Our eyes incorporate a flexible lens (the crystalline lens) - it allows us to change our focus to far or near objects (accommodation). The lens is surrounded by a ring muscle (the ciliary muscle), which relaxes to allow the lens to flatten or contracts to cause the lens to bulge, changing its refractive power.

Problems

Unfortunately, there are a number of conditions that the eye can suffer from. Most people will suffer as their lives go on from presbyopia - a loss of ability to change the power of the lens due to hardening of the lens tissues, leading to a lack of ability to focus on close objects, such as when reading.

Other common conditions that affect large numbers of people are:

• Myopia: (short-sightedness) Rays of light from a distant object are focussed in front of the retina. Myopia occurs either because the eye is too long or the refractive elements (the cornea and crystalline lens) too powerful to produce a clear image at the retina.
• Hyperopia: (long-sightedness) Rays of light from a distant object are focussed behind the retina, either because the eye is too short or because the refractive elements of the eye are not powerful enough to bring an image into focus at the retina.
• Astigmatism: Astigmatism occurs when the curvature of any given refractive element of the eye differs across its surface (i.e. the lens in the eye is shaped more like a rugby ball than a football). This lack in uniformity across the surfaces of the eye's refracting elements prevents it from producing a sharply focused image at the retina. Astigmatism usually produces an image that is more defocused in one direction than another.

Fluid-filled lenses use the incompressibility of liquids to change the physical shape of the lens. The lens is constructed with two flexible membranes on the optical surfaces, held in place by a solid surround.

The power of the lens is changed by pumping fluid into or out of the central lens reservoir - less fluid causes the flexible membranes to bow inwards due to the reduced internal pressure, and more fluid causes the membranes to expand outwards, as shown in the diagram. This causes the lens to change refractive power, which makes it suitable for adjustable eyeglasses.

Advantages

• Simple actuation mechanism - pumping fluid from external reservoirs (e.g. a syringe)
• Suitable for fitting into spectacle frames
• Can be made to different sizes relatively easily
• Tough and durable if the membranes are placed behind plastic covers

• Current technology only allows the lenses to be circular
• Astigmatism of the eye cannot presently be corrected

Electrowetting lenses rely on the surface tension of liquids to produce their curvature. The walls of the electrodes in the diagram repel the conducting fluid - when the electric field is switched on the interfacial tension between the conducting fluid and the electrodes is changed, leading to a change in surface contact angle where they meet and thus a change in curvature of the overall lens system.

Advantages

• Very large range of lens powers available - often in the range of +/- 50-100 dioptres
• High speed alteration of lens power

• Difficult to manufacture in large sizes - limited to the range over which surface tension provides curvature and are thus unsuitable for glasses

Applications

Electrowetting lenses are currently being developed for small-size lens applications, especially those in camera phones, small consumer devices and other OEM applications.

Electroactive lenses rely on the principle that certain liquid crystals change their refractive index (and thus its refractive power) when an electric field is applied across them. By applying different types of electric field, different electroactive lens technologies can be used.

The top half of the above diagram illustrates one type of electroactive lens - in this case an internal element with (when deactivated) an identical refractive index to the surrounding material. When switched on, a fixed refraction is added to the overall lens, changing the overall power of the lens.

The bottom half of the diagram illustrates an alternative and more complex method - a varying electric field is applied across a liquid crystal element, causing the refractive index to vary across the surface. This is a 'graded index' lens, which bends light differentially depending on the refractive index that the light interacts with, focusing incoming beams to a point.

Advantages

• Very high speed power change - almost instantaneous
• Could be made thin and large enough to place into glasses
• No external reservoirs of liquid needed

• Currently difficult and expensive to manufacture - no commercial products are currently based on this technology

Alvarez lens systems are two lenses with special surface shapes - a cubic polynomial function. When moved relative to one another as shown in the diagram, the overall thickness of the combined lenses changes to produce different amounts of refraction and lens power.

Advantages

• Can be made cheaply and relatively easily
• Simple actuation mechanism
• Any shape possible - there are no restrictions on lenses being round

• Limited power range possible whilst keeping the lens thin - less than that of fluid-filled lenses
• Optical quality currently perhaps not as high as other lens technologies

Applications

Alvarez lenses have been used in various applications over the years - most famously as the lens for the Polaroid Spectra series of cameras.