Biomedical Engineering Reference
In-Depth Information
Fig. 8.1
(
a
) Reflection from Bragg gratings within the object and (
b
) the wave vector diagram
Scattered and Reference Beams
Object
Laser
< 1µm Pinhole
CCD Detector
Fig. 8.2
Point-source, in-line digital holography
of DHM is a coherent reference field which is made to interfere with the scattered
field. Provided that the phase and amplitude of the reference field is known, that of
the object field can be reconstructed.
Point source in-line holographic microscopy is based on the original idea
proposed by Gabor [
30
,
31
] in an effort to get away from the limitations imposed
by using physical lenses, and more specifically, the very severe limitations of
electromagnetic lenses used in electron microscopy. In the configuration shown
in Fig.
8.2
, the illumination is provided through a pinhole which is less than
a wavelength of light in diameter. The pinhole acts a source of spherical wave
fronts that expand as they propagate from the pinhole. The expanding nature of the
wave fronts provides magnification. If a CCD detector is illuminated by this wave
front it will be uniformly exposed, but if a small scattering object is placed in the
field, interference between the direct wave front and the scattered wave fronts
is recorded by the CCD.
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