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Liquid Lenses Offer Big Gains for Parcel Processing and Package Inspection

Industrial Photonics
Jan 2017
SHILPA VARGHESE, PIXELINK

Industrial vision cameras are a mainstay in post offices and shipping companies around the world. Gone are the days when the bulk of the items processed at these facilities were letters or postcards. Today’s post office is a hub for personal and commercial trade; packages must be processed by size, shape and color. Complicating matters, bar codes are often located on multiple sides of packages that are moving swiftly on a high-speed conveyor belt.

Multiple industrial vision cameras are typically required to capture information reliable enough to process packages correctly, segmenting items according to size and shape and then capturing identification information in order to sort for processing. Considerable investments are needed to enhance speed within sorting applications.

That’s posed limitations for facilities relying on traditional fixed-focus lens cameras. Autofocus technology signifies a substantial advance in industrial camera design. Systems typically include a camera, a control system and a lens capable of automatically focusing on a selected point or region of interest.

Integration of a liquid lens, which relies on an electrical charge to expand and contract the lens, allows for significantly sharper images, from less than 2 cm to infinity in less than 20 milliseconds. CMOS cameras within such systems offer significant advantages over CCD cameras, shortening exposure time using binning, snapping to focal points and increasing the frame rate through a reduced region of interest.

A Varioptic liquid lens on the PixeLINK PL-D 725.
A Varioptic liquid lens on the PixeLINK PL-D 725. Courtesy of Pixelink.

Today’s autofocus, liquid lens-based camera imaging systems are compact and can easily be integrated into space-restrictive systems, including conveyor belt monitors and parcel processing devices. Cameras fitted with autofocus liquid lens technology can handle myriad challenges within a single system, capturing multiple images at different focus positions on objects from 100 mm to infinity. The variable focus liquid lenses feature no moving parts, are often quicker and more resilient, and consume less power than traditionally triggered autofocus lenses. Adjustments can be made automatically for lighting conditions, with spontaneous corrections for gain, exposure, contrast and white balance.

The optical quality of the liquid lens is often close to the diffraction limit, and therefore will not impact the associated fixed lens quality. Liquid lenses are transparent in the near-infrared, even though they are designed for visible light. The wide focus range allows for focus adjustment on a vast distance range of the object, from a few centimeters to infinity. The focus can be adjusted for closed loop search of the target, or in open loop when target distance information can be acquired from another sensor. The unique steadiness of the liquid lens enables it to function in this open loop mode.

An autofocus camera available enclosed or board level.
An autofocus camera available enclosed or board level. Courtesy of Pixelink.

Liquid lenses also have the ability to process at high speeds. While traditional algorithms typically use a step-by-step method to achieve the best position of focus, liquid lenses can make small and big steps rapidly, enabling smarter focus searches with nonsequential processes.

The liquid lens technology used in an autofocus camera is based on an electro-wetting capability, which allows variable focal lengths with no moving parts. While the typical lifespan of a mechanical lens system is a few hundred thousand cycles, liquid lenses have been tested over 100 million cycles without performance degradation. Liquid lenses also can reconfigure themselves in tens of milliseconds, making them ideal for high vibration environments and applications that require fast focus changes.

A liquid lens integrated with a high-speed camera brings cutting-edge solutions to industrial and medical applications.

Sharp image, moving object

Autofocus systems with liquid lenses feature fairly simple mechanical components, requiring only software programming while avoiding complex modifications in the mechanical design. From a design perspective, it’s fairly simple to incorporate a liquid lens with a camera system, ensuring that the inherent structure of the system remains unchanged. Today, a few manufacturers have already developed cameras with inbuilt autofocus algorithms.

These autofocus imaging systems are typically integrated with a lens algorithm that includes controls driven from the camera itself, enabling the system to automatically adjust the optics as necessary, maintaining the target in focus.

A camera integrated with an autofocus lens can capture a sharp image even when the target object is moving. It also can capture different target objects that are at varying distances from the optical system. An active autofocus system measures the distance to the target, with the software built into the camera, whereas a passive autofocus system will analyze the images from the target. When using a phase detection autofocus sensor in an industrial setting, for example, the autofocus imaging system can achieve autofocus via contrast detection.

An autofocus algorithm can be implemented using a microprocessor, a digital signal processor, a field-programmable gate array or in software. Many different algorithms can be used to calculate contrast. A 3 × 3 or 5 × 5 Sobel filter for edge detection is a common solution.

One-shot autofocus is easily implemented in an existing application for still subjects. For moving objects, a continuous autofocus algorithm can be used at the expense of added complexity. The autofocus system determines if the image has achieved maximum contrast. If not, this feedback is provided to the optics to adjust the focus.

Other popular applications that benefit from autofocus technology are machine vision, biometrics and biotechnology. Consider, for example, the manufacturing of circuit boards in a machine vision application. Printed circuit boards are becoming smaller and more complex and require autofocus to enable the best quality control. An autofocus system allows for evaluation of component placement prior to reflow. It also enables bare board and solder paste inspection, in addition to checking for open or short circuits.

Additionally, liquid lenses are silent when in operation, a benefit in areas where noise could be an issue, especially in a biotechnology or biometric application environment.

Industrial applications, such as in a machine vision or biotechnology environment, have high intensity requirements from a camera system. In these environments specifically, autofocus technology has evolved significantly this decade, and deploying vision systems with precision and ease is now more doable than ever before. This development has filled a technology gap in the imaging world; a high-quality solution is now available in an industry where applications are often saddled with the issue of mild to intense vibrations affecting the outcome of the imaging system.

With the sharp focus and ultrahigh speed of an autofocus camera system, building these vision applications just got easier.

Meet the author

Shilpa Varghese is the marketing manager at Pixelink in Ottawa, Ontario, Canada; email: shilpa.varghese@pixelink.com.



How does autofocus technology work?

An autofocus system uses a camera, control system and lens to automatically focus on a selected point or region of interest (ROI).

By integrating a liquid lens, which uses an electrical charge to expand and contract the lens, cameras are able to get sharp images from near or far in faster time.

Cameras using liquid lens technology can focus faster than those using mechanical autofocus lenses.

Autofocus liquid lens technology is well-suited for high-vibration environments.

CMOS cameras have the ability to shorten exposure time using binning, snap to focal points and increasing the frame rate through a reduced ROI.


CMOSimagingindustrial imagingmachine visioncamerasCMOS camerasCCDSobel filterFeatures

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