Advanced Imaging


Advanced Imaging Magazine

Updated: January 12th, 2011 10:01 AM CDT

Making a Splash in Optics

Autofocus liquid lenses provide variable focal lengths with no moving parts
Liquid lens technology
© Varioptic
Liquid lens technology is characterized by a very wide range of focal lengths that enable very close focus, high optical quality, fast and consistent response, and are very robust with no moving parts to wear or break.
The liquid interface
© Varioptic
The liquid interface is in contact with a conical metallic part insulated by a thin film. When an electrical voltage is applied charges accumulate on both sides of the film applying a force to the place where the interface rests. This becomes deformed and assumes a convex shape. The effect is rapid but also very reversible because the system has a very weak hysteresis.

By Nathalie Combrousse, Varioptic

Liquid lenses provide key advantages over traditional, mechanical actuators in three important areas:

Reliability and robustness: 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, and continuous autofocus for video systems. Mechanical robustness also is a strong concern for both consumer and industrial applications. Liquid lenses are unaffected by vibrations or drop tests, and can sustain a wide range of temperatures. They can operate to specification from -20░C to +60░C, and storage conditions can go from -40░C to +85░C.

Response time: Fast response time is a key requirement in many applications, such as biometrics, bar code and webcam applications. Traditional solutions usually are plagued by two critical issues. First, high-hysteresis (a retardation of effect) due to a changing magnetizing force. requires system designers to use "tricks" to avoid missing the best-focus position. The liquid lens doesn't suffer from hysteresis so control systems are much simpler and faster. Second, long settling timeŚmechanical actuators all use the same principle of moving 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 which takes around 50ms for each step (and there typically are up to 20 steps).

This limits the time-to-focus (TTF) for still images and the frame rate for the system if it is in video mode. For example, 25 frames per second (fps) requires frame capture within 40ms (1/25 seconds) so a 50ms settling time would mean the auto-focus algorithm can only run every other frame, or to reduce the frame rate to 20fps (but probably 15fps to be safe) to be able to run auto-focus using every frame. The liquid lens can run comfortably at 40ms giving 500ms TTF and good video mode performance.

Repeatability: A critical factor in open-loop applications, repeatability is the ability of an actuator to constantly and accurately return to the same position for the same input command. To be classified as repeatable, an actuator must not suffer from hysteresis, and also should not suffer drifts in time. It should also be possible to model the response curve in temperature.

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