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OPTICS
TINY SHAPE-SHIFTING
LENSES THAT MIMIC THE
LENS OF THE HUMAN EYE
COULD TRANSFORM THE
MULTIBILLION-DOLLAR
CAMERA-PHONE MARKET
BY BENNO HENDRIKS
& STEIN KUIPER
32 IEEE Spectrum | December 2004 | NA
THE CAMERA PHONE is one of the hottest-
ILLUSTRATIONS: BRYAN CHRISTIE
selling items in all of consumer electronics, with
anticipated sales this year of 170 million units. The little gadgets have become so ubiquitous that hardly anyone finds it odd anymore to see tourists squinting with
one eye while pointing their cellphones at a Buddhist
temple, a Greek statue, or a New York City skyscraper.
It’s easy to see why analysts expect that this year camera
phones will outsell conventional digital cameras and
traditional film cameras combined.
But as anyone who has ever seen them can attest,
the images that come out of camera phones leave plenty
to be desired. Part of the problem is their CMOS imaging chips, which typically have a sensor array of only
about 300 kilopixels—a quarter or less of the number
in a low-end digital camera. Of course, semiconductor
industry fundamentals ensure that 1-megapixel camera
phones will soon be the norm. When they are, however,
the only thing we may see more clearly is the other weakness of these cameras: their tiny, fixed-focus lenses,
which have poor light-gathering and resolving power.
We have a solution. It’s modeled on the human eye,
with its remarkable optical capabilities. We call it the
FluidFocus lens. Like the lens of the eye, this lens, which
we built at Philips Research Laboratories, in Eindhoven,
the Netherlands, varies its focus by changing shape rather
than by changing the relative positions of multiple lenses,
as high-quality camera lenses do. Our tests of a prototype FluidFocus lens showed that it can be made nearly
as small as a fixed-focus lens. Fixed-focus lenses use a
small aperture and short focal length to keep most things
in focus, but at the sacrifice of light-gathering power and
therefore of picture quality.
At the same time, our prototype lens delivered sharpness that is easily on a par with that of variable-focus
lenses. In fact, the optical quality of a liquid lens combined with a megapixel imaging chip could soon give
cellphone snapshots quality that rivals images from
conventional—and much bulkier—digital cameras.
The superior capabilities of FluidFocus lenses should
make them ideal not only in camera phones but also
in products whose design constraints demand a tiny
but capable optical system. Just a few examples are
pocket-size conventional digital cameras, PDA cameras, webcams, hidden security cameras, DVD recorders,
and endoscopes. And with extensive bioengineering,
it’s even possible to imagine these lenses being a key
component of a future implantable artificial eye—long
a dream of ophthalmologists and science-fiction writers. The superhuman, zooming vision first popularized
by the hero of the 1970s U.S. TV series “The Six Million
Dollar Man” is still far off, but now, at least, we have
an idea of how it might be achieved.
SHAPE SHIFTER: The FluidFocus lens comprises a volume of water [blue]
covered by a volume of oil [tan] inside a glass cylinder [light blue]. At the inner
surface of the glass are cylindrical layers of an electrode, an insulator, and, on
the very inside, a water-repellent material.
With no voltage on the electrode, the water surface is convex [top]. And
because the refractive index of oil is greater than that of water, parallel light
rays passing through the meniscus—the interface between the water and the
oil—spread out.
A voltage on the electrode attracts water molecules toward the cylinder’s
surface, making it act less repellent, and the water surface becomes concave
[next diagram]. Here, parallel light rays passing through the meniscus converge
at a focal point.
Water-repellent
coating
Oil
Insulator
Glass
Water
Electrode
Incident light
Voltage
As the voltage on the
electrode increases, the
meniscus between the oil
and water changes from
convex to concave.
0V
CONVENTIONAL AUTOFOCUS SYSTEMS are not practical
in today’s camera phones and other portables, because
they use motors and gears to shift the position of the
lenses. Those assemblies are difficult to miniaturize and
are vulnerable to wear. But our liquid-based lens has no
50 V
75 V
December 2004 | IEEE Spectrum | NA 33
moving parts or mechanical actuation, which makes it more efficient and potentially much longer-lived. Such features are a big plus
in security cameras, for example, which are constantly refocusing.
The human eye focuses on objects at different distances by contracting and expanding muscles attached to the lens. The muscles
change the shape of the lens and alter its focal length.
Our FluidFocus lens, on the other hand, uses electrostatic forces
to alter the shape of a drop of slightly salty water inside a glass
cylinder 3 millimeters in diameter and 2.2 mm long. One
end of the cylinder points toward the image plane;
the other is directed at the subject being imaged
[see diagram, “Shape Shifter”].
The cylinder containing the water drop is
filled with oil. Around the inside walls of
EYE OF THE BEHOLDER: In the human
eye, the cornea provides the main optical
power, and the lens supplies the variable
focus. Muscles attached to the lens change
its curvature to focus on objects at different distances. The closer an object is, the
more rounded the lens must be in order to
project a focused image onto the retina at
the back of the eye.
the cylinder is a water-repellent Teflon-like
coating, and behind this coating is an electrode.
Basically, the water and the oil make up the lens,
and the shape of the interface between the two—the
meniscus—determines its focal length. Changing the voltage on the electrode changes the shape of the interface and alters
the focal length of the lens.
The lens exploits surface-tension characteristics of fluids. The
surface of a column of water in a clean glass cylinder forms a bowlshaped meniscus. Because the molecules in the glass attract water
molecules, the liquid surface curves upward near the clean cylinder wall. If the glass is greasy, the water surface curves downward
near the wall, because grease repels water.
At the center of the meniscus, the water surface is nearly flat
because of gravity. Without gravity the water surface would be spher-
ABOUT THE AUTHORS
BENNO HENDRIKS joined Philips Research Laboratories, in
Eindhoven, the Netherlands, to work on electron optics in
1990. He switched to optical recording six years later and in
2002 joined the FluidFocus project, specializing in the optics
of the liquid lens. He received his Ph.D. in quantum optics at
Utrecht University, the Netherlands, in 1989.
STEIN KUIPER is the project leader of the FluidFocus
effort, which he started in 2000. Earlier that same year, he
obtained his Ph.D. in physics from the University of Twente,
in the Netherlands.
34 IEEE Spectrum | December 2004 | NA
ical—the ideal shape for a focusing lens. In our lens, we cancel the
effect of gravity by keeping the drop small and covering it with oil,
which doesn’t mix with the water. To completely cancel the effect
of gravity, the oil must have the same density as the water, because
only then does gravity attract the oil and the water with equal force.
In our lenses, we use a mixture of special silicone oils (phenylmethylsiloxanes) with that identical density. The result is a waterto-oil interface whose shape will hold with any
orientation of the cylinder but can be changed
by a voltage on the surrounding electrode.
The optical power of the lens
Lens
that forms at the surface beCornea
tween the oil and the water
depends on two things: the
curvature of the meniscus
and the difference between the refractive indices of the oil and
water. The refractive index—the ratio of the
Aqueous fluid
speed of light in a vacuum to its speed in the
medium—characterizes
the amount by which
light bends when it passes
from one medium to another. The curvature of the
meniscus depends on the
diameter of the cylinder and on
how strongly the cylinder wall
repels or attracts the water. That
attraction or repulsion changes with the
voltage on the electrode.
In our lens, the coating on the inside walls of the cylinder repels
water so strongly that the water does not even touch it: there is a
very thin oil layer between the coating and the water. So the water
touches the cylinder only at the flat surface on one end, which has
no water-repellent coating. With no voltage on the electrode, the
meniscus is hemispherical, with the center bulging outward beyond
the ring where the water comes closest to the cylinder. However, a
voltage on the electrode attracts the water and produces a concave
meniscus, forcing the edges beyond the center.
If liquid-filled lenses are such a great idea, why weren’t they perfected long ago? There are three main reasons: the difficulties of
counteracting the effects of gravity, of containing the fluid so its
shape can be precisely controlled, and of then deforming the fluid
in a controlled way.
In the 17th century, the English scientist Stephen Gray built
microscopes using water drops, creating water-drop lenses with a
diameter so small—about 0.3 millimeters—that their curvature was
not strongly influenced by gravity. Gray found that the images these
lenses created were quite good, thanks to the smooth surface of the
drop. He kept the drops from moving around by placing them in
holes drilled in a plate. Different hole diameters led to different drop
curvatures and different magnification factors.
BRYAN CHRISTIE
WITHIN THE NEXT YEAR OR TWO OUR FLUIDFOCUS LENSES MAY BE ENHANCING
THE RESOLUTION OF PICTURES FROM CELLPHONE AND PDA CAMERAS
ALL IN FOCUS: An actual variable-focus camera [bottom, right] built with a
FluidFocus lens [bottom, left] is only 5.5 millimeters high. In the schematic drawing
of the liquid lens [top], a plastic lens at the aperture provides the main optical
power, while the glass lens below it makes the camera’s focal length independent of
wavelength. The camera captures images with a CMOS sensor.
Aperture
Plastic lens
Meniscus
Oil
Glass
lens
Water
Plastic lens
CMOS
sensor
TOP: BRYAN CHRISTIE; BOTTOM: PHILIPS RESEARCH
In 1940, Robert Graham, working at Ohio State University, in
Columbus, attempted to make a humanlike lens by changing the
amount of liquid between two flexible membranes. He was not
successful. The liquid leaked out of the membranes, and worse,
elastic tension in the membranes made it impossible to control
the lens shapes with enough precision to produce undistorted
images. Moreover, the effects of gravity made the lens’s shape
dependent on its orientation. (For the human eye, these problems
are not severe, because the lens is surrounded by fluid. Since the
membrane that surrounds the lens is very thin, distortions due
to elastic tension are small.)
Our approach in designing the prototype lens starts with Gray’s
17th-century idea of keeping the drop centered in a hole. It also borrows from the discoveries of two 20th-century researchers, Aleksandr
Froumkine and Christopher Gorman.
In 1936, Froumkine began experimenting with the use of electric fields to change the shape of a water drop sitting on a metal surface. This phenomenon, in which an electric field pulls the drop
toward the plate, is called electrowetting: the drop wets, or contacts,
the surface better when it is attracted by an electric field.
Using the electrowetting technique, Gorman and his colleagues
at Harvard University, in Cambridge, Mass., made the first variable-focus lens in 1995 by replacing the metal plate with a transparent conducting plate. They believed that these lenses could
have applications in adaptive optics for astronomical telescopes,
a technique that compensates for atmospheric effects by dynamically changing the shape of a mirror [see “Taking the Twinkle
out of Starlight,” IEEE Spectrum, December 2003]. An intrinsic
problem with such lenses is that they are difficult to sabilize
because they do not center themselves about the optical axis—
the line that runs through the center of the aperture.
In 2000, Bruno Berge and Jérôme Peseux at the Université Joseph
Fourier, in St. Martin d’Hères, France, improved on Gorman’s design
by covering the transparent electrode with an insulating film, and
by adding the means for centering the drop. Berge is currently pursuing commercialization of his liquid lens approach at Varioptic S.A.
in Lyon, France, a company founded in 2002.
Our solution to the centering problem combines electrowetting with Gray’s old concept of centering the drop in a cylindrical hole—in our case, the glass cylinder—and placing the electrode that creates the electric field around the inside of the cylinder,
instead of on the ground plate.
ONE IMPORTANT ADVANTAGE of our liquid lens is that it can be very
small. In fact, as Gray showed us, smallness is inherently advantageous, because it minimizes the effects of gravitational pull on the
liquid. In addition, miniaturization makes liquid lenses more powerful, because the electrostatic forces between the liquid and the
inner surface of the cylinder become stronger as the lens size shrinks.
This property makes small electrowetting lenses very fast. Our
prototype can refocus in 10 milliseconds, much faster than the human
eye, which can refocus in about 200 ms. Scaled to the size of a human
eye lens, the refocusing time would increase to 50 ms, which would
still be four times faster than that of the eye.
So how good are these lenses? The optical power of a lens is specified in diopters, a measure of how much the lens can bend light. The
dioptric value of a lens is proportional to the inverse of the radius
of curvature of the lens in meters. The closer objects are to a lens,
the more the lens must bend the light to bring them into focus. So
when an object is far away, a lens needs less optical power to bring
it into focus than it does when the object is near. Our liquid lens
changes its focus by changing its optical power through the change
of the water drop’s radius of curvature with voltage on the electrode.
The strength of eyeglasses is also expressed in diopters. So, for
example, eyeglasses of +2 increase the optical power of the eye by
2 diopters, allowing the wearer to see things that are close.
The optical power of our lens, with its inner cylinder diameter of
3 mm, can vary over a range of 150 diopters. This is accomplished
by changing the meniscus between hemispherical (its radius equal
to half the diameter of the cylinder) and concave (its radius approximately equal to the diameter of the cylinder). If it were the same
size as a human lens, its optical power range would be about
50 diopters—12 times as large as the optical power of the human eye,
which has a range of about 4 diopters.
The main optical elements of the human eye are the cornea,
the iris, the lens, and the retina. The cornea is the transparent dome
that covers the front of the eye and serves as its outer window. Next
comes the iris, the colored part of the eye, which forms the variable
aperture of the system—that is, the pupil, which opens or closes
to admit more or less light. Behind the iris is the deformable lens,
the part of the eye that is analogous to our liquid lens, which focuses
the light on the curved retina at the back of the eyeball.
December 2004 | IEEE Spectrum | NA 35
36 IEEE Spectrum | December 2004 | NA
STEIN KUIPER
In the human eye, the
main optical power of the
lens system comes from the
cornea and is about 40 diopters [see diagram, “Eye of the
Beholder”]. The typical
deformable human lens is
about 9 mm wide and 4 mm
thick. The lens has an optical power of between +20
and +24 diopters to bring
into focus objects that are
FROM HERE TO INFINITY: Two photos made with the authors’ liquid-lens camera show how the focal length can change to
closer or farther away.
bring each of two objects into focus. The genie is 50 centimeters from the camera, and the ladybug is 5 cm away.
The human lens consists
of several fine layers of
transparent tissue with different indices of refraction ranging in the other to refocus the image. In principle, changing the magnifithe various layers from 1.406 in the center to about 1.386 in the cation by moving the lens throws the image out of focus.
outer layer. This gradient in the refractive index makes the focal Conventional cameras keep the image in focus through a system
length independent of wavelength—that is, it bends all colors of of rods that connect the separate lenses.
light equally. It is an important property for creating sharp images.
We are presently designing a zoom lens system that uses two
liquid lenses in series. This lens will work by changing the shapes,
TO DEMONSTRATE THE ADVANTAGES of our liquid lens, we built and therefore the optical power, of the two lenses, rather than by
a digital camera just 5.5 mm high and 4 mm across [see diagram, changing their locations. Compared with conventional zoom lenses,
“All in Focus”]. At the back of the camera is a CMOS imager with liquid lenses will have two advantages: no moving parts and a very
a 640-by-480-pixel sensor array. Directly in front of the CMOS small size at, potentially, a very low cost.
imager is a plastic lens, which allows the image to be projected
It is even possible to apply the liquid lens in high-quality optisharply onto the flat CMOS image sensor. The eye does not need cal recording systems like DVD recorders, because its resolution can
such a lens because the image sensor in the eye (the retina) is curved. be controlled so it is not limited by lens imperfections but only by
In front of this plastic lens is the liquid lens in its cylindrical diffraction, which restricts the resolution of all lens systems.
glass housing, with the cylinder’s outer diameter measuring 4 mm
The lenses have other intriguing possibilities, too. Replacing the
and its inner diameter 3 mm. The oil side of the liquid lens is close electrode that encircles the inner wall of the glass cylinder with multo the imager. A glass plate seals the liquid lens on the side near the tiple vertical electrodes and adjusting their voltages separately allows
imager, and a truncated glass sphere mounted on a flexible mem- tilting of the interface between the liquids, offering the ability to
brane seals it on the opposite side.
image in directions that are at an angle to the lens axis. A lens that
The truncated sphere allows the focal length of the camera to be can be tilted and focused could let engineers design video cameras
independent of wavelength—as with the human eye. This property and binoculars that would compensate precisely for hand moveis important because it focuses all the wavelengths that make up the ment and other undesired motions.
image at the same point, leading to a sharp image. The membrane
We expect that within the next year or two our FluidFocus lenses
allows the volume of the liquids to expand or contract depending on will be enhancing the resolution of pictures taken with cellphone
the temperature. In front of the truncated glass sphere is another and PDA cameras.
plastic lens, which, like the cornea of the eye, provides the main optiBecause the liquid lens is based on materials that are, at least
cal power. In front of this plastic lens is the fixed aperture.
in theory, biocompatible, and because refocusing the lens requires
By changing the voltage on the electrode of the liquid lens, we very little energy, we can envision (pardon the expression!) future
were able to focus on objects at distances anywhere from 2 cen- applications to replace a malfunctioning human eye lens. With a
timeters up to infinity. To do so, we varied the focal length from zooming feature, we might even far surpass it. Imagine being able
2.85 mm to 3.55 mm [see photos, “From Here to Infinity”].
to read a car license plate from a kilometer away or the menu in a
In contrast to the human eye, which is embedded in a restaurant window without getting out of your car.
temperature-controlled system, our lens must operate over a range
We have plenty of work to do before that can happen. But to
of temperatures. For portable applications, the lens must work make this science-fiction dream a reality, what we’ll need most
■
between –30 °C and +60 °C and survive temperatures between of all is vision.
–40 °C and +85 °C. Because such a wide range requires special liquids, we added large amounts of salt or antifreeze to the water in
TO PROBE FURTHER
our prototype camera lens to lower the freezing point sufficiently
Bruno Berge and Jérôme Peseux reported on their liquid
without adversely affecting the image quality.
lens work in The European Physical Journal E 3 (2000), p.
There is, however, one property for which our lens probably can’t
159.
beat the human body, and that is lifetime. No autofocus camera that
Technical details of the FluidFocus lens are published in
we know of can operate all day long, every day, for 80 years or more.
S. Kuiper and B.H.W. Hendriks, Applied Physics Letters 85
We have, however, varied the focus of our liquid lens—from one
(2004), p. 1128.
end of its range to the other—more than a million times without
A clear explanation of how human eyes and camera
any decrease in performance.
lenses work is available at http://www.umiacs.umd.edu/
We are now working on a FluidFocus zoom lens. Optical zoom~ramani/cmsc828d/lecture3.pdf.
ing requires at least two lenses—one to change the magnification,