Digital holographic microscopy

Digital Holographic Microscopy (DHM) is digital holography applied to microscopy. Digital holographic microscopy distinguishes itself from other microscopy methods by not recording the projected image of the object. Instead, the light wave front information originating from the object is digitally recorded as a hologram, from which a computer calculates the object image by using a numerical reconstruction algorithm. The image forming lens in traditional microscopy is thus replaced by a computer algorithm. Other closely related microscopy methods to digital holographic microscopy are interferometric microscopy, optical coherence tomography and diffraction phase microscopy. Common to all methods is the use of a reference wave front to obtain amplitude (intensity) and phase information. The information is recorded on a digital image sensor or by a photo detector from which an image of the object is created (reconstructed) by a computer. In traditional microscopy, which do not use a reference wave front, only intensity information is recorded and essential information about the object is lost.

Holography was invented by Dennis Gabor to improve electron microscopy. Nevertheless, it never found many concrete and industrial applications in this field.

Actually, DHM has been mostly applied to light microscopy. It has shown in this field unique applications for 3D characterization of technical samples and enables quantitative characterization of living cells. In material Science, DHM are used routinely for research in academic and industrial labs, depending of the application microscopes can be configured both in transmission and in reflection. DHM is a unique solution for 4D (3D + time) characterization of technical samples, when information needs to be grabbed very quickly. It is the case for measurements in noisy environment, in presence of vibrations, when the samples move, or when the shape of samples change due to external stimuli, such as mechanical, electrical, or magnetic forces, chemical erosion or deposition, evaporation, … . In life sciences, DHM are usually configured in transmission mode. They enable label free Quantitative Phase Measurement (QPM), also called Quantitative Phase Imaging (QPI), of living cells. Measurements do not affect the cells, enabling long term studies. It provides information that can be interpreted into many underlying biological processes as explained in the section “Living Cells Imaging” here below.

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