Spectacle-wearers will soon be able to say goodbye to the humble bifocal lens, trusted friend of ageing eyes. Scientists have developed new flat lenses that can change their focusing power at the flick of a switch, allowing the wearer to change their focus from the horizon to something a few inches away in a split second.
The lenses are made from liquid crystal - a type of material behind modern computer displays which changes the alignment of its molecules when an electrical current is applied, changing properties such as transparency or focal power.
The inventors say a small battery set in the frame of glasses could be used to power the lenses and predict they could eventually replace standard bifocal lenses.
As eyes age, their outer layers lose flexibility, leading to an inability to shift focus from distant to near objects - a condition called presbyopia. Bifocal lenses, where the top and bottom of the spectacles have different focal lengths to help eyes deal with differing distances, are useful for presbyopes but the wearer has to shift their gaze between the different parts of the lens for different tasks.
"The field of view is limited in such eyeglasses, requiring the user to gaze down to accomplish near-vision tasks and in some cases causing dizziness and discomfort," said Guoqiang Li and Nasser Peyghambarian, of the University of Arizona, in a paper published yesterday in the Proceedings of the National Academy of Sciences.
Their solution is a lens that consists of transparent concentric rings which control the focusing power of a thin layer of liquid crystal sandwiched between a couple of layers of glass.
When an electrical current is passed through them, the concentric rings, which are electrodes operating at low voltage, take less than a second to change the focal power of the liquid crystals. Dr Li's technology allows the whole area of the lens to be used to focus at different distances. If electrical power is suddenly lost, the lens becomes as transparent as everyday glass. The researchers said that this makes the new lens safe for use while driving.
"These flat lenses are highly promising to replace conventional area division refractive, multifocal spectacle lenses used by presbyopes," said the researchers.
They added that the technology could revolutionise spectacle lenses when combined with some sort of automatic control of the electrical current, and therefore the focusing power of the liquid crystals, in the lenses.
The liquid crystal lenses were tested by a model of the human eye (a CCD camera with a filter attached that matched human light response). The researchers simulated reading by placing an image 30cm in front of the lens. The model eye could not focus on the image but, when the lens was switched on, the image was brought into sharp focus. Initial results with people wearing the lens attached to real spectacles were also reported as successful.