Biomedical Engineering Reference
In-Depth Information
digital holographic microscopy (SALDHM), provides the generation of a synthetic aperture
(SA) that expands the spatial cutoff frequency of the imaging system and, thus, its spatial
resolution limit, in comparison with the case where no SA enlargement is considered. The SA
generation is based on the combination of angular- and time-multiplexing incoming from
tilted beam illumination (angular-) sequentially performed (time-) in order to multiplex
different object's spatial regions. This strategy has been successfully validated to improve the
NA value in DHM
[52]
and now it is applied to LDHM.
The proposed SALDHM concept is validated using two different approaches. The first one
concerns the case of imaging essentially transparent (low concentration dilutions) and static
(slow dynamics) samples
[53]
. In other words, it is applied in the Gabor's regime where
holography rules the electronic recording process. However, as it was stated in Ref.
[24]
,
diffraction dominates the process preventing an accurate recovery of the object's complex
wave front when the amount of light blocked by the object, for high density samples, is
significant. For these kinds of samples, a different strategy must be used to permit imaging
capabilities in LDHM. The use of spatial light modulators in combination with a
phase-shifting algorithmic is a possibility
[54,55]
. Moreover, time multiplexing SA
generation incoming by CCD shift at the recording plane has also been reported
[56]
.
Nevertheless, the classical way to circumvent the Gabor's concept is by reinserting an
external reference beam at the recording plane in a way similar to the one reported by Leith
and Upatnieks
[6
8]
. In that case, a beam splitter usually is placed in front of the digital
recording device (typically a CCD camera) is usually placed to allow a reference beam for
holographic recording. The inclusion of a beam splitter in the setup prevents the sample
from being placed close to the CCD camera (at least at a distance equal to the face length
of the beam splitter cube). This fact, in addition to the restricted CCD sensor size, limits the
experimental NA value that can be achieved. Thus, our second reported approach operates
in this situation of working for any type of sample due to the external addition of a
reference beam
[57]
.
In both presented approaches
[53,57]
, preliminary tests and calibrations are performed using
a synthetic object (1951 USAF resolution test target) in order to quantify the resolution
improvement. Then, the approaches are implemented on biological specimens (sperm cells
and red blood cells, respectively).
9.2 SALDHM Inside the Gabor's Regime
In this section, we demonstrate the generation of an SA having a high SNA value in DIHM
incoming from time and angular multiplexing of the sample's spectrum. The experimental
setup is depicted in
Figure 9.1
for the two cases, (A) on-axis and (B) off-axis illuminations.
Essentially, a divergent spherical wave illuminates the sample and a CCD device records
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