Biomedical Engineering Reference
captured projection holograms along a single axis with a limited angular range
50 ı , since the sensor offered poor response at higher illumination angles.
Therefore, a significantly better axial resolution could be achieved using a dual-
axis tomography scheme as already demonstrated in [ 22 ], as well as using different
sensors offering better angular response (i.e., larger pixel NA), that could permit
recording of holograms at larger angles, for example, 70-80 ı .
Holographic Lensfree Optofluidic Microscopy
In the imaging devices described above that utilize multiple-shifted frames and PSR
algorithms to create a higher resolution image, the spatial shifts between different
frames were achieved by utilizing multiple-shifted light sources or equivalently by
mechanically shifting a single source of illumination.
A different method of obtaining these spatial shifts required for PSR is to shift the
object being imaged, rather than the light source. Considering the wide interest in
microfluidic devices in recent years for biological and chemical analysis of biochips
[ 61 ] and even optical devices [ 62 ], utilizing the flow of an object in a microfluidic
channel appears to be a promising approach for generating the shifts required for
PSR. This would allow incorporating optical imaging into existing microfluidic
devices with our simple on-chip in-line holography scheme and extending the
possible functionalities of microfluidic devices in general.
This optofluidic approach has been implemented to create a lensless holographic
optofluidic microscope on a chip [ 21 , 63 ]. The system configuration is similar
to the on-chip in-line holography configuration described earlier in this chapter.
A microfluidic channel where the objects are to flow is placed atop a CMOS sensor,
with the illumination filtered through a large aperture placed a few centimeters away
from the channel, as shown in Fig. 4.17 . The objects flow through the microchannel,
either due to a pressure gradient or electrokinetically driven, and the digital sensor
continuously captures holograms, which are shifted due to the movement/flow of
the objects. Since the shifts between consecutive lensfree holograms are calculated
solely from the captured images, the fluidic flow does not need to be particularly
uniform in speed or direction. The requirements on the object while it flows are that
it is rigid and does not rotate out of plane.
These multiple-shifted holographic frames during the flow of the objects are
processed in a similar fashion to the processing done in the portable super-resolution
microscope discussed earlier. The shifts between different frames are estimated, and
then, the PSR algorithm finds the best high-resolution hologram compatible with all
the shifted images. To obtain microscopic images of the flowing objects, performing
PSR for only a single viewing angle (e.g., vertical illumination) can be sufficient,
while the illumination angle can also be varied to obtain multiple SR projection
images if tomographic microscopy is of interest. In Fig. 4.17 , the imaging result for