Digital holography is the technology of acquiring and processing holographic measurement data, typically via a CCD camera or a similar device. In particular, this includes the numerical reconstruction of object data from the recorded measurement data, in distinction to an optical reconstruction which reproduces an aspect of the object.
Digital holography typically delivers three-dimensional surface or optical thickness data.
There are different techniques available in practice, depending on the intended purpose.

Phase-shifting holograms

The phase-shifting digital holography process entails capturing multiple interferograms that each indicate the optical phase relationships between light returned from all sampled points on the illuminated surface and a controlled reference beam of light that is collinear to the object beam (in-line geometry). From a set of these interferograms, holograms are computed that contain information defining the shape of the surface. Multiple holograms gathered at multiple laser light wavelengths are then combined to compile the full shape of the illuminated object over its full dimensional extent.

Off-axis configuration

At the off-axis configuration where a small angle between the reference and the object beams is used.
In this configuration, a single recorded digital hologram is sufficient to reconstruct the information defining the shape of the surface, allowing real-time imaging.

Multiplexing of holograms

Digital holograms can be numerically multiplexed and demultiplexed for efficient storage and transmission. Amplitude and phase can be correctly recovered. The numerical access to the optical wave characteristics (amplitude, phase, polarization) made digital holography a very powerful method. Numerical optics can be applied to increase the depth of focus (numerical focalization) and compensate for aberration.

Wavelength

Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

 multiplexing of holograms is also possible in digital holography as in classical holography. It is possible to record on the same digital hologram interferograms obtained for different wavelengths.
) or different polarizations

Super-resolution in Digital Holography

Superresolution is possible by means of a dynamic phase diffraction grating for increasing synthetically the aperture of the CCD array

Optical Sectioning in Digital Holography

Optical sectioning, also known as sectional image reconstruction, is the process of recovering a planar image at a particular axial depth from a three-dimensional digital hologram. Various mathematical techniques have been used to solve this problem, with inverse imaging among the most versatile.

Extending Depth-of-Focus by Digital Holography in Microscopy

By using the 3D imaging capability of Digital Holography in Amplitude an Phase it is possible to extend the depth of focus in Microscopy.

Combining of holograms and interferometric microscopy

The digital analysis of a set of holograms recorded from different directions or with different direction of the reference wave allows the numerical emulation of an
objective with large numerical aperture

Numerical aperture

In optics, the numerical aperture of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. By incorporating index of refraction in its definition, NA has the property that it is constant for a beam as it goes from one...

, leading to corresponding enhancement of the resolution.
This technique is called interferometric microscopy

Interferometric microscopy

Interferometric microscopy or Imaging interferometric microscopy is the concept of microscopy whichis related to holography, synthetic-aperture imaging, and off-axis-dark-field illumination techniques....

.

See also

• Computer generated holography
  Computer generated holography
  Computer Generated Holography is the method of digitally generating holographic interference patterns. A holographic image can be generated e.g...
  
  
• Digital holographic microscopy
  Digital holographic microscopy
  Digital holographic microscopy is digital holography applied to microscopy. Digital holographic microscopy distinguishes itself from other microscopy methods by not recording the projected image of the object...
  
  

External links

• Lyncée Tec DHM^TM Instruments for Biomedical and Metrologic Applications
• Phase Holographic Imaging Digital holographic microscopy