Introduction



Introduction

Nowadays cameras are one of the hottest selling items in all of consumer electronics. But as anyone who has ever seen them can attest, the images that come out of these camera phones leave plenty to be desired. Part of the problem is their CMOS imaging chips, which typically have a censor array of only about 300 kilo pixels-a quarter or less of the number in a low-end digital camera. But the major problem is their tiny, fixed focus lenses. These fixed focus lenses are very small but they have poor light gathering power and resolving power.

Conventional auto focus systems used in high quality digital cameras use motors and gears to shift the position of the lenses. They have high quality, but are difficult to miniaturize because of the gears and motors.

‘FLUID FOCUS LENSES’ can combine both these qualities. It is a special type of lens developed by Philips Research Laboratories. It uses the principle of a human eye. Like the lens of a human eye it focuses on objects at different distances by varying the shape of the lens rather than by varying the relative positions of multiple lenses. It uses electrostatic force to alter the shape of a drop of slightly salty water inside a cylinder 3 millimeters and 2.2 mm long. So it can be made to be very small and the images taken by using these lenses will be having very high quality.

These superior capabilities of ‘FLUID FOCUS LENSES’ should make them ideal not only in camera phones but also in products whose design constraints demand a tiny but capable optical systems.

Camera

A camera is a device that captures an image on a film for an optical camera, or a CCD(charged coupled device) for a digital camera. A simple camera consisting of a lens, a shutter, a media holder, and a viewfinder. The main part of a camera is lens. A lens is an optical device that focuses light rays. In cameras, the lens is the device on the front face (or in a tube extending from the front face) that gathers the incoming light and concentrates it so that it can be directed toward the film (in an optical camera) or the imaging device (in a digital camera).

The term focus means to move the lens or film/image sensor in order to record a sharp image.Then the term focal length describes the distance from the surface of the lens to the focal point or center point at which light rays converge; the focal length determines the length of the lens.

Image Formation by A Lens

Image formation by a lens depends upon the wave property called refraction. Refraction may be defined as the bending of waves when they enter a medium where their speed is different. Since the speed of light is slower in a glass lens than in air, a light ray will be bent upon entering and upon exiting a lens in a way that depends upon the shape and curvature of the lens. In the case of a converging lens such as the double convex lens shown below, parallel rays will be brought together at a point.

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The distance from the lens to this principal focus point is called the focal length of the lens and will be designated by the symbol f. A converging lens may be used to project an image of a lighted object. For example, the converging lens in a slide projector is used to project an image of a photographic slide on a screen, and the converging lens in the eye of the viewer in turn projects an image of the screen on the retina in the back of the eye.

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There is a geometrical relationship between the focal length of a lens (f), the distance from the lens to the bright object (o) and the distance from the lens to the projected image (i). The relationship between the distances illustrated in Figure 2 can be expressed as :

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This relationship will be used to determine the focal length of a glass lens, and will be used as the basis for a qualitative investigation of image formation by the eye with the use of a large eye model. The calculation of focal length of a lens is described below.

1. Position the lens and white screen on the optical bench and place them so that the distance from the lighted "object" to the lens can be measured on the bench scale. Adjust the screen to get a clear image.

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Determine the object distance and image distance, o and i, and calculate the focal length from the lens relationship. Describe the appearance of the image, compared to the object (e.g, larger, smaller, erect, inverted). Adjust the object distance to a different value and repeat the process with a different set of measurements.

|Object distance |Image distance |Focal length |Description of image |

|(1) | | | |

|(2) | | | |

2. What is the average of your focal length measurements, expressed in meters?

3. The lens strength in diopters is defined as S = 1/f(in meters). The unit is 1/m but this unit is commonly called a "diopter".

Then these lenses can be classified into two:

1. Fixed focus lens

2. variable focus lens

A fixed focus lens is a lens in which the focus is preset and is not adjustable.

But the focal length of a variable focus lens can be changed for the need of zooming.

Nowadays these fixed focus lenses are widely used in camera phones, pocket size conventional digital cameras, webcams ,hidden security cameras,DVD recorders.and endoscopes.But the images that come out of these equipments 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. Also these have poor light-gathering and resolving power. These 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.

Fluid focus lens is a solution for this. This functions like our eye. It varies its focus by changing shape rather than by changing the relative positions of multiple lenses, as high-quality camera lenses do. Fluid Focus lens can be made nearly as small as a fixed-focus lens.

Fluid focus lens delivered sharpness that is easily on a par with that of variable-focus lenses.

Thus to study about fluid focus lens we have to analyze the functioning of human eye.

Eye Anatomy

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When you look at an object, light rays are reflected from the object to the cornea, which is where the miracle begins.  The light rays are bent, refracted and focused by the cornea, lens, and vitreous. The lens' job is to make sure the rays come to a sharp focus on the retina. The resulting image on the retina is upside-down.   Here at the retina, the light rays are converted to electrical impulses which are then transmitted through the optic nerve, to the brain, where the image is translated and perceived in an upright position!

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Think of the eye as a camera.  A camera needs a lens and a film to produce an image.  In the same way, the eyeball needs a lens (cornea, crystalline lens, vitreous) to refract, or focus the light and a film (retina) on which to focus the rays.  If any one or more of these components is not functioning correctly, the result is a poor picture.  The retina represents the film in our camera.  It captures the image and sends it to the brain to be developed.

Theory Behind Fluid Focus Lens

A fluid focus lens varies its focus by changing its shape. 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.

Fluid focus 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.

The lens exploits surface-tension characteristics of fluids. The surface of a column of water in a clean glass cylinder forms a bowl-shaped 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.

Fluid focus lens uses the phenomenon called electrowetting .In electrowetting electric fields are used change the shape of a water drop sitting on a metal surface. The drop wets, or contacts, the surface better when it is attracted by an electric field.

Electrowetting

With electrowetting a voltage is used to modify the wetting properties of a solid material. An example of such increased wettability is illustrated in the photographs of figure . The left hand side shows a water droplet on a hydrophobic surface. The water droplet does not like to be in contact with the surface and therefore minimizes the contact area.

In the photograph on the right hand side, a voltage difference is applied between the electrode in the water and a sub-surface electrode present underneath the hydrophobic insulator material. As a result of the voltage, the droplet spreads, i.e. the wettability of the surface increases strongly.

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Figure – water droplets on hydrophobic surface without and with voltage

applied.

When the voltage is removed, the droplet returns to the original state indicated on the left hand side.

Structure Of A Fluid Focus Lens

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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 figure]. 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.

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A voltage on the electrode attracts water molecules toward the cylinder's surface, making it act less repellent, and the water surface becomes concave. Here, parallel light rays passing through the meniscus converge at a focal point.

Working Of A Fluid Focus Lens

The cylinder containing the water drop is filled with oil. Around the inside walls of 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.

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The lens exploits surface-tension characteristics of fluids. The surface of a column of water in a clean glass cylinder forms a bowl-shaped 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 spherical—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 water-to-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 that forms at the surface between 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 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.

Advantages Of A Fluid Focus Lens

One Important Advantage of liquid lens is that it can be very small. 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. Fluid focus lens 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.

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An actual variable-focus camera [bottom, right] built with a FluidFocus lens [bottom, left] is only 5.5 millimeters high.

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.

To demonstrate the advantages of liquid lens, we can consider a digital camera just 5.5 mm high and 4 mm across. At the back of the camera is a CMOS imager with a 640-by-480-pixel sensor array. Directly in front of the CMOS imager is a plastic lens, which allows the image to be projected sharply onto the flat CMOS image sensor. The eye does not need such a lens because the image sensor in the eye (the retina) is curved.

In front of this plastic lens is the liquid lens in its cylindrical glass housing, with the cylinder's outer diameter measuring 4 mm and its inner diameter 3 mm. The oil side of the liquid lens is close to the imager. A glass plate seals the liquid lens on the side near the imager, and a truncated glass sphere mounted on a flexible membrane seals it on the opposite side.

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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.

The truncated sphere allows the focal length of the camera to be independent of wavelength—as with the human eye. This property is important because it focuses all the wavelengths that make up the image at the same point, leading to a sharp image. The membrane allows the volume of the liquids to expand or contract depending on the temperature. In front of the truncated glass sphere is another plastic lens, which, like the cornea of the eye, provides the main optical power. In front of this plastic lens is the fixed aperture.

By changing the voltage on the electrode of the liquid lens, we were able to focus on objects at distances anywhere from 2 centimeters up to infinity. To do so, we varied the focal length from 2.85 mm to 3.55 mm .

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Two photos made with the liquid-lens camera show how the focal length can change to bring each of two objects into focus. The genie is 50 centimeters from the camera, and the ladybug is 5 cm away.

In contrast to the human eye, which is embedded in a temperature-controlled system, our lens must operate over a range of temperatures. For portable applications, the lens must work between -30 degrees C and +60 degrees C and survive temperatures between -40 degrees C and +85 degrees C. Because such a wide range requires special liquids, we added large amounts of salt or antifreeze to the water in our prototype camera lens to lower the freezing point sufficiently without adversely affecting the image quality.

There is, however, one property for which this lens probably can't beat the human body, and that is lifetime. But we can vary the focus of this liquid lens—from one end of its range to the other—more than a million times without any decrease in performance.

The lenses have other intriguing possibilities, too. Replacing the electrode that encircles the inner wall of the glass cylinder with multiple vertical electrodes and adjusting their voltages separately allows tilting of the interface between the liquids, offering the ability to image in directions that are at an angle to the lens axis. A lens that can be tilted and focused could let engineers design video cameras and binoculars that would compensate precisely for hand movement and other undesired motions.

Conclusion

It is sure that, the liquid lenses will overcome the problems associated with today’s camera phones. It is even possible to apply the liquid lens in high-quality optical recording systems like DVD recorders, because its resolution can be controlled so it is not limited by lens imperfections but only by diffraction, which restricts the resolution of all lens systems.

So we can expect that within the next year or two these Fluid Focus lenses will be enhancing the resolution of pictures taken with cell phone and PDA cameras.

Because the liquid lens is based on materials that are, at least in theory, biocompatible, and because refocusing the lens requires very little energy, we can envision future applications to replace a malfunctioning human eye lens. With a zooming feature, we might even far surpass it.

References

➢ IEEE SPECTRUM December 2004/volume 41/number 12/International edition

➢ The European Physical Journal E 3 (2000), p.159

➢ Applied physics Letters85 (2004), p. 1128

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