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Digital
Holographic Microscopy (DHM) generates, in real-time, high resolution 3D digital
images of a sample using the principle of holography. Holograms are generated by
combining a coherent reference wave with the wave received from a specimen (see
figure below). They are recorded by a video camera and transmitted to a computer
for real-time numerical reconstruction. DHM software procedures allows
computation, from a single hologram acquired in a few microseconds, of the
complete wavefront emanating from an object and provides:
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Intensity
images providing the same contrast as with classical optical
microscopy,
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Phase
images providing quantitative data, defined at a sub-wavelength scale,
used for accurate and stable measurements. In reflection, the phase
image reveals directly the surface topography with a sub-nanometric
vertical resolution. In transmission, the phase image reveals the
phase shift induced by a transparent specimen, which depends on its
thickness and refractive index.
This
digital approach to holography allows the application of computer based
procedures at a level unreached in video-microscopy. In particular the DHM
principle features software compensation of optical aberrations, digital
image focusing and numerical compensation for sample tilt and environmental
disturbances, making DHM a robust and easy to use method for routine
inspections at the nanometer and micrometer scale. |
