From physics to products
Electrowetting based variable focus lenses
Electrically controlled deformable lenses are a topic where a lot of different technologies have been investigated these last ten to twenty years. Among all of them, only electrowetting technology is showing a strong success on the market today, thanks to its unique features such as vibration and shock resistance, low power consumption and no wear.
But how does electrowetting works? A drop of insulating liquid (oil for example) is deposited on a flat surface, made of a conductive material covered with an insulating and hydrophobic layer. All this is immersed in a conductive liquid. Voltage is applied between the conductive substrate and the conductive liquid. The shape of the drop then changes as voltage increases. In above structure, it is difficult to avoid the drop of insulating liquid moving from left to right when the voltage changes; the stability of the optical axis being an obvious requirement for optical systems, the Varioptic lenses include a conical centering that prevails changes of the optical axis trough focus. Another challenge of deformable lenses in general is to prevent the lens from being deformed by gravity, or by acceleration in general. Unlike other technologies, the lens design includes two liquids of equal density; therefore, the interface between the two liquids is both insensitive to orientation, but also insensitive to vibrations and mechanical shocks, both strong requirements in industrial devices. As for the mechanical structure of the lens, the concept is close to a button-cell battery, ensuring ease of integration (two electrical contacts on the top and the bottom of the lens), and low cost, high volume production capability.
Advantages of electrowetting
Electrically controlled variable focus has been used since decades in consumer type cameras: stepper motors, piezo driven motors, and voice coil motors have been used successfully in such devices. However, the spread to industrial devices has been very limited, mainly linked to the intrinsic limitations of these technologies: limited number of cycles, weak resistance to mechanical shocks, high power consumption, space constraints … No wear: due to the fact that liquid lens has no moving parts, there is no friction in the device, leading to no wear. For example, the Arctic 316 lens had been tested up to 500 Million cycles with no degradation, which is equivalent to a three cycle/second operation for five years 24/7 – at this stage, the limits of the technology in terms of number of cycles are unknown. This is key for markets such a factory automation or logistics, where the throughput would lead to a failure of mechanical systems in days or weeks. Shock resistance: due to the iso-density of the two liquids, the liquid lens is extremely resistant to mechanical shocks: a 2000g/0.25ms/200shocks (100 horizontal, 100 vertical) test has been performed on 100 liquid lenses with no failure and no change in the performance. These kinds of requirements are typical for markets such as barcode readers and for handheld devices in general. Vibration resistance: for the same reason, the liquid lens is also very resistant to vibrations – this means that it is possible to make an image perfectly in focus even if the camera is shaking – which is typically the case in industrial equipment like conveyor belts or in automated manufacturing equipment (Pick&Place for example). Low Power consumption: the lens being an electrostatic device, and its electrical model being a capacitor, it is a high voltage (up to 70V AC) but low current flow device, leading to extremely low power consumption: the lens itself is in the range of 1mW, and including the driver the typical power consumption is in the range of 15-20mW. Besides the requirements of battery operated devices to be low power, the fact that the lens itself dissipates no power leads to no internal heating, leading to extremely high stability of the optical power. Compact size: unlike motorized solutions, where the motor needs to be placed next to the imaging lens, the lens is placed directly into the optical stack, leading to no extra XY space compared to a motorized lens. Cameras down to 8.5×8.5mm² have been designed. No Noise: this feature is required typically in compact video cameras, where the proximity of the motor to the microphone can lead to crosstalk.
A full range of products has been designed, from the bare liquid lens, for customer willing to design their own ultra-compact camera, to M12 modules for easy integration on a camera board, and to C-Mount lenses with an embedded liquid lens and driver circuit. Also, a range of drivers are available, generating the required high AC voltage required by the liquid lens – these drivers are typically driven by a 2.8 to 5.5VDC and have either an analog input or an I²C input. As for the C-Mount lens, the liquid lens driver is directly integrated in the C-Mount – no additional components are required. Due to its high shock resistance, low power consumption, and compact size, the products have been designed in number of handheld products: from 2D barcode readers, to intraoral cameras or low vision devices for visually impaired. But the market is not limited to handheld devices: its unique shock and vibration resistance suits well the strong requirements of defense or machine vision applications.