Liquid lens technology mimics the human eye to achieve very rapid focus change. They can be easily combined with conventional lenses and mounted on equipment such as robot arms as Gardasoft’s Jools Hudson explains.
In the machine vision environment, there is constant demand for increased camera performance. Camera manufacturers are always searching for ways to improve parameters such as field of view, scanning speed, and speed of focusing so that the vision system can process more items in the same amount of time. A conventional camera setup uses a fixed-focus optical system but these have a limited depth of field which means that different object distances must be accommodated using multiple static imaging systems. The result is increased cost and complexity of the system.
To address the challenge of fast focus change, Optotune has develop a range of robust focus tunable lenses that can be added to a standard imaging system comprising a camera and imaging objective. These tunable lenses can change focus in under 10 ms and enable rapid, electrical tuning of working distance while preserving resolution and field of view of the original system. Using this cutting-edge technology allows end users to eliminate the need for multiple objectives and cameras, or the need to manually adjust focus. The result is a simpler and cheaper solution.
Tunable lenses: an artificial eye
Tunable lenses are conceptionally like the human eye. An elastic polymer membrane covers a reservoir of optical liquid, much like the lens of the eye. A voice coil actuated system exerts pressure on the outside of the membrane which alters the shape of the reservoir, thereby changing the optical power of the tunable lens. The optical path through the reservoir is shown in Figure 1. Optotune currently offers liquid lenses with clear aperture sizes of 3, 10 or 16 mm which suit a wide range of applications.
Figure 1: Working principle of a typical liquid lens. Left: A parallel beam is transmitted though the liquid lens. The arrows indicate where the actuator exerts pressure. Right: When actuated, the curvature of the lens and the optical power change.
Implementation of tunable lenses
Tunable lenses can be mounted either in front of or behind the conventional lens. Where the vision system has a focal length of between eight to 50mm, the Optotune liquid lens is best mounted in front of the conventional lens which provides a wide focus range from infinity down to 100mm. For extremely compact systems it is possible to combine the Optotune liquid lenses with M12 board lenses directly on a C-mount or even an S-mount camera, reaching an extreme compact and cost-effective design. A common application for this would be package sorting where bar codes on packages of varying sizes are read and tracked.
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