Abstract
Auto-focusing in imaging systems depends on
the determination of the correct image focus criterion. In this
research, the image captured by a liquid-filled diaphragm (LFD)
fluid lens was analyzed to determine a focus measure criterion
that can be used to establish the correct focus and thus quantify
the image quality. The LFD lens was actuated using a steppermotor
driven syringe mechanism. The lens diaphragm was made
of polydimethylsiloxane (PDMS) polymer that exhibit good
optical properties. The lens focal length was controlled by
varying the fluid volume within the diaphragm lens. A CCD
camera was attached to the fluid lens to capture live images of a
binary target. The edge slope width (ESW) of the pixel intensity
profile across the white-to-black transition region in a binary
target was used as the focus measure. The experiments carried
out showed the viability of the proposed focus measure criterion
for automatically focusing the image formed by a diaphragmtype
fluid lens.
Keywords—liquid-fill, diaphragm, lens, focus
I. INTRODUCTION
Optical devices such as cameras, microscopes and other
imaging systems use various types of variable focusing
mechanisms that have been developed in the past. The
conventional methods of varying the lens focal length, i.e. by
using motors, slides and gears, is still widely used in most
imaging systems. The main disadvantage of the conventional
focusing mechanism is that it requires several interlinking
mechanical components. The inherent contact-friction subjects
these components to wear, thus affecting the focus quality of
the captured images. Since there is a general advancement
towards making smaller cameras and imaging systems,
accommodating the various moving parts in the conventional
focusing mechanism faces space and manufacturing
constraints. Therefore, a new focusing mechanism with fewer
moving parts is required for enhancing the performance of
optical devices.
Fluid lenses have great potential for replacing the
traditional glass lenses in achieving variable focusing. In a fluid
lens system a single lens changes its shape continuously to
achieve varying focal lengths, similar to the human eye. In one
popular design of the fluid an elastic transparent diaphragm
that is pressurized by an optically transparent fluid lens ise used
to achieve variable focusing. The fluid pressure causes the
diaphragm to produce various curved profiles, thus resulting in
continuously varying focal lengths. Unlike the conventional
variable focal length lenses (such as a zoom lens) where the
distances between multiple lens elements are varied, the fluid
lens accomplishes variable focusing using a single lens.
Because of this significant advantage of the fluid lens
compared to a glass lens, a number of studies relating to
diaphragm-based liquid-filled lenses have been undertaken in
the past.
Shaw and Sun [1], studied the optical properties of different
liquid-filled fluid lenses to obtain the best membrane shape.
The effect of diaphragm clamping boundary conditions,
diaphragm thickness and diaphragm cross-section shape were
studied using a non-linear finite element method. Shaw and Lin
[2] designed and analyzed an asymmetric diaphragm type fluid
lens. By controlling the tilt angle of a pressure ring it was
possible to control the light direction or the focal length. Zhang
et al. [3] developed a variable focus zoom lens on a chip with a
wide field-of-view tuning range using the standard microfabrication
process. The lens was characterized using an array
of LEDs as the object, placed at a fixed distance from the lens.
Ahn and Kim [4] proposed a crystalline human eye’s lens-like
variable focusing lens using square glass diaphragms. The
square boundary conditions, however, causes the deflected
diaphragm to take a cushion-like shape and thus introduce
distortions in the image. Several other researchers have looked
into various aspects of the fluid lens, such as lens on a microfluidic
chip [5-7] and obtaining high zoom ratio and wide
tuning range [8].
Several authors used various types of actuation mechanisms
for the LFD lens to change the lens focal length. For instance,
Gunasekaran et al. [9] used a syringe pump to actuate the fluid
lens, Kuwano et al. [10] implemented a stepper motor driven
syringe mechanism to change the liquid pressure inside the
lens, Ren et al. [11] used a servo motor to squeeze an elastic
rubber membrane wrapped around the fluid lens, thus varying
the pressure within the lens and Lee et al. [12] developed a
fluid lens that was tuned using an electro-magnetically actuated
diaphragm pump attached to the main lens chamber. Shaw and
Lin [13] developed a novel method to actuate the diaphragm
lens by pressurizing an O-ring attached to the fluid lens. A
thermal-based fluid actuation for a diaphragm micro-lens of 1.9
mm diameter


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