Biomedical Engineering Reference
In-Depth Information
transformation must be applied to the synthetic hologram to avoid geometric distortion
when recording holograms at high NA (outside paraxial approximation). This procedure is
crucial for obtaining optimal reconstruction incoming from the numerical propagation of the
synthetic hologram
U
0
S
to focus the sample.
Another different strategy that can be conducted in DIHM was reported by the research
group of Prof. Ozcan at the UCLA and named lensless on-chip holographic microscopy
(LOHM)
[34]
. Briefly, instead of placing the illumination pinhole close to the sample to
achieve high magnification, the sample in LOHM is placed on the top of the electronic
recording device defining typical values for
z
S
and
z
D
as between 5 and 10 cm and around
1 mm, respectively
[35]
. This configuration defines a magnification factor close to 1, so the
resolution limit becomes restricted by the pixel size of the electronic device limiting the
minimum period of the interferometric fringes that can be sampled by the method.
Although it is theoretically possible to have a value of NA which is close to 1 or even
higher, in practice the typical values of NA are around 0.2 since the sample is placed very
close to the recording device and the space between them is filled with several materials
(plastic covers and glass protective layers) having a refractive index higher than 1. This
modest NA value also limits the spatial resolution provided by LOHM. However, the entire
area of the electronic sensor is dedicated for imaging purposes, enabling a wide object field
of view in comparison with the one provided in DIHM. LOHM has been applied in several
applications such as cytometry
[36]
and blood analysis
[37]
, fluorescent imaging
[38,39]
and DIC microscopy
[40]
, portable applications
[41,42]
as well as for detection of
waterborne parasite
[43]
and semen analysis
[44]
. Chapter 8 explains the LOHM principle
and presents the semen analysis results.
However, both DIHM and LOHM lack high values of NA, especially the latter one. In
order to improve the NA value in DIHM, Garcia-Sucerquia et al.
[45]
replaced the air gap
between the sample and the CCD sensor by a high refractive index medium. This strategy
directly increases the NA value since the refractive index of the surrounding medium
explicitly defines the NA. NA improvement from 0.39 to 0.55 was validated. Instead of
modifying the experimental setup, Kanka et al.
[46
48]
have proposed several approaches
to provide high-contrast high-resolution images in DIHM based on improvements in the
numerical reconstruction procedure. Using this methodology, Kanka et al.
[46
48]
have
demonstrated imaging capabilities in DIHM incoming from NA values between 0.6 and 0.7.
Concerning LOHM, Ozcan et al.
[49
51]
also have demonstrated improvements in the NA
value in an approach based on the concept of multi-angle illumination and pixel
superresolution, where the NA value has been approximately enhanced from 0.2 to 0.5
[50]
and to 0.4
[51]
.
In the current chapter, we report on a different direction for improving the NA value in
lensless holographic microscopy. The proposed method, named synthetic aperture lensless
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