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Soft focusing

作者:仇隹    发布时间:2019-03-08 05:20:01    

By Duncan Graham-Rowe LIQUID lenses that can change their shape —and hence their magnification—at the flick of a switch have been developed by researchers in France. The lenses could replace slow and bulky focusing mechanisms in a wide range of imaging devices such as endoscopes. Developed by Bruno Berge, a physicist at Joseph Fourier University in Grenoble, the lenses are marvels of simplicity. They consist of a tiny drop of silicone oil suspended between two small volumes of salty water in a cylindrical container (see Diagram). The oil drop sits on a transparent polymer film and is kept in the centre by a hydrophilic coating around the edge of the film. The shape of the drop can be accurately changed in a few milliseconds by varying the voltage applied to the volumes of salty water on either side of the oil drop. The whole liquid lens arrangement acts like a capacitor, a device that stores charge in electronic circuits. In a normal capacitor, two metal electrodes are separated by an insulator. In the liquid lens, the two layers of water act as the electrodes, and the oil and polymer together form the insulator. When a voltage is applied, charge builds up on the interfaces between the water and the insulator, and the resulting electrostatic repulsion between like charges on the surface of the insulator makes the oil drop bulge outwards. By varying the voltage, the degree of bulging—and thus the focal length of the entire lens—can be precisely controlled. The effect works with other types of liquid besides silicone oil and water, provided the drop is made from an insulator and the other liquids are conductors. But to make a powerful lens, there must be a large difference between the refractive properties of the two liquids. The droplet also has to be dense enough to hold its shape when the lens is moved about, says Berge. Even with silicone oil, which is relatively dense, too much vibration can cause the two liquids to mix, forming a mayonnaise-like suspension. Berge, now at Ecole Normale Supérieure in Lyon, is trying to scale the lens up: at present, the larger the drop, the more difficult it is to contain. He is also trying to make the lenses vibration-proof. If this were possible, the device could be used to create a bionic eye. A neural implant could change the voltage and allow the wearer to focus on near and far objects at will. At present, the most common form of variable focus lens uses mirrors, says Chris Dainty, an expert in adaptive optics at Imperial College, London. Dainty says that while other researchers are looking into the use of liquid crystals to guide light in a variable focus lens, Berge’s liquid lenses are likely to be much cheaper. “I think this could certainly be useful. But we would want to know what the optical qualities of the lens are,

 

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