Multimodal Biomedical Imaging Systems - Biomedical Optical Imaging Technologies: Design and Applications - page 337

Considering the spectrum bandwidth of UV LEDs, the effect of excitation UV LEDs
on the fluorescence signal is negligible.
Because the polarizer and the emission filter are the first and second elements
in the detection path, there is no any special requirement on material, coating,
and mounting for optical elements in the imaging path. One shortfall with this
configuration is that half of the fluorescence signal is blocked by the polarizer.
To overcome this limitation, multiple UV LEDs are needed for fluorescence
imaging.
Key parameters for designing an area imaging system are NA and FOV. For
fluorescence imaging, the NA should be as large as possible to maximize light-
collection efficiency because the fluorescence signal is very weak. However, large
NA means shallow depth of field. Therefore, a trade-off must be made when
determining the NA. The large FOV will help screen large areas faster, but the
resolution is lower given the fixed number of pixels in the detector. Also, the large
FOV means the probe will be larger, which is not desirable because the probe will
be inserted into the oral cavity. In order to quickly screen teeth without missing
regions, the minimal FOV should be larger than a tooth.
NA and FOV are also two key parameters when designing OCT imaging optics.
The larger the NA is, the higher the resolution. However, large NA means shorter
working distance and/or a larger scan lens, which should be avoided in handheld
devices. Similarly, large FOV requires a larger scan lens because the scan lens is
typically designed as telecentric on the tissue side.
With the above considerations, the FOV of the imaging lens for fluorescence and
polarized reflectance imaging is determined as 18 13:5 mm, and the NA is 0.02.
The system can cover one full tooth and two half adjacent teeth and can obtain a
sharp image of the occlusal surface of the molar, which has a couple of millimeters
between the cusp and pit. The selected NA of the OCT scan lens is 0.09 and the
FOV is 5 5 mm. The lateral resolution of the OCT scan lens is 10 m, determined
by the wavelength and NA, and the axial resolution is 12 m in air, determined by
the coherence length of the light source.

9.4.6 Optical Design

Generally, special attention must be paid to the selection of optical materials,
coatings, lens shapes, and lens mounting methods for fluorescence and polarized
imaging systems. For this multimodal system, the analyzer and the emission filter
are the first and second element in the detection path, therefore, conventional
materials can be used and coatings can be designed without consideration of
phase shift.
Because the imaging system for fluorescence imaging and reflectance imaging
has a relatively small FOV and low NA, a triplet lens format with three singlets