Biomedical Engineering Reference
In-Depth Information
Fig. 4.13
A photograph (
left
) and a schematic illustration (
right
) of a field-portable lensfree
optical tomographic microscope (weighing
110 g) are shown. Twenty-four LEDs, butt-coupled
to individual multimode optical fibers, are mounted along an arc to automatically and sequentially
provide multiangle illumination within an angular range of
50
ı
. The movable fiber tips are
electromagnetically actuated to record multiple sub-pixel shifted holograms for each viewing angle
to digitally synthesize pixel super-resolved projection holograms of the objects
˙
reconstructed. Therefore, a back-projection approach can be used to calculate the
3D object function [
22
,
49
,
55
,
59
].
Based on our initial promising results with lensfree optical tomography [
22
],
we demonstrated a cost-effective and field-portable implementation of an optical
tomographic microscope that employs a single axis of illumination spanning an
angular range of
50
ı
[
20
]. This compact, cost-effective, and high-throughput
lensfree tomographic microscope weighs only
˙
110 g(seeFig.
4.13
) and achieves
1m lateral resolution and <7m axial resolution over a large imaging volume
of
20 mm
3
[
20
]. To implement this portable tomographic microscope, we used 24
LEDs (<0.3USD per piece) that are individually butt-coupled to an array of multi-
mode fiber-optic waveguides (with a core diameter of
0:1 mm) tiled along an arc as
illustrated in Fig.
4.13
. These LEDs provide partially coherent illumination at differ-
ent angles along a single axis. To automate the illumination process, an inexpensive
microcontroller and a custom-built LabView interface is employed that sequentially
turns on these LEDs and captures projection holograms at different angles. To
slightly increase the temporal coherence of illumination (i.e., to create a sufficiently
rich hologram at especially large illumination angles, where temporal coherence
requirements increase proportional to the path length increase between the object
and the sensor planes), we used interference-based color filters centered at
640 nm
with
10 nm bandwidth (<50USD total cost), which are mounted on a piecewise
arc to match the arc-shaped geometry of the fiber-optic array (see Fig.
4.13
).
To increase the 3D spatial resolution, in addition to multiangle illumination, we
also utilized pixel SR techniques for
each
projection image. To achieve this, we
chose to devote a single optical fiber to each angle and then physically displace
the fiber tips by small amounts (<500m) to record sub-pixel shifted holograms
at each viewing direction. In this scheme, the optical fibers are connected to a
