Multimodal Biomedical Imaging Systems - Biomedical Optical Imaging Technologies: Design and Applications - page 335

the fluorescence signal is usually very weak. For a handheld device, a color CMOS
detector is commonly used because of its compactness and low cost. One problem
with the use of CMOS in fluorescence imaging is low signal-to-noise ratio. To
remedy this effect, stronger UV light excitation is necessary. For OCT, an InGaAs
photodetector is selected; this device has high responsivity at 1,310 nm (0.95 A/W),
low DC offset, and low noise.

9.4.4 Illumination Path

One basic requirement of an illumination system for fluorescence imaging and
polarized reflectance imaging is illumination uniformity. With LEDs as the light
sources, there are several approaches to achieve uniform illumination. The first
approach is using lenslet array. The light from the LED is collimated and then
divided into subsections by the lenslet array, and then a field lens overlaps the light
from each lenslet onto its focal plane. The second approach uses a light pipe to
uniformize the light from LEDs, and then the uniformized output is relayed to the
tissue surface. The third approach uses a freeform lens to redistribute the light from
the LED to generate uniform illumination. The fourth option uses LEDs directly,
without any additional optical element.
For this multimodal imaging system, the last approach is used. The drawback
of this approach is that uniformity is not as good as the other three methods.
Because there is no additional optical component to generate uniform illumination,
the divergent angles of LEDs should be optimally selected and the positions and
orientations of the LEDs should be optimized to maximize light efficiency and
illumination uniformity.
Even UV LEDs can be regarded as monochrome light sources, but there are
some very weak tails in the emission spectrum. Therefore, a bandpass filter, whose
transmission is shown in Fig. 9.14 a, is used to block the LED light in the emission
spectrum. A polarizer is placed in front of the white LEDs to provide polarized light
illumination. Since it is in the illumination path and there is no special requirement
on transmitted wavefront, a sheet polarizer is a good candidate because it has a
flexible shape, large acceptance angle, and low cost.
The OCT subsystem in this multimodal system is a point scanning system; it
is necessary to raster scan the focused point across the tissue to generate 3-D
images. Even two optically coupled scan mirrors are a better solution; two close-
coupled mirrors are used because the scan angle is not large and they occupy less
space. To combine OCT imaging and area imaging modalities, a dichroic mirror
is employed. The transmission of this dichroic mirror is plotted in Fig. 9.14 b. This
mirror has very high reflectivity from 1,200 to 1,400 nm and high transmission from
460 to 1,150 nm. This dichroic mirror also has reasonable transmission from 320
to 410 nm and a second reflection band from 430 to 455 nm. Therefore, it reflects