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Small, durable lenses set to revolutionise machine vision systems

If you are developing an optical system – a machine vision system for example – the starting point would typically be the optical components.

More specifically glass optical components with a fixed focal length. However, an electrically controlled unit based on polymer lenses has now been developed that improves on some of the features of traditional optics that were previously thought as immovable.

While potential applications are plentiful, it is probably designers of machine vision systems that will see the most obvious benefits of a new technology for optical lenses. The enabling technology behind these lenses is an elastic polymer which encloses an optical fluid, both of which have optical qualities better than glass. The physical properties of the lens allow it to be controlled accurately to provide different focal lengths.

There are versions of the lens that are available for manual tuning applications, but it is the electrically controlled versions that are likely to be of greater interest to system designers.

The assembly essentially comprises the lens, which is surrounded by a static ring housing an electromagnetic coil. The coil expands when a current is passed through it and this presses down on the edge of the polymer. As the lens has to retain its volume, the squeezing of its outside edge causes the centre to rise or fall, depending on the pressure exerted by the coil. As the shape of the lens changes, its focal length is also changed.

Depending on the product selected, the focal length can be adjusted from -500mm to +50mm or from 20mm to +120mm. What is more interesting is the transition from one end of the scale to the other can take place in as little as 10ms, which is why designers looking for innovative machine vision solutions may be interested.

Alongside machine vision, other applications for the 'fast focus' electrically controlled versions include: low power laser systems used for etching, where accurate depth settings are important; retinal scans; and microscopy. There is also a manually tuneable version of the lens that may have application in such areas as ophthalmology and LED lighting in museums.

The system has been developed by Swiss company Optotune, which was established by a team who studied nanotechnology at ETH Zurich. The company, which critically holds the IP for the lens materials, has recently taken production of this range of products out of the lab and into the factory, also in Switzerland.

Matthew Ashton, divisional manager of Pacer (Optotune's UK representative), highlights another advantage of the technology. "It is a very thin lens with effectively no mechanical parts, apart from the polymer," he commented. "What it replaces is a series of lenses that would move relative to each other – much like the lenses in an SLR camera. These systems have motors and moving parts that can wear out. So this is the equivalent of taking all those components and replacing them with one of these Optotune lenses. The lenses will go through many millions of retuning cycles without any mechanical issues."

The change in the focal length achieved by the Optotune lenses would require a conventional glass lens assembly to move by several centimetres. Adopting this approach would allow the overall length of an optical assembly to be reduced significantly. Mechanical wear is said to be minimal, with the lenses rated as lasting 1billion cycles (at 10% to 90% of focal length). Another important property of the polymer is that it is non absorbent, so it doesn't go yellow with age.

There are a number of devices in the range, but the basic electrically tuneable version (the EL-10-30) has a total diameter of 30mm (10mm of which is the actual lens aperture) and depth of 9.7mm. The only input to the lens is two wires, through which the 0 to 5V, 0 to 400mA control current is applied. The assembly represents a load of 12.5?.

Because the lens will heat up during a period of operation, the maximum voltage must not be exceeded and designers should adhere to a typical operating power limit of 1W. This translates to an effective maximum input current of 280mA, which is enough to control the maximum focal length of the lens (approximately 20mm to 120mm in the EL-10-30). Dedicated control electronics can also be designed; for example, by using pulse width modulation. The frequency in this case should be set between 20kHz and 50kHz.

Using the technology to its maximum advantage will involve integrating the lenses in an equally responsive system. Ashton continued: "It is up to the designer to control the lens in order to get the picture they want. So, in a machine vision system where you are looking at video capture or if you are looking at video recognition, the designer would have to create a control loop to make sure that they get a sharp image.

It works with the system's vision software to do this automatically and there would be a feedback loop to adjust the lens. Alternatively, it may be possible to use a barcode to set the parameters. So the lens becomes part of the wider machine vision system, but what it does is allow the designer to find the focus very quickly and repeatedly.

"You would use one of these devices where you had a special requirement for a high end lens when you didn't want to use existing technology," commented Ashton. "If you have a system that you can't afford to have wear out after 1000 hours of operation, or don't want a telescopic system that is 3m long, that is when you need to look at alternatives."

Ashton admits electronic lenses are not as cheap as glass when it comes to manually tuneable versions, but points out that, in higher end systems, electrically focused lenses have lower power requirements and are lighter than motorised lensing systems and don't have as many moving parts. "In this instance," he said, "I think the lenses are probably about parity for pricing.
Either way, there are lot of customers coming to us who are excited by what they can do and who are prepared to spend a little bit more money to get it."

One application for the electronic lens which is currently being evaluated is with thermal cameras performing an infra red profile. Ashton said users want to be able to focus quickly on a point to take spot thermal measurements: sometimes without getting too hot and sometimes to measure inaccessible pieces of plant.

Designing a fast, small and lightweight lens into portable systems such as this are typical of the specialised and industrial applications that a potential customer is currently targeting. System designers interested in evaluating the lens quickly can use a newly introduced USB driver, which allows immediate tuning of the lenses.

Tim Fryer

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