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Optical design using liquid lenses | OFH

Liquid lenses, are a type of optical lens that have a tunable focal length by shaping a liquid electronically. These lenses can be based on voice coil actuation, or based on electrowetting technology, which involves manipulating the surface tension of a conductive liquid droplet by applying an electric field.

 

The core component of an electrowetting-based is a transparent container filled with two immiscible liquids, typically water and oil, with the conductive liquid droplet residing between them. The droplet acts as a lens by controlling the curvature of its surface. By applying voltage to the conductive droplet, the shape of the droplet can be modified, allowing for adjustments in the focal length.

 

One of the significant advantages of liquid lenses is their ability to change focus rapidly and smoothly, making them suitable for applications such as image stabilization, barcode scanning, and auto-focus in cameras and microscopes. They offer a compact and robust alternative to traditional mechanical lens systems.

 

While liquid lenses offer unique advantages, they also come with a few challenges and limitations. Here are some of the problems that can arise when working with liquid lenses:

 

Optical Performance: liquid lenses may have limitations in terms of optical performance compared to conventional lenses. They can exhibit certain optical aberrations, such as spherical aberration, astigmatism, or distortion, which can affect image quality. The resolution and clarity of the images obtained with liquid lenses may not be as high as those achieved with traditional lenses.

 

Limited Aperture: liquid lenses typically have a fixed aperture, which means they may not be able to achieve very wide or very narrow aperture settings. This limitation can impact the control of depth of field and the amount of light entering the lens.

Optical design using liquid lenses 1Figure 1. A liquid lens can change shape by adjusting the curvature of a liquid interface. This allows for dynamic focusing and zooming, making liquid lenses useful in microscopy, machine vision, and mobile cameras 

To overcome the limitations of liquid lenses, several approaches can be considered. One important approach is through careful design of an add-in optical component.  Through careful optical design, it is possible to compensate for some of the inherent aberrations and limitations of liquid lenses. Advanced optical modeling and simulation techniques can be employed to optimize the lens design, minimize aberrations, and improve overall image quality. This can involve using additional optical elements or incorporating software algorithms for image correction and enhancement.

For example, at OFH, we embarked on a collaborative endeavor with a client to address the specific performance requirements of a liquid lens within a particular system. Our client presented us with the task of a specific combination of  focal length (FL), F-number (F#) and MTF performance for a unique set of liquid lenses. To meet this demand, we devised a comprehensive solution by incorporating an advanced system, as depicted in Figure 2.   Through this integration, we successfully achieved a focal length of 12.5 mm and an F-number of 6.6, aligning with the client’s specifications.

Optical design using liquid lenses 2

Figure 2. Add-on design at OFH for a liquid lens to achieve specific performance parameters.

By carefully configuring the components and optimizing their interactions, we were able to achieve improvements in the lens’s performance. The primary objective was to achieve a specific focal length simultaneously and  F-number to enhance the system’s overall imaging capabilities.

Through this collaboration, we were able to provide our client with a tailored solution that met their expectations. The integration of the advanced system, as exemplified in Figure 2, demonstrated the potential for pushing the boundaries of liquid lens capabilities and unlocking new opportunities for diverse applications.

Brands

Varioptic lenses and Optotune lenses are two brands of liquid lenses in the market. Both types of adaptive lenses can change their focal length and provide variable focus capabilities. However, they differ in their underlying technology and the mechanisms by which they achieve this adaptive functionality. Let’s compare them in various aspects:

1. Technology:

   – Varioptic lenses: Varioptic lenses are based on the principle of electrowetting. They consist of a transparent liquid droplet that is enclosed between two immiscible fluids, typically water and oil. By applying an electric field across the droplet, the contact angle between the liquids can be modified, thereby changing the curvature of the lens and its focal length.

      – Optotune lenses:  Optotune lenses are shape-changing lenses based on a combination of optical fluids and a polymer membrane. The core element consists of a container, which is filled with an optical liquid and sealed off with a thin, elastic polymer membrane which is pushed by a circular ring that shapes the tunable lens. The change in shape alters the curvature of the lens, leading to a variation in its focal length.

2. Response Time:

   – Varioptic lenses: Varioptic lenses typically have fast response times, often in the range of milliseconds. They can rapidly change their focal length, making them suitable for applications that require quick adjustments.

– Optotune lenses: Optotune lenses have faster response times compared to Varioptic lenses, typically in the range of a few milliseconds. The shape changing mechanism is often faster than the electrowetting technology. For example the response of Optotune EL-3-10 is 3-4ms while the response of Varioptics A-25H is 100ms!

3. Power Consumption:

   – Varioptic lenses: Electrowetting-based lenses, such as Varioptic lenses, are known for their low power consumption. They require power only during the focal length adjustment process and can maintain their focal length without continuous power input.

   -– Optotune lenses: Optotune lenses can consume higher power compared to electrowetting lenses. Since they rely on applying an electric current to push the lens shaper, they require a continuous power source to maintain the adjusted focal length

 

4. Optical Performance:

   – Varioptic lenses: Varioptic lenses offer good optical performance, with low aberrations and good image quality. They can provide a relatively large range of focal lengths, making them suitable for various imaging applications.

– Optotune lenses: Optotune lenses also provide good optical performance, with a bigger clear aperture and focal length range. Optotune lenses can have a clear aperture up to 50mm. However, due to the very small capillary forces, the electrowetting technology only allows for up to about 6mm apertures

In summary, both Varioptic lenses and Optotune lenses are adaptive lenses that can change their focal length. Varioptic lenses utilize electrowetting technology and offer fast response times, low power consumption, and good optical performance. On the other hand, Optotune lenses are shape changing lenses based on voice coil acutation, have slightly faster response times, bigger clear apertures, but higher power consumption. The choice between them depends on the specific requirements of the application at hand

 

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