A liquid lens consists of two liquids (aqueous or oil, i.e. not mixing) with the same density as possible , but with different refractive indices and electrical properties in one chamber. An electrical voltage of up to 60 volts (between a ring electrode in the chamber and a transparent indium tin oxide (ITO) electrode on the outside of the chamber window) controls the electric field, which, via the effect of electrowetting, the contact angle of a drop of the polar liquid (water) inside changed on this transparent chamber window. As a result, the hemispherical drop becomes wider on the inner wall and thus reduces the curvature of its surface (the phase boundary which is effective for refraction), as a result of which the lens changes its focal length . A focus or a zoom can thus be implemented.
Four radially arranged electrodes also allow a controlled, wedge-like distortion or displacement of the drop. Optical image stabilization can be achieved with the help of the (auto) focus liquid lens when the camera shakes or when the object is moving. This technology was presented by Varioptic in February 2010.
As lenses with variable refractive power, liquid lenses can be made particularly small, which is especially necessary for cameras in flat cell phones. They are fast, energy-saving and stable against vibrations. The solution of salts (such as potassium sulfate) in the water in the lens makes it conductive and lowers its freezing point and its vapor pressure sufficiently to cover a wide temperature range for operation (−20 to +60 ° C) and storage (−40 to +85 ° C) ensure (ie the water neither freezes nor evaporates). The following technical data are typical:
- 8 mm diameter
- 2 mm thick
- 2.5 mm optical aperture with a refractive power adjustable between −2 ... +10 D
- Response time around 1 ms.
As early as 1988 Werner B. Schneider specified an arrangement for demonstration purposes for physics lessons with which the focal length of a lens made of water could be adjusted over a wide focal length range by means of a voltage. The technical exploitation of this idea was only realized later with the development of small digital cameras. The American scientist Tom Krupenkin and his colleagues produced the first liquid lens in August 2002. At the computer fair CeBIT 2005, the company introduced Varioptic an application of the liquid lens in the form of an optical zoom and an auto focus function in a camera phone before. Research under Bruno Berge at the University of Grenoble (F) from 1997 to 1999 led to fundamental patents (for example on the chamber shape and centering of the drop). These patents are now held by Varioptic; this company is now operating the further development. Today, liquid lenses are used for numerous applications in which automatic, fast focusing is necessary. This is the case, for example, in industrial image acquisition for monitoring production processes ( Industry 4.0 ) or in medical technology.
Other types of liquid lenses
A second type of liquid lens is based on a liquid crystal layer between glass plates, in which the optically active molecules are not rotated by an applied electrical voltage, but instead concentrate more radially outwards. The necessary diffusion process takes time and then causes a higher refractive index (and thus a higher diffusing effect) on the outside of the lens. In 2006 a laboratory sample with a diameter of 1 cm was able to change its refractive power within 3 minutes; Miniaturization (0.5 to 1 mm diameter) suggests a response time of 1 s. A first reference to this technology can be found in an article by LG Commander et.al. from 1995.
As early as 1940, R. Graham published the article "A variable focus lens and its use" on a liquid-filled deformable chamber. All three types of liquid lenses have a liquid optically active core.
- the natural model of the human eye, for example, in which an elastic lens in a fluid-filled chamber is mainly involuntarily deformed by muscle power for focusing
- the optical image through falling, lying or floating water drops
- the deformation of liquid surfaces by stirring, rotating or by applying an external magnetic field
- Retroreflection of sunlight caused by dew drops on grass
- Disturbance of the retroreflection on traffic signs through fine drops of condensation water, raindrops or a film of water
- Liquid level indicator through sight glasses with 90 ° ribbing on the inside, which only leads to total or retroreflection if there is no liquid in contact with a sufficiently high refractive index
- Collective mirror effect of large display panes under the effect of ventilation or wind pressure as well as double-glazed insulating windows, the surfaces of which bulge slightly under the influence of internal temperature, gas filling, diffusion and external atmospheric pressure, resulting in focal lengths in the 10-meter range that block the sunlight Bundle light patterns on the street.
- Tunable liquid microlens ( Applied Physics Letters , January 20, 2003)
- Liquid lens focuses light on command (Wissenschaft.de, January 28, 2003)
- Liquid lens with optical zoom (avguide.ch, March 13, 2005)
- Liquid lens brings autofocus and zoom into cell phones (Welt am Sonntag, March 13, 2005)
- Almost fluid and yet super sharp (Wissenschaft.de, May 25, 2006)
- Varioptic company (developer and manufacturer) - explanations, demo videos
- Liquid lens with electrically adjustable focal length (PDF; 143 kB) Publication in Physik und Didaktik 1988
- A Lens with an Adjustable Focal Length (PDF; 190 kB) Publication in Physics Teacher 1993
- Dierk Fricke, Evgeniia Denker, Annice Heratizadeh, Thomas Werfel, Merve Wollweber, Bernhard Roth: Non-Contact Dermatoscope with Ultra-Bright Light Source and Liquid Lens-Based Autofocus Function . In: MDPI (Ed.): Applied Sciences . tape 9 , no. 11 , 2019, pp. 2177 , doi : 10.3390 / app9112177 ( mdpi.com [accessed June 3, 2019]).