• Liquid lens for AutoFocus (AF)

    Outstanding quality from digital pictures comes from a precise control of the focus: a good picture requires the main subject to be very sharp, and the rest of the picture more blurred. An effective AF capability is thus essential to obtain a superior photo. Varioptic's liquid lens technology delivers fast, continuous and precise focus control. This is for use in all situations where the digital capture is critical:

    • Digital photography
    • Mobile photography
    • Industrial data capture
    • Difficult barcode reading (1D and 2D)
    • Videos with large amplitudes of object distance variations

    The first liquid lens using the concept of Electrowetting: Varioptic's "ARCTIC" Auto-Focus (AF) lens

    A sealed cell contains 2 liquids which are immiscible, oil and water. The shape of the oil's drop is changed by electrowetting. Both liquids are transparent, with different index of refractions but exactly the same density. The cell contains only those two liquids and this density matching makes the whole lens very stable against mechanical shocks. The liquid lens external shape is fixed, there is no moving part: only the internal liquids change shape. The Arctic lens is used for high performance Auto-Focus:

    • Focus changes from -10 diopters to +30 diopters
    • Continuous, highly reversible
    • Fast changes, even for large steps of focus

    Imaging applications

    AF Liquid lens is compatible with all the highest quality miniature camera modules, from small sensor formats to large ones (up to 1/2.5 inchs), for all resolutions, from 3Mp to 16Mp and highest. Liquid lens optical quality is often close to the diffraction limit, thus not impacting the associated fix lens quality. The liquid lens is made for visible light, but it is transparent as well in the near infrared.

    The large focus range enables focus adjustment for a wide range of object distance, from a few centimeters to infinity.

    The lens can be used for closed loop search of the focus (AF), or in open loop when focus information can be obtained from another sensor. The unique stability of the liquid lens makes it the only actuator enabling this open loop mode.

    Another unique property of the liquid lens is its ability of achieving very fast random steps: AF algorithms typically use a step by step approach of the best focus. Varioptic liquid lens, being as fast to make big steps as well as small steps, enables smarter focus search with n on sequential algorithms.

    Design rules for using a liquid lens into a camera module are simple and easily derived from the product's datasheet. Optically the choice of liquid lens location comes from the overall system organization and function: for an optimal pure focus control, liquid lens should be placed as close as possible from the plane of the aperture stop: this can be in front of the lenses (add-on design), inside the optical stack or behind it.

    As the liquid lens has no moving parts, the mechanical design is much simpler, enabling a shorter design cycle: using the liquid lens for AF, complex actuator designs are unnecessary and replaced by straightforward electronic architecture and simple software programming.

    Demos

    此页面上的内容需要较新版本的 Adobe Flash Player。

    获取 Adobe Flash Player

    Click on – and + to see the focus control

    The focus control changes continuously by an electrical command through the electrowetting principle.

    How it works

    Get Adobe Flash Player.

    Adobe Flash Player

    Click on – and + to see the focus control

    The focus control changes continuously by an electrical command through the electrowetting principle.

    Electrowetting

    For centuries scientists have combined lenses of very different optical powers, from diverging lenses to highly converging ones, to design and manufacture complex optical instruments. Until now the optical properties of all these lenses, either made from glass or plastic and typically requiring months to be produced, were always fixed. Varioptic has now created a « smart lens » that can be reconfigured on demand with just a variation of voltage. The lens can adapt rapidly and continuously from diverging to converging and be modeled to support all key optical functions, starting with Auto-Focus and Optical Image Stabilization.

    The technology uses the principle of "Electrowetting" and a combination of transparent and optically defect-free liquids to create a lens and change its characteristics in real time. Liquids have been used since 40 years in optical systems for high-end products such as goggles or camcorders but Varioptic's innovation is to have created a real-time programmable platform that offers to change the shape of the liquids in a very fast, repeatable, precise and controlled way.

    Main advantages of liquid lenses:

    • Large range of optical variations:

      Displacing a liquid interface allows large phase shifts variations, more than 200 waves in a typical "Arctic" lens, where typically liquid crystals or other adaptive systems are limited to about 10 waves. When used in focus variations, that means for instance 40 diopters (10 times the eye accommodation) over a 3mm diameter aperture.

    • Robustness:

      Liquid lenses have been tested for over 30 millions cycles without any performance degradations, while mechanical systems such as piezo, motors and VCM (Voice Coil Motors) are typically limited to a few hundred thousand cycles.

    • Shock resistance:

      Under drop tests Liquid lenses behave much better than mechanical solutions like VCM. Repeatability of LL response curve is excellent before, and after shock tests. In a VCM system the lens is suspended to a soft spring, resulting in offset positions changing after every shock, causing increasingly degraded focus performance.

    • Speed:

      Liquid reconfiguration can be as fast as tens of milliseconds. These time scales are well adapted to human interfaces, video, photo capture etc…

    • Low power consumption:

      Liquid Lenses will typically dissipate about 15mW, driver included, about 10 times lower than VCM, piezos or motors.

    • Design flexibility:

      One can change at will the diameter of the liquid lens, the zero-voltage value of the optical power, the index of fluids, the range of dioptric corrections as well as the position of this range regarding to an offset position.

    • Programmability:

      Camera module manufacturers can use liquid lens for AF and benefit also from free-focus integration to suppress costly individual adjustment during manufacturing.

    Liquid Lens Applications

    Electrowetting enables to manipulate the liquid interface shape, in order to create various optical functions or optical corrections.

    Possible applications of liquid lenses are too many and too diverse to be all mentioned, but the list below can serve as a starting point for innovative thinking for new products, new applications or improved performance.

    Digital imaging applications:

    • Very fast image capture
    • Very fast AF mode (non sequential algorithms)
    • Capturing multiple images at different focus positions, from infinity to 3cm object.
    • Reconstruct a 3D map from captured depth information
    • Correct for handshake in real time
    • Super-resolution, zoom

      • Non-imaging applications:

    • Laser focusing XYZ (AF+OIS liquid lens)
    • Laser de-cohering
    • Wave front corrections (adaptive optics, ophtalmology)
    • Light dispersing for illumination
    • Longitudinal spectrometry
    • Feedback loops for fine optical adjustments
    • Active control of beam deviations

    Multiple liquid lens configurations in a single body

    The flexibility of liquid lens packaging allows having a single external body for several products, allowing very high flexibility in product design and evolution:


    Varioptic liquid lens external packaging

    Inside the liquid lens, the different optical functions can be produced using different designs with specific electrodes patterns and different liquids

    A few examples:
    Single electrode lens:
    Liquid interface is a sphere, changing radius as a function of voltage     		Quad-Electrode lens:
    the liquid interface is a sphere of adjustable radius and arbitrary tilts     		Multiple electrode lens:
    With eight electrodes, the liquid interface can include Zernike corrections for:
    • Sphere
    • Tilt
    • Cylinder (or astigmatism)
    Fast focusing of the light for imaging or beam control applications Fast focusing and tilting of the light beam for XYZ control of the optical rays.  


    Electrowetting: A voltage variation induces a change in the contact angle of the fluid and effectively creates a lens with the precise desired properties.


    In the drawing below, Electrowetting is explained in a very basic configuration: a drop of oil is deposited on a planar surface , made of a flat electrode covered with a thin insulating and hydrophobic layer, all this being immersed in a water based conducting liquid. Voltage is applied between the flat substrate electrode and another electrode dipped in the conducting liquid (not shown on the photographs).



    A basic experiment enables to measure very accurately the contact angle versus voltage shown on the curve below:

    The curve shows the drop shape variation as a function of voltage during a voltage cycle. The dotted square shows the region of interest in a liquid lens. This basic experiment emphasizes two important characteristics of the Electrowetting fluid actuation: linearity (precision) and reversibility (or hysteresis free). Both are key for making a liquid lens, in addition to a number of other suitable design choices.

    Liquid Lens for Optical Image Stabilization (OIS)

    A second generation of liquid lens has been developed for mobile digital photography, in all situations where the quality of digital picture counts, and/or when one cannot afford to try several times to get the good shot, especially with children, friends etc…

    • Mobile photography
    • Mobile photography in medium light (restaurant, indoor situations…)
    • Mobile video

    The progresses made in camera module's optics quality have made the intrinsic blur coming from the camera itself negligible. Major source of blur are now extrinsic:

    1. Focus blur, when the imagers are not equipped with an AF actuator impacting visualpicture quality due to the lack of precise control of the focus.

    2. Handshake blur: the miniaturization of camera phones have degraded their light sensitivity and under medium light illumination, images are blurred due to a longer time exposure than the one experienced with a Digital Still Camera (DSC). Optical Image Stabilization is a key requirement to produce a good picture and, Varioptic OIS liquid lens is the ideal solution to achieve a high image quality while keeping a very small camera format.


    Varioptic is the first company to have developed a lens that effectively and simultaneously performs superior Auto-Focus and Optical Image Stabilization, correcting both major sources of blur.


    This new generation of liquid lens, with a multi- electrodes design (see picture below), allows tilting the light in all directions of space and, at the same time, to refocus the light.

    The ability to tilt the light enables camera module manufacturers to use our liquid lens to activate Optical Image Stabilization by measuring the handshake and compensating in real time with the liquid lens. This lens uses 4 electrodes to enable tilts and non symmetrical shapes necessary to simultaneously support Auto-Focus and Optical Image Stabilization:

    The optical design of a camera module with the liquid lens OIS+AF obeys to the same general rules as for the AF case: locate the liquid lens as close as possible to the aperture stop plane. Electronic design uses information coming from a gyroscope, either "image dedicated", or shared with multimedia applications. The gyroscope signal processing is very similar to DSC stabilizing systems, with some minor adaptations.

    Main properties of the OIS liquid lens are:
    • Effectively corrects most of the handshake blur: with picture integration time up to 400ms, 90% of pictures have their maximal handshake blur within the tilt correction range of the liquid lens.
    • Response frequencies optimized for mobile photography.
    • Long exposures enabled (up to 1s)
    • In AF macro mode, focus range is maintained very wide, enabling excellent focus at distances from 3cm to infinity.