Biomedical Engineering Reference
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(A)
Off-axis reference beam
(B)
Off-axis reference beam
Fourier
transforming
lens
Fourier
transforming
lens
Illumination
prism
Input
object
Input
object
CCD
CCD
Laser
d
beam
d
Laser
beam
d
d
Displacement and
rotation stage
Figure 9.5
Experimental setup for SALDHM out of the Gabor's regime: (A) on-axis illumination case and
(B) one possible off-axis illumination case.
can be recovered at the Fourier domain of the recorded hologram since off-axis recording
avoids the overlapping between the different holographic diffraction orders.
Using this experimental assembly, the FT of the recorded hologram performs the recovery
of the focused bandpass image of the object under test. When on-axis illumination is used
(
Figure 9.5A
), the focused image at the Fourier domain becomes the conventional image
provided by the proposed imaging system architecture. That image will be limited in
resolution due to the low NA that is defined by the imaging system and which results from
both the reduced CCD size and the distance between the input object and the CCD. When
angular multiplexing incoming from the tilted beam illumination (
Figure 9.5B
)is
considered, a different bandpass image of the input sample will be accessible at the Fourier
domain. The illumination stage is composed from a prism that can be shifted and rotated to
provide sequential oblique illumination onto the input sample, thus, the shift allows tilted
beam illumination at a given angle (defined by the deviation angle of the prism) and the
rotation allows 2D spatial-frequency space coverage around a circle where the deviation
angle of the prism is constant. Let us call this circle an illumination ring. Thus, once on-
axis illumination is considered, the wedge prism is shifted to off-axis positions and rotated
according to such positions in order to illuminate obliquely the input object. This
illumination procedure permits the sequential recovery of different spectral bands of the
object's spectrum when performing digital FT of every off-axis recorded hologram and a
filtering process over one of the diffraction hologram orders. Thus, different rectangular
elementary apertures containing a different spatial-frequency range of the object's spectrum
are recovered in time sequence. By providing tilted beams around the illumination ring, 2D
Fourier space coverage is ensured. This additional information can be properly managed to
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