Page 237 - Optofluidics Fundamentals, Devices, and Applications

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track length. Similarly, the world’s shortest (< 1.5 cm) 3× optical zoom
lens can also be achieved with fluidic lenses.

9-2-1 Auto-Focusing Miniaturized Universal Imager
Fluidic lens has the inherent capability of changing its focal distance
over a wide range. However, a single lens, being tunable or not, is not
sufficient for producing high-quality image on a solid-state image
sensor. As mentioned previously, one needs to introduce additional
surfaces to compensate for the aberrations. One major aberration that
can be easily understood is field curvature, which is the deviation of
the focal point from the image plane as the ray incident angle
increases. This can be seen in Fig. 9-6.
In human and most mammals’ eyes, the retina is curved to accom-
modate for the large field curvature [51]. However, current image
sensors, both complementary metal-oxide-semiconductor (CMOS)
and charged-coupled device (CCD), are flat. The limitations imposed
by current image sensor technologies require additional lenses to cor-
rect the effect of field curvature. This leads to a hybrid optical system
consisting of fluidic lenses and fixed lenses. The former produce tun-
ability and the latter correct aberrations such as field curvature. One
example of such fluidic/fixed lens system is shown in Fig. 9-7. In this
design, the fluidic lens is placed in front of a Cooke Triplet consisting
of three fixed lenses. When the fluidic lens has zero power, the system
is optimized at an object distance of 10 m. The simulated image height
is 4.4 mm (the diagonal of the CMOS image sensor), corresponding to
a 1/4 in optical format. By varying the power of the fluidic lens, we
show that such an optical system can form images of objects at infin-
ity and as close as 2 cm from the lens, as a result of the wide tuning
range of the fluidic lens. When the object is at infinity, the simulated
system has a resolution of 205 line-pairs/mm (diffraction-limited) on
the axis, 148 line-pairs/mm at 50% field, and 95 line-pairs/mm at
100% field. For an object distance of 2 cm, the resolution is reduced to
56 line-pairs/mm on the axis, 88 line-pairs/mm at 50% field, and

Field curvature Field curvature
Object at 2 cm
Object at infinity

1 mm 1 mm
FIGURE 9-6 Ray tracing of a single lens imaging system. Any single lens imaging
system is riddled with aberrations. One easy-to-spot aberration is the fi eld
curvature. One can easily observe that the focus moves away from the image
plane as the ray incident angle increases.