reconstructed object beam (diffraction order m = 1) 3) “conjugate” reconstruction beams that form a real “image” that is inside out! (diffraction order m = -1) What coherence lengths are needed for reconstruction beam?

  10. Inside-out image: reversed 3D perception “pseudoscopic” images appear inside out, for example: a box on a floor would appear as a box shaped hole in the floor.

  11. Hologram as superposition

  12. Pieces of the same hologram

  13. Film requirements Resolution: 1000-2000 fringes/mm (almost down to l) Need a special nonlinear transparency of film: Normal (linear) film: so Holography needs so

  14. Reflection holography Object and reference beam come from opposite sides of holographic plate. Reconstructing beam from same side as viewer: reflection

  15. Reflection holography

  16. White-light holograms Recorded with coherent light. Reconstructed with incoherent light 15-20mm thick (“volume” hologram). Use “Bragg-plane” interference to give constructive interference for only a narrow band of wavelengths (like photonic crystal)

  17. Rainbow thin reflecting holograms for white light reconstruction Slitnear object. 3-D appearance only along one axis. Rainbow in other direction.

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