The industrial fixed-mount ID readers from Cognex Corp., Natick, Mass., use a liquid lens to focus the device’s sensor on a bar code or other printed ID. The lenses use no moving parts, so they are rugged, compact, require little power, and can change focal length quickly.
The lenses use two immiscible fluids, oil and water, each with a different refractive index, to create variable-focus lenses as small as 10 μm. The two liquids are placed in a roughly trapezoidally shaped container (see illustration). Water is electrically conductive and the oil used is not. So applying a voltage across the two liquids makes the water more attracted to the surface of the container, a process called electrowetting. As the water adheres to the walls of the container, the oil is squeezed into a round droplet. The curved shaped of the droplet lets it bend incoming light and, thus, serve as a lens. And as the voltage increases, the oil droplet becomes thicker as the attraction between the water and wall becomes stronger and water moves farther down the wall, changing the shape of the droplet and its focal length.