Holography

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Holography is a technique for recording and reconstructing the complete wavefront of light, including both amplitude and phase information.

Principle

Unlike photography which records only intensity, holography records the interference pattern between:

  • Object wave: light scattered from the object
  • Reference wave: a coherent reference beam

Recording

The hologram transmittance encodes the interference pattern:

$$ t(x,y) \propto |U_{\text{obj}} + U_{\text{ref}}|^2 $$

This contains terms proportional to:

  • $|U_{\text{ref}}|^2$ (DC term)
  • $|U_{\text{obj}}|^2$ (halo)
  • $U_{\text{obj}} U_{\text{ref}}^*$ (virtual image)
  • $U_{\text{obj}}^* U_{\text{ref}}$ (real image)

Reconstruction

Illuminating the hologram with the reference wave reconstructs the original object wave, creating a 3D image.

Types

  • On-axis (Gabor) hologram: Reference and object beams colinear
  • Off-axis hologram: Angled reference beam separates image terms
  • Reflection hologram: Viewable in white light

Requirements

  • Coherence: Source must have sufficient temporal and spatial coherence
  • Stability: Sub-wavelength stability during recording