Liquid lenses poised to revolutionise variable focus optics

Citation

(2006), "Liquid lenses poised to revolutionise variable focus optics", Sensor Review, Vol. 26 No. 4. https://doi.org/10.1108/sr.2006.08726daf.001

Liquid lenses poised to revolutionise variable focus optics

Liquid lenses poised to revolutionise variable focus optics

Keywords: Cameras, Optics

Unlike humans, with our soft, pliable lens which can change focus, literally in the blink of an eye, man-made optical devices have no such facility. A problem common to all variable-focus optical systems is that to change the focal length or magnification, one or more lenses must be moved, either relative to each-other or with respect to a fixed imaging device. Now, the revolution in digital photography has highlighted the need for something better. Present generations of camera phones are fitted with simple, fixed-focus lenses which severely compromise image quality and digital cameras, particularly DSLRs, are fitted with large and bulky variable-focus lenses, driven by power-hungry electromechanical systems. All of this is now about to change, thanks to the development of adaptive liquid optics, commonly known as liquid lenses.

The principle of the liquid lens is simple and exploits a phenomenon termed electro-wetting. When a droplet of water deposited on a metal substrate is covered with an insulating layer and a voltage is applied to the substrate, the contact angle of the droplet changes. In a liquid lens, two liquids with similar densities but differing optical properties are used as the conductor and insulator and variations in the applied voltage cause a change in the curvature of the liquid/liquid interface, which is manifest as a change in the focal length (Figure 1).

Figure 1 Schematic of a liquid lens (Varioptic)

The key benefits of this technology are that the lenses are compact, rugged, inexpensive and consume very little power (͂1mW) and are, therefore, well suited to battery- powered imaging applications such as digital cameras. Commercialisation is already underway: the French company Varioptic was founded in 2002 and is developing a range of patented products based on the results of a decade's research at two academic institutions – the Université Joseph Fourier and the Ecole Normale Supérieure de Lyon. Varioptic has teamed-up with the US company Sunex, Inc., a specialist in cost- effective optical solutions for the digital imaging industry, and announced its first auto-focus liquid lenses at the end of 2005. In early 2006, and after having worked closely with Samsung Electro-Mechanics, the company commenced production of its “Arctic 320” auto-focus lens and has the capacity to manufacture several million units each month. This is aimed at the low end of the digital imaging market, notably camera phones with 3 megapixel resolution and CMOS sensors. The specification is shown in Table I. Partner Sunex has combined the Arctic 320 with one of its fixed lenses to produce the “ALF 872” which is a ready-to-use auto- focus lens module with an aperture of F2.8 that can be fitted to any standard M8×0.35 lens mount.

Table I Specification of the Varioptic “Arctic 320” liquid lens

Varioptic is now in the preliminary stages of designing a zoom lens which it hopes will enter production by the end of 2006. It will comprise two liquid lenses and one conventional lens and the aim is to achieve an optical zoom ratio of ×2.5, a focal length of 4.26-10.64mm (35mm film camera equivalent=42-105mm), and an aperture varying from F2.8 to F4 or 5.

Other companies are also working on liquid lenses: although not yet in production, Philips Research showed a prototype of its “FluidFocus” lens in 2004. As with the French products, these consist of two immiscible fluids with different refractive indices, one being an electrically conducting aqueous solution and the other a non-conducting oil. These are contained in a short tube with transparent end caps. The internal surfaces of the tube wall and one of its end caps are coated with a hydrophobic (water-repellent) coating that causes the aqueous solution to form into a hemispherical mass at the opposite end of the tube, where it acts as a spherically curved lens. A sectional schematic and the voltage- induced changes in the lens geometry are shown in Figure 2 and Plate 1 shows the actual lens. By increasing the applied electric field, the surface of the initially convex lens can be made completely flat (i.e. with no lens effect) or even concave. As a result, it is possible to design lenses that smoothly and rapidly (i.e. in <10ms) make the transition from being convergent to divergent and back again.

Plate 1 Philips researchers show the miniature variable-focus lens and the camera that contains it

Figure 2

It is evident that the first products to benefit from this technology will be camera phones, followed by digital cameras, but in the longer term many other applications are anticipated in such diverse areas as healthcare and security (Table II).

Table II Future applications for liquid lenses

Contact: Varioptic SA, Bâtiment Tony Garnier, 8B Rue Hermann Frenkel, 69007 Lyon, France. Tel: +33 (0)437 653531; web site: www.varioptic.com