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The most important features of the light source are its wavelength and coherence length. The coherence length determines the width of the correlogram, which relies on the spectral width of the light source, as well as on structural aspects such as the spatial coherence of the light source and the numerical aperture (NA) of the optical system ...
Figure 2. File:Twyman-Green interferometer set up as a white light scanner. Vertical scanning interferometry is an example of low-coherence interferometry, which exploits the low coherence of white light. Interference will only be achieved when the path length delays of the interferometer are matched within the coherence time of the light source.
Optical coherence tomogram of a fingertip. It is possible to observe the sweat glands, having "corkscrew appearance" Interferometric reflectometry of biological tissue, especially of the human eye using short-coherence-length light (also referred to as partially-coherent, low-coherence, or broadband, broad-spectrum, or white light) was investigated in parallel by multiple groups worldwide ...
The advantages of white light, which produced a distinctive colored fringe pattern, far outweighed the difficulties of aligning the apparatus due to its low coherence length. [29] This was an early example of the use of white light to resolve the "2 pi ambiguity".
Early implementations of ms/LCI were in the time domain using lock-in detection in order to take advantage of long scanning depths as well narrow detection bandwidths. [2] As in traditional OCT, the beam interference coherence gates the light in order to filter out photons that have not traveled a sufficient path length.
If the surface is rough enough to create path-length differences exceeding one wavelength, giving rise to phase changes greater than 2π, the amplitude, and hence the intensity, of the resultant light varies randomly. If light of low coherence (i.e., made up of many wavelengths) is used, a speckle pattern will not normally be observed, because ...
The interferometer has been used, among other things, to measure phase shifts between the two beams caused by a sample or a change in length of one of the paths. The apparatus is named after the physicists Ludwig Mach (the son of Ernst Mach ) and Ludwig Zehnder ; Zehnder's proposal in an 1891 article [ 1 ] was refined by Mach in an 1892 article ...
This is important because two waves of different frequencies do not produce a stable interference pattern. The coherence length of the laser determines the depth of field which can be recorded in the scene. A good holography laser will typically have a coherence length of several meters, ample for a deep hologram.