Most visual imaging applications require a well focused image at differing distances. Traditionally, good focus at multiple depths required either using large lenses with suitable depth of field or using mechanically-driven zoom or auto-focus lenses.
More recently tunable lenses have been introduced to the market and deliver a lot of advantages compared to traditional solutions. Smaller, faster and more compact than traditional autofocus designs, these lenses are finding use in a number of machine vision, handheld devices and microscopy applications.
In previous project OFH has designed optical systems that integrate tunable lenses, most recently in the application of barcode reading.
There are several approaches to consider when searching for a tunable lens - including electro-optical, electro-mechanical and acousto-optical techniques. Below we present three of the best known and explain how they work.
1.) Varioptic (owned by Corning) produces electro-optical tunable lenses (Pic.1) based on an electro-wetting principle. These liquid lenses use a sealed cell containing two liquids with different indexes of refraction. By applying an electric field, the radius of curvature of the meniscus between the two fluids changes, resulting in a change in the focal length of the lens.
Pic.1 Varioptic lens . Principle of work
2.) Optotune uses electro-mechanical techniques. The design of lens consists of a container, which is filled with an optical fluid and sealed off with a thin elastic polymer membrane (Pic.2). The deflection of the membrane is proportional to the pressure in the cell, which is controlled by an electromagnetic actuator (EL series of lenses) or manual pressure ring (ML series of lenses).
Pic.2 Optotune lens. Principle of work
3.) TAG Optics uses acousto-optical technique. This lens is a type of gradient index of refraction (GRIN) lens that uses standing sound waves to produce a constantly changing gradient index of refraction within a liquid contained in the lens (Pic.3). The sound waves send a vibration through the liquid and force the molecules move closer together, it allows to change the refraction index of material depend on location of sound waves. By controlling the shape and location of these sound waves, it is possible to establish an index of refraction profile that looks like a simple lens in the center of the lens.
Pic.3 Acousto-optical tunable lens. Principle of work
Which solution is best will depend on your application requirements and target price.