Biomedical Engineering Reference
In-Depth Information
Fig. 8.6
TF of a coherent
microscope
Measured
Frequencies
k
r
−
k
i
k
k
z
k
0
k
x
A DHM is therefore sensitive to spatial frequencies within the object function
that lie on a spherical shell of radius equal to the wave number that is translated
from the origin by a vector equal to
k
i
. This is illustrated in Fig.
8.6
for the case
of axial backscatter illumination, where
k
i
¼
o
.
Inverse Fourier transformation of the TF gives the PSF and provides further
insight into the imaging process. This has been computed for a back-scatter DHM
configuration with NA
k
0
^
¼
0.55 operating at
l ¼
600 nm and a section through this
distribution is shown in Fig.
8.7
.
By definition the PSF of an instrument provides the image of a weakly scattering
point object. For DHM it is found that the intensity of the PSF decreases as the square
of the distance in the observation direction from its center, in exactly the same way as
a focused beam. This means that the total power in any plane perpendicular to
the optical axis remains constant and as such a DHM does not produce a 3D image
in the strictest sense [
5
]. Nevertheless, if it is used to investigate sparsely seeded
flows, individual particles can be distinguished as bright points in the reconstruction
and their 3D position can be measured [
34
].
A good example of DHM taken from a study of underwater microbiology by
Jorge Garcia-Sucerquia et al. [
6
] is shown in Fig.
8.8
. This image is a reconstruction
from an in-line hologram.
8.4 Optical Coherence Tomography
Optical Coherence Tomography (OCT) differs fundamentally from DHM because
it measures the coherent response of the object
to light of different wavelengths
.
Imaging OCT was originally proposed by Huang et al. [
13
] and has been practiced
in many forms. OCT has major applications in opthalmology where it is used
extensively to diagnose retinal disorders. For these applications it is convenient
to use galvanometer scanning mirrors to scan the retina as shown in the so-called
fundus camera illustrated in Fig.
8.9
.
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