Improved image quality from your camera phone
The demand for consumer electronics to get smaller, lighter and cheaper, is a stimulus for great ingenuity. Cell phones are a classic example where electrical engineers and designers are constantly working to put a whole lot more into ever smaller spaces. And now that cameras are almost as standard a feature in these devices as the ring tone, optical engineers must also devise increasingly clever ways to shrink the optics while improving their performance.
One of the difficulties of miniaturising cameras is that the moving parts needed to focus an image do not easily scale down. Optical engineers have therefore looked to nature and tried to emulate the way in which the eyes of animals perform this function by deforming the shape of the lens.
Various techniques exist for doing this with lenses made from liquids or polymers, which are deformed either by applying pressure or using an electric field to distort the shape of a liquid layer. Now engineers at the National University of Singapore have produced a hybrid lens which exhibits excellent optical quality over a wide range of focal lengths.
The hybrid lens system is comprised of water contained within a 5 mm diameter transparent polymer cell, the front surface of which is very thin and forms a flexible membrane. The back surface is fabricated with minute concentric ring features, which form a Fresnel lens.
The diagram below shows how a collimated beam of light, such as one emitted by a laser, is focused by the lens. The water-filled cell has a curved front to it and focuses light at a distance f. That value can be varied by changing the curvature of the membrane, which depends on the pressure of the fluid inside. This is accomplished by regulating the intake and outlet of fluid in the cell using valves.
The concept of tunable focusing with a single-fluid lens is not a new one, but chromatic aberration - an effect whereby different wavelengths of light are focused at different distances - restricts the focal range over which they can be tuned. The practical consequence of this is that a colour image can only be brought into sharp focus within a narrow range of values of f. By introducing the Fresnel lens, however, these researchers have improved on the quality of the conventional, single-fluid lens.
The Fresnel lens was designed to correct for the chromatic aberrations of the fluid lens. To assess the performance of their new lens, they compared two lens systems fabricated in almost exactly the same way: the hybrid lens which includes the Fresnel lens, and a second water-filled cell without the Fresnel lens. Chromatic aberrations were far smaller for the hybrid lens over a focal range from 10 to 20 mm.
The paper's authors suggest that such a lens is no more complex to make than a conventional, single-fluid lens, because of the novel fabrication process they used, involving diamond turning and lithography. It therefore promises all the benefits of being small, light weight and tunable, but with better quality and no increase in cost to manufacture. If you worry that phones are getting so small you'll never be able to find them, rest assured that they'll probably still be able to see you.
Reference: Zhou, G., Leung, H., Yu, H., Kumar, A., & Chau, F. (2009). Liquid tunable diffractive/refractive hybrid lens Optics Letters, 34 (18) DOI: 10.1364/OL.34.002793