Liquid Lens for Auto Focus (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
- industrial data capture
- barcode reading (1D and 2D)
- biometric data acquisition
- videos with large amplitudes of object distance variations
The first liquid lens using the concept of Electrowetting: ARCTIC Series
A sealed cell contains 2 liquids which are immiscible, oil and water. The shape of the oil drop is changed by electrowetting. Both liquids are transparent, with different indices of refraction 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
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 inch), for all resolutions, from 3Mp to 10Mp and higher. Liquid lens optical quality is often close to the diffraction limit, thus not impacting the associated fix lens quality. The liquid lens is designed 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 object distance 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 to achieve very fast random steps: AF algorithms typically use a step by step approach of the best focus position. Varioptic liquid lens, being as fast in making big steps as small steps, enables smarter focus search with non-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 to the plane of the aperture stop: this can be in front of the lenses (add-on design), inside the optical stack or even 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 electronics architecture and simple software programming.