Glossary

Application Programming Interface

High level set of building blocks to develop a computer program.

Region of interest. A specific area in an image where a computation or processing takes place (the rest of the image is ignored).

Software Development Kit

Low level set of building blocks to develop a computer program.

The amplitude image is the image as we are used to get from a standard camera or optical microscope.

The amplitude image is reconstructed (mathematically computed) from the hologram image.

The Coherence Distance is the maximal possible difference between the optical path lengths of two laser beams for them to be coherent and thus interfere to produce a hologram. The Coherence Distance is a property of the source.

Digital Holographic Microscope

Digital holographic microscopes distinguish themselves from other microscopes by not recording the projected image of the object. Instead, the light wave front information originating from the object and a reference wavefront is digitally recorded as a hologram, from which a computer calculates the object image by using a numerical reconstruction algorithm to obtain amplitude (intensity) and phase information.

The hologram is the image directly returned by your DHM's camera.

To produce a hologram (i.e. an image containing the information necessary to reconstruct the amplitude and phase images), the image beam and reference beam must be coherent so that they interfere and produce the characteristic fringed image of a hologram.

The non-measured points are points in the image that are considered not to contain measurement information.

In Koala and the SDK, pixels are considered as non-measured points either via thresholding of the intensity image, or a thresholding of the phase gradient or manually (user-defined mask).

This is the distance as it is seen by the light wave. It takes into account the optical characteristics of the medium in which the wave has traveled, that is the refractive index and the geometrical propagation distance. Generally a beam goes through several optical elements (lenses,...), separated by air. During certain measurements, particularly in transmission measurements, the beam also goes through the sample. All those elements and their optical characteristics influence the value of the Optical Path Length.

The phase image is a representation of the difference between a reference surface and the wavefront deformation following its reflexion or travel through the sample. This phase shift is represented modulo 2 Pi. For homogenous samples, it is directly linked to the topology of the sample, for non-homogenous samples the physical characteristics of the surface have an influence.

The phase image is reconstructed (mathematically computed) from the hologram image.

The "contour lines" in the phase image, due to its modulo 2 Pi representation.

Phase jumps depend from the topography of the sample. In some cases it can be minimized or avoided by setting some parameters such as the offset, or by unwrapping the phase image.

The reconstruction distance is an artificial distance representing the focus of the intensity image.

Contrarily to the working distance it is a pure mathematical form of fine-tuning the focus and does not require to move the sample. It can therefore be performed offline on previously acquired holograms.

The artificial wavelengths resulting from the interference of beam of two different wavelengths. This produces a low frequency wave and a high frequency one which can both be used as additional wavelengths of the DHM expanding the range of imaging characteristics.

The unwrapping is the usage of a mathematical method to eliminate the phase jump and reconstruct a continuous surface.

It gives best results on phase images of smooth surface without noise.

The working distance is the physical distance of the sample to the optics.

It can be adjusted by moving the sample vertically (Z) in order to bring the image plane in the coherence zone to produce a usable hologram and to focus the intensity image (not the hologram image, this does not make sense for holography).