Pushing for Change in Magnifying Lenses

Michael J. Lander

As scientists have realized for many years, the fluid nature and natural magnification of water could make it perfect for use in liquid lenses with an adjustable focal length. Yet prototypes developed using water and other liquids have enjoyed few practical uses because of a limited aperture size, lack of portability and other problems. At the University of Central Florida in Orlando, researchers have made a lens that may overcome these issues and that could find use in cell phones and other devices.

Using a liquid lens based on pressure-induced liquid redistribution, researchers magnified an image with a high degree of resolution. Courtesy of Hongwen Ren and Shin-Tson Wu.

Hongwen Ren and Shin-Tson Wu constructed the lens assembly using two round transparent glass plates glued together with a 1-mm gap between them. Before gluing, they bored a 5-mm hole into the bottom plate and sealed a plastic membrane over its inside surface. This would serve as the lens hole. In the upper plate, they created another hole offset from the bottom one, covering it on its top side with an elastic membrane. The researchers filled the space between the plates with water and sealed the sections together.

While illuminating the assembly from above with a Newport HeNe laser, the scientists gently exerted pressure on the elastic membrane with a ball-headed mechanical lever. This caused the plastic membrane to bulge outward and form a lens. To evaluate its effectiveness, they generated intensity profiles of light passing through it and, using a CCD camera from Santa Barbara Instrument Group, took images of objects placed underneath it. Response time was calculated from transmission measurements taken with a digital oscilloscope.

In the nonfocusing state, intensity profiles revealed a beam diameter of just over 5 mm. Upon deformation to a focusing condition, the diameter decreased significantly to approximately 200 μm. According to the images, the lens had a resolution of more than 25 line pairs per millimeter. Effective focal length ranged from infinity to 10 cm — close enough for most picture-taking applications. Transformation to or from the focused state took about 40 ms, a time that could allow high data-acquisition rates.

“The design is particularly attractive for cell phones and digital cameras as an optical zoom lens,” Wu said, especially because it is thinner than previous liquid lenses with a peripheral elastic membrane. Existing systems that rely on electrowetting also require a relatively high voltage to function. To create an array of lenses, multiple membrane-covered holes could simply be made in the lower plate.

The researchers face several challenges, however, because they have yet to develop compact and efficient mechanisms for membrane depression. Currently, they are exploring the use of artificial muscles for this application. Lenses composed of silicon oil or liquid monomer could permit a larger zooming ability than the water lens, which experiences deformation at short focal lengths. Working with an industrial partner, the investigators hope to have a cell phone camera lens ready for testing in the future.

Optics Express, May 14, 2007, pp. 5931-5936.