Electronic Design

Chip Drives Liquid Lenses To Stretch Battery Life In Cell-Phone Cams

Maxim Integrated Products has teamed up with Varioptic to pioneer a complete package for digital still and video camera and cell-phone makers that want to replace powerhungry, noisy hard lenses with liquid lenses. The problem with hard lenses? Focusing them involves physically moving the lens elements with motors— most recently voice-coil affairs that drain batteries and tend to leave distracting noises on video soundtracks.

What’s a liquid lens? Essentially, it’s formed by the curved interface between an oil drop and a water drop. A process called electrowetting can electrostatically deform the shape of the interface. Though seldom used, electrowetting has been understood as a process since at least 1875.

The partnership marks a trend in the industry as traditional analog companies deal with new economic realities by bundling up their skill sets and taking them wherever they can be used effectively to solve new problems (see “Analog Survival Means Learning To Be Adaptable,” Jan. 15, p. 17).

In this case, companies that want to improve their cameras by using liquid lenses need to generate and control a relatively high dc voltage (roughly 42 V) efficiently inside a device as small and crowded as a cell phone, with optimum power management, the smallest possible footprint, and the lowest possible external parts count.

The partnership also reflects new business realities. The device has a Maxim part number, MAX141515, which is a custom job destined solely for Varioptic’s auto-focus Arctic 314 liquid lenses. In turn, these lenses generally will be built into camera modules for Asian original design manufacturers. There are still plenty of design wins the old-fashioned way, but the times are a-changing.

Maxim calls the MAX14515 a “highvoltage liquid-lens driver.” It supplies a high-voltage (0- to 47-V) differential output, which is controlled through an I2C interface, using a charge-pumpbased boost converter and an integrated H-bridge. It also includes an 8-bit digital-to-analog converter (DAC). The MAX14515 will run on one or two lithium batteries. It comes in a 1- by 2-mm package.

Maxim is very proud of how it fit all of those elements into such a tiny package and cut the external bill of materials down to two components. However, the company would not discuss how it achieved this feat under any circumstances. Similarly, Varioptic somewhat explains how the liquid lenses change shape in the presence of an electrostatic field on its Web site, but the company won’t give away all of its trade secrets either.

Still, interested researchers can glean a great deal by combing through the conference papers and bibliographies that Varioptic also has posted.

For example, the company’s 2006 presentation for the Optical Society of Japan (www.varioptic.com/res/documents/0802.pdf) offers some key insight into the lens construction (see the figure).

The lens itself, the thing that refracts light, comprises two non-miscible liquids that are trapped in a closed cell between a pair of glass windows. One liquid is waterbased and electrically conductive. The other is some kind of oil that is apolar and non-conductive. The natural interface between the liquids is curved and the index of refraction of the two materials is different, so there’s your lens. Now you have to be able to control its degree of curvature.

The process of electrowetting makes that control possible. Electrowetting can be defined as the change in solid electrolyte contact angle due to the applied potential difference between the solid and the electrolyte. A dc potential of 0 to 60 V across the lens can significantly and controllably change its shape. Varioptic demonstrates the range of the shape change in a video on its Web site available at www.varioptic.com/en/tech/Electrowetting.avi.

How good are liquid lenses optically? According to Varioptic, at the current full-production capacity of millions of units per month, you’re looking at something that will match image quality with a 2-Mpixel imager and a conventional f 2.8 0.33-in. lens system. Zoom-wise, liquid lenses provide variable power with a typical range of 20 diopters (–5 to about +15). This allows most optical systems to focus from infinity to around 5 cm. Light transmission is 97% (at 587 nm – green light).

Since the system doesn’t use voice-coil technology to focus with rigid lenses, and its mass is next to nothing, you’ve picked up some advantages in rapid focusing. In normal conditions, the response time is between 50 and 100 ms. Varioptic demonstrates this response in a video at www.varioptic.com/en/tech/Focus_Fast-Divx.avi. Another advantage of charge-based focusing is the absence of camera noise on the soundtrack when shooting video.

But the obvious question about water-drop lenses, ever since Leeuwenhoek, is what happens when it gets cold. (Actually, the story you might have heard in high school is probably apocryphal. Leeuwenhoek knew how to make good glass lenses. The question about freezing is still valid.) Varioptic specifies –20°C to 60°C for operation and –40°C to 85°C for storage. That’s because there are some ingredients in the fluid mix the company isn’t talking about. Varioptic does admit that response time due to viscosity changes does vary a little over the operating temperature range.

But what about gravity? What if users shake it? Varioptic says that the density of both liquids is equal at working temperature. The gravitational force on both liquids is the same, so they retain their shape. In addition to designing the liquids with equal densities, the company also formulated them so they don’t mix. Electrostatic force and surface tension keep the interface between the liquids stable, and there is no noticeable impact on the image quality. From a reliability viewpoint, Varioptic guarantees that the lens will be robust and reliable against shock and vibration.




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