Endomicroscopy - Biomedical Optical Imaging Technologies: Design and Applications - page 273

Fig. 8.11 Schematic of three types of GRIN or GRIN-related lenses used in endomicroscopy:
a single element GRIN, b combination of two GRIN lenses, and c hybrid hemisphere-GRIN lens
objective. Note that same length of systems was presented for simplicity, while in fact the length
of objectives will vary between the types

out of focus light emanating from above and below the focal plane in thick tissues
will degrade image contrast. Methods to reduce out-of-focus light which have been
implemented on benchtop microscopes have also been adapted to endomicroscopy
systems (e.g., structured illumination, confocal, or nonlinear imaging modes). To
achieve optical sectioning by confocal or nonlinear methods, a focused beam can be
scanned over a plane proximal to a GRIN lens assembly, which will relay this plane
to the tissue surface.
A more advanced realization of GRIN-lens-based objectives combines a GRIN
component with a hemispherical lens element Fig. 8.11 C. Such objectives can reach
0.8-0.85 NA in water. The interesting feature of this design is that optical design
software can indicate an appropriate ion exchange process to correct for some
aberrations of the system [ 52 ]. Refractive index distribution of n.r / D n 0 C n r2 r 2

C
n r4 r 4 can be considered, where n r2 , n r4 are experimentally derived coefficients to
optimize it for operation with refractive first component lens.
Note that the commercially available lenses of this type allow 0.8 NA at working
distances of 80 m (fluorescence endomicroscopy design) and 200 m (2-photon
design). The lenses have a variable performance across the field and at the 50 m
field point system performs at diffraction limited performance drops so effective
NA is in 0.4-0.5 range [ 56 ]. A graph showing this dependence for fluorescence
design from GRINTECH is presented in Fig. 8.12 . This suggests that the useful FOV
for in vivo imaging might be limited only to 60-80 m, for example, in 2-photon
applications [ 52 ]. A brief comparison of selected lenses for all three GRIN lens
types is shown in Table 8.5 . The comparison includes commercially available lenses
and examples of lenses reported in literature for endoscopic applications.

Miniature Compound Lenses

Another class of miniature objective lenses involves systems comprised of several
individual lens elements. This approach parallels traditional microscope objective