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Autofocus liquid lenses target new applications

19 Oct 2007

Liquid-lens technologies designed to meet the insatiable demands of the mobile-phone industry are now being deployed in other imaging systems. Philippe Ruffin explains to OLE how Varioptic is adapting its products for an increasing number of uses.

The mobile-phone market has driven many of the changes that imaging systems have undergone in recent years. The industry continues to require cameras not only with enhanced performance but also modules that are cheaper and smaller than their predecessors. These constant demands have inspired new technologies, many of which can be deployed in other imaging applications. Miniature variable focus optics is just one of these technologies.

At Varioptic, we are pioneering liquid-lens solutions and have seen that markets such as webcams, barcode readers, security and medical are launching or developing products that include variable focus. Here, we outline why we believe that moving to an autofocus set-up based on liquid lenses would benefit these key sectors.

Autofocus with liquid lenses
Autofocus is a standard feature in all of today's digital cameras. These systems use electromechanical technologies such as stepper or piezo motors to precisely translate the lenses with respect to each other. Although these systems work well, they are expensive, difficult to miniaturize, have a limited number of cycles, and can fail to meet the tight optical tolerances required because of friction between the mechanical components.

Our liquid-lens solution uses electrowetting technology to produce variable focal length lenses with no moving parts. These lenses have been commercially available for over a year for a variety of applications including mobile phones. Liquid-lens technology is characterized by a wide range of focal lengths that enable very close focus; high optical quality; fast and consistent response; and finally by its robust nature with no moving parts to wear or break.

Varioptic's lenses use a watertight cell that contains two nonmiscible liquids. The shape of the liquid interface is controlled by a voltage, which when applied, changes the power of the lens. The interface is in contact with a conical metallic part that is coated with an insulating film several microns thick (see figure 1).

When an electrical voltage is applied, charges accumulate on both sides of the insulating film. This applies a force to the interface, which deforms and assumes a convex shape. This effect is both reversible (because the system presents a very weak hysteresis) and rapid.

In this configuration, manufacturers benefit from an extremely robust module that enables a rapid autofocus of <500 ms; uses very little energy (around <15 mW); and can be manufactured at low cost in very large numbers (<1 million per month).

Since the launch of our first liquid lens in January 2006 (the ARCTIC 320), we have continually added to our portfolio.

All liquid-lens products share very high performance and quality levels on critical parameters, either optical (such as transmission and wavefront error) or electrical (such as low dissipated power). Typical specifications are given in table 1, p17.

Advantages of liquid lenses Liquid lenses provide advantages over traditional mechanical actuators in the three critical areas outlined below.

Reliability and robustness
Whereas mechanical systems are rated to a few hundred thousand cycles, liquid lenses with no moving parts have been tested to more than 30 million cycles. This is particularly useful for very high cycle-rate applications, such as barcode reading systems.

Mechanical robustness is also a serious concern for consumer and industrial applications. Liquid lenses are not affected by vibrations or drop tests and can operate from –20 to 60 °C and can be stored in conditions from –40 to 85 °C.

Response time
Fast response time is a key requirement in many applications, including biometrics, barcode readers and webcams. The two critical issues that usually plague traditional solutions are hysteresis and long settling times.

A high hysteresis requires system designers to use tricks to avoid missing the best focus position. A liquid lens does not suffer from hysteresis so the design is much simpler and more reliable.

Mechanical actuators move the lens up and down the optical axis to vary focus. However, before the system can capture the images necessary to perform the auto-focus calculations, the lens must reach a stable, settled state that takes around 50 ms for each step (and there are typically up to 20 steps). This limits the time-to-focus (TTF) for still images and the frame rate of the system if it is in video mode.

For example, a rate of 25 frames per second (fps) requires frame capture within 40 ms, so a 50 ms settling time would mean that the autofocus algorithm can only run every other frame. One option is to reduce the frame rate to 20 or 15 fps and to run the autofocus with every frame. In comparison, the liquid lens can run comfortably at 40 ms giving 500 ms TTF and good video-mode performance.

Repeatability
Repeatability is the ability of an actuator to constantly and accurately return to the same position for the same input command and is critical in open-loop applications. To be classified as repeatable, an actuator should not suffer from hysteresis or from drifts in time. It should also be possible to model the response curve in temperature. The response of a liquid lens is highly repeatable and linear in the main operating range of –5 to 13 d optical power with a slope equal to 1∂ per 1 Vrms.

Autofocus applications
Webcams
Webcams were designed originally for low pixel counts mainly associated with the video transmission requirements of the CIF format. Poor quality images at very low frame rates were acceptable due to transmission speed constraints. With the increased availability of broadband access (meaning larger image files can be transmitted faster), high-definition products and megapixel screens, users now expect better image quality, and this is driving the move to higher-pixel webcams.

A typical webcam is placed in front of a laptop and must show everything from a close-up view of an object to overviews of a scene. With these factors and an increase in pixel count, an autofocus soon becomes mandatory. Other important considerations for webcam imaging systems are a fast response time to maintain good focus when the webcam is in use, and silent operation to avoid an irritating background noise every time the focusing system moves.

Barcode readers
Barcodes readers are increasingly required to scan 2D barcodes. Using standard fixed-focus lenses, manufacturers usually develop several designs to cater for both long- and short-range requirements. With autofocus, it is possible to have a single product that covers both needs, enabling users to scan items from as close as 5 cm to infinity. Critical factors for this application are response time, repeatability (as barcode readers work in an open-loop architecture) and reliability (with in excess of 10 million cycles).

Biometrics readers
Face-, iris- and fingerprint-recognition systems are being marketed for applications such as law enforcement, immigration control and military, as well as increasingly for consumer applications such as access control in hotels or private homes and device security in place of PIN codes. Current fixed-focus imaging systems can be cumbersome with the holder having to follow a series of instructions to move closer or further away to "manually" adjust the focus. Using autofocus gives a much better user experience, provided that the variable focus system has: a fast response time (in a closed-loop system); good repeatability; is easy to integrate into the existing system; and is both reliable and robust (as it may be unattended and exposed to harsh conditions).

Medical applications
Endoscopes, fibrescopes and dental cameras all rely on the user to adjust the focus manually and a reliable autofocus makes these tools less tedious to handle. The requirements on the autofocus system are a very close distance to focus (often below 1 cm), reliability (to comply with medical environmental targets) and silent operation.

Low-vision systems are designed to help people with reduced vision from glaucoma or cataracts. These can be hand-held or fixed for manual or computer-assisted use. Such instruments require an autofocus to capture the target scene or image prior to displaying it with the required improvements. Given this usage scenario, a very high level of repeatability and reliability is expected from the autofocus actuator as well as the ability to focus at distances below 10 cm.

Future development
Varioptic is continuing to broaden its range of liquid-lens products and to monitor the new opportunities that are emerging with the development in CMOS sensor technology. Here, pixel sizes are moving to 1.75 µm for a 0.25 inch 3 Mpixel sensor, and the next milestone is a 1.4 µm pixel size. We are also developing larger optical systems for imagery and ophthalmology applications. Today, Varioptic has a range of available products to meet current market demands and has also developed a clear roadmap to meet future market requirements.

• This article originally appeared in the October 2007 issue of Optics & Laser Europe magazine.

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