Endomicroscopy - Biomedical Optical Imaging Technologies: Design and Applications - page 274

Fig. 8.12 Dependence
between diffraction limited
performance at different field
locations for equivalent NA.
Data is presented for
fluorescence GRINTECH
hybrid objective
GT-MO-080-0415-488 [ 56 ]

0.8
0.7
0.6
0.5

0.4
0.3
0.2
0.1
0.0

0 10 20

radial object field height [ µ m]

30 40 50

design and allows the designer more degrees of freedom to satisfy criteria including
FOV, NA, image quality, field flatness, and correction of chromatic aberrations.
These systems are usually custom made, quite expensive to prototype, and tedious
to build, with assembly usually the primary cost-driving factor. The design itself is
a standard process, as are the capabilities of today's fabrication processes (grinding
and polishing, diamond turning and injection molding). In addition, these modern
fabrication techniques allow creation of aspherical optical surfaces that can simplify
the overall optical design [ 58 ]. A comparison of selected lens implementations
is presented in Fig. 8.13 and Table 8.6 . Note that numbers in figure and table
correspond to the same lens and that schematics of optical layouts in Fig. 8.13 are
not to scale between each other.
Lenses 1 and 2 were built with traditional technologies (grinding and polishing,
using glass materials). Lens 1 uses 8 spherical components, is corrected for single
wavelength illumination, and can image up to 450 m in depth. This system was
designed with hydraulic suction to change the distance between the focal plane and
the tissue to enable optical sectioning at different axial positions when coupled with
a confocal platform. Lens 2 is particularly interesting though quite complicated and
costly to manufacture. It provides a NA of 0.46 over a 450 m FOV. This system
is achromatized for significant spectral range spanning over the 480-660 nm. The
aberration correction was accomplished through the use of multiple singlet, doublet,
and triplet lens components and different glasses (SFL6, N-PSK53, and F2). To
enable imaging at adjustable depths, the last lens can be moved, allowing the object
plane to be translated from 0-200 m.
Objectives 3, 4, and 6 were designed with the intention of simplifying the
assembly process and enabling mass production techniques to be applied. The
ultimate targeted fabrication technique was injection molding, while Lens 4 and
6 prototypes had elements prototyped with diamond turning in plastic. Lens 4
also incorporated a single low-tolerance spherical glass lens. For assembly, Lens
3 used kinematic mounts embedded in the lens itself. This feature increased the
overall diameter to 8 mm while the clear aperture was actually 2.75 mm. Lenses
4 and 6 were assembled using self-centering springs [ 34 ] to automatically place
components in the correct lateral position. Note that use of plastic materials