Presbyopia, short-sightedness brought on by aging, is very common and applies to most of the population of age over 45 years. When working optimally, the human eye can focus on objects ranging from around 25 cm away to infinity. This is accomplished by the ciliary muscles changing the shape of the lens inside the eye to maintain the focus of the image onto the retina for objects at different distances. During aging the ciliary muscles suffer from a reduced ability to deform the lens and the eye progressively loses the ability to focus on closer objects.
Presbyopia has traditionally been addressed by using additional lenses in front of the eye, such as a pair of ‘reading glasses’ which are used when looking at close objects, or additional lenses integrated into an existing pair of glasses, such as ‘bifocals’ or ‘varifocals’. These additional lenses can be used by glancing down.
More recently, liquid-crystal technology, allowing for a controllable change in refractive index by applying an electric field, has been introduced for glasses and contact lenses (see, e.g., “Liquid crystal lens with large focal length tunability and low operating voltage”, by H. Ren, D. W. Fox, B. Wu, and S. T. Wu, Optics Express, Vol. 15, Issue 18, pp. 11328-35, OSA Publishing, 2007; “Electronic liquid crystal contact lenses for the correction of presbyopia”, by H. E. Milton, P. B. Morgan, J. H. Clamp, and H. F. Gleeson, Optics Express, Vol. 22, Issue 7, pp. 8035-8040, OSA Publishing, 2014). This allows a controllable change in the optical properties of a lens by application of a voltage or current. Such lenses may be controlled either by a manual switch or by utilizing an orientation sensor detecting when a person wearing the lenses is moving her head, as is described in U.S. Pat. No. 6,517,203 B1.
In U.S. Pat. No. 7,656,509 B2 devices for determining a distance of an object a user of an electro-active lens is looking at are presented. The optical power of the electro-active lens may be altered based on the determined distance to ensure that the object is correctly focused, without requiring the user to operate a switch, glance down, or dip her head.