Page 240 - Optofluidics Fundamentals, Devices, and Applications

Bio-Inspir ed Fluidic Lenses for Imaging and Integrated Optics 215

9-2-2 Fluidic Zoom Lens
Animal eyes have an elegant way to produce accommodation. How-
ever, no animal eyes possess the capability of optical zoom where the
magnification of the system can be changed without varying the
object distance. Optical zoom is an essential feature for most modern
imaging systems including digital still cameras and camcorders. In
this section, we demonstrate that the bio-inspired fluidic lens can
provide compact optical zoom systems with high image quality.
Optical zoom refers to the change of magnification or image size
without varying the object distance. Equivalently, one may think of
optical zoom as varying the field of view. For a given image sensor, a
larger image means a smaller field of view and a smaller image means
a larger field of view. Optically, the field of view has a nearly propor-
tional relation with the effective focal length (EFL) of the optical
system, and the zoom ratio of an optical system is defined as the ratio
between the maximum EFL and the minimum EFL [44]. In a greatly
simplified model, conventional zoom lenses comprise two mov-
able lenses. In such a two-lens system, the total power of the
system is related to the power of each lens in the following way:
⋅
Φ = Φ + Φ − ⋅ Φ Φ where Φ and Φ are the power of individual
d
T 1 2 1 2 1 2
lenses and d is the distance between the lenses. The only way for a
traditional optical system to change the EFL is to change the distance
between the lenses. Moving lenses along the optical axis is a compli-
cated process, making today’s zoom lenses bulky, expensive, and
hard to manufacture. Optical zoom can be achieved without varying
the lens distance provided fluidic lenses are used. Considering a sim-
ilarly simplified optical system comprising only two fluidic lenses,
one can change the EFL and the field of view by varying the lens
shapes instead of the lens spacing. A longer EFL or narrower field-of-
view is achieved if the front lens has a convex shape and the rear lens
has a concave shape. Conversely, a shorter EFL or larger field of view
is achieved if the front lens has a concave shape and the rear lens has
a convex shape. The elimination of axial lens movements greatly sim-
plifies the fabrication of zoom lens system, and the large tuning range
of fluidic lenses reduces the system’s total track length.
One example of 1/4-in format fluidic zoom lens system is shown in
Fig. 9-11. In Fig. 9-11a, the front lens is tuned to a convex lens and the
back lens is tuned to a concave lens. Such a configuration is equivalent
to a telephoto system with a diagonal full field of view (FFoV) of 13°. By
changing the curvature of the fluidic lenses, as shown in Fig. 9-11b, a
reversed telephoto system consisting of a concave front lens and a con-
vex back lens can be created. The FFoV becomes 60°. Ignoring aberra-
tions, the field of view of an optical system is determined by its effective
focal length (EFL) and the diagonal of the image sensor (i.e., optical
format). The EFL of each configuration is 14.2 and 4.1 mm, respectively.
This corresponds to greater than 3× optical zoom.