🔮 Volumetric Holography: Beam Interference Patterns

Holography requires coherent light — photons with a fixed phase relationship over time (temporal coherence) and across the beam (spatial coherence). Lasers provide both, allowing stable interference fringes to form during the exposure time. Ordinary white light has too short a coherence length.

Coherence length is the maximum path-length difference between the reference and object beams over which interference fringes remain visible. It must exceed the depth of the scene being recorded. HeNe lasers have coherence lengths of tens of centimetres; diode lasers vary from millimetres to metres.

Full-colour holograms are recorded with three lasers at red, green, and blue wavelengths simultaneously. Each wavelength creates its own set of fringes at slightly different spacings; illuminating with the same three wavelengths reconstructs all three colour channels, combining to reproduce the original colours.

Holographic interferometry compares two wavefronts recorded at different times or under different conditions. Any deformation or vibration of the object changes the optical path lengths, creating interference fringes in the reconstructed image that map displacements with nanometre precision.

Hologram resolution is limited by the grain size of the recording medium (typically 1–3 µm for high-resolution film), the coherence of the light source, and mechanical stability during exposure (vibrations larger than λ/4 wash out fringes). Digital holograms are additionally limited by the pixel pitch of the camera sensor.