A liquid crystal eye for detail

Lenses can be made to change their focus without use of moving parts by electrically addressing arrays of liquid crystals. Unlike conventional lenses, they can be made to have different focal lengths in different areas to allow zooming in on part of an image or correction of images blurred by smoke or distance. UK Police and organisations concerned with surveillance are queuing up to be first to receive commercial products, now that the idea is coming out of the laboratory and transferring to industry. It is also possible that those of us who have problems with focussing their eyes at different distances could soon be wearing them. The original idea was invented by Dr Alexander Naumov and his team at the P.N.Lebedev Physics Institute in Samara, Russia, and was patented by them in 1999. The present development is in the hands of Dr Gordon Love, in the Department of Physics in the University of Durham and Dr Gleb Vdovin, one of the original inventors, now based in the Department of Electronic Instrumentation at the Technical University of Delft and proprietor of OKO Technologies. Liquid crystal displays include both resistive and capacitative elements. The resistive elements arise from the transparent electrodes use to excite them, and the capacitative elements are a consequence of the liquid crystals. If the display is driven by an alternating voltage applied to a circular driving electrode, the end result is a voltage that varies from the maximum at the outer edge to a minimum at the centre. By varying the applied voltage, amplitude and frequency, it is possible to change the focal length of the lens. Typical driving voltages are a few volts, and typical driving frequencies, 1kHz to 9kHz. Because voltage can be applied by a patterned array of electrodes, it is equally possible to manufacture a lens which has different focal lengths in different areas and which could be used to zoom in on part of an image, or greatly sharpen images blurred by atmospheric effects. While the manufacturing costs of such devices is trivial, the complexity of the design problem is considerable. Seeing an obvious plethora of commercial applications, the idea is now being actively researched by quite a number of people and institutions, both academic and commercial. Dr Love's department is currently engaged in three projects. One is a Particle Physics and Astronomy Research Council project with CRL-Opto on constructing liquid crystal lenses. Another is a Smart Optics Faraday Partnership project on using variable focus lenses in optical systems for space based images and industrial applications. A third is a National Institutes of Health funded collaboration with the University of California Berkeley on using Liquid crystal lenses in 3D vision systems. Lenses for astronomical applications have been made up to 100mm across, but these have very low optical powers. More typical lenses have a diameter of 5mm and a focal length of 500mm to infinity, but much larger areas, can, of course, be made up of arrays of small lenses, each of which can be driven separately. Higher optical powers can be achieved by stacking lenses on top of each other, but there is then some loss in optical transmittance, although it is now said to be possible to achieve better than 90% transmittance per lens. There is also the added complication that liquid crystals cause polarisation, so it is important to ensure that each layer does not interact with the others in such a way as to cause cross polarisation. Design problems may soon be eased with the news that the Zemax Development Corporation in San Diego is developing a suite of suitable design software. Dr Steve Welch, Faraday Partnership Technology Translator and member of the Mullard Space Science Laboratory at University College London gave Eureka some indication of the very wide range of applications for the technology, when encountered at a just held conference. He explained that potential applications ranged from micro cameras for use in space to medical endoscopes and surveillance. Readers may have noticed that the UK boasts more CCTV systems in both public and private places than any other country in the world. As well as applications in private security systems, very strong interest has been shown by both the Defence Science and Technology Laboratory at Fort Halstead, and the Police Scientific Development Branch. The latter would particularly like to be able to zoom in and read car number plates and capture the faces of vandals in town centres. Dr Welch told Eureka that his role was to qualify the technology for space use, enabling satellites to use the technology for adaptive optics to produce sharper pictures, in a much smaller envelope than is required by systems using deformable mirrors. In some situations, single layer devices will probably be quite adequate. However, in most applications, especially in industrial vision systems, and the next generation of digital cameras, Dr Welch was of the opinion that it would probably be best to combine variable liquid crystal lenses with fixed focus conventional lenses. One of the most interesting potential applications is to correct vision. Dr Vdovin, and Drs Mikhail Loktev and Alexander Naumov have published a paper in which they consider the technical possibility of using the technology in an adaptive contact lens or an adaptive eye lens implant based. They report that their first 5mm demonstrator can be controlled via a wireless capacitative link. It is also possible that similar technology might be used to enhance ordinary spectacles, allowing them to adjust focal length to range detected by other means, and even work in conjunction with images perceived externally on the eye retina. It may be possible to sharpen up images in a similar manner to that presently used in large telescopes, but at a tiny fraction of the cost. Whatever the outcome, the fact that manufacturing of liquid crystal arrays is in the digital watch display range of costs, means that we can expect the technology to quickly become equally ubiquitous. Hopefully, at least some of the profits will be retained within the UK. Dr Gordon Love's home page Dr Gleb Vdovin's home page OKO Technologies Smart Optics Faraday Partnership Zemax Development Corporation Eureka says: Since it was the UK that originally pioneered liquid crystal displays, it is highly appropriate to find that it is commercially oriented British research that is leading the field in their latest application to the next generation of vision systems Pointers * Electronically driven liquid crystal arrays can be used to focus light * Driving voltages are a few volts and driving frequencies, a few kHz * Typical lens element diameters are 5mm, with focal lengths electronically adjustable from 500mm to infinity.