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How technology is bringing sharper images to users of smartphone cameras and videos

Optical Image Stabilisation (OIS) is coming to a smartphone near you; indeed, a few models already incorporate the technology, which allows the capture of significantly improved and blur free images and shake free videos.

It's a variation on technology that has long been the mainstay in all digital still cameras, but why is it taking so long for it to get established in the burgeoning mobile phone smartphone markets, where almost all models now boast a decent camera?

As the acronym attests, OIS uses optical methods to stabilise an image. Typically, a gyroscope sensor is used to detect and measure the vibration on the camera lens and optical components are moved to compensate for this motion. The technique already offers an improvement – and its proponents suggest that ongoing advances will offer an even more significant improvement – on either accelerometer based digital stabilisation, where software adjusts the image framing in response to left-right and up-down inputs, or purely software based digital stabilisation, where high contrast objects in the frame are tracked and used as reference points. The latter approach – Electronic Image Stabilisation – is used in Apple's current iPhone range.

According to HTC, one of the biggest complaints about smartphone cameras has been blurry photos and it says there are two main reasons:
the length of time required to capture an image results in misaligned light streams coming into the sensor; and the physical shaking of the hand holding the camera or pressing the shutter button.

To address the first issue, HTC's camera can capture full size photos in up to 1/48 of a second, rather than the 1/30s in the HTC One X.

The second issue is dealt with using optical image stabilisation (OIS), something which HTC says is a 'significant milestone' for smartphone cameras.

OIS, it continues, is more effective than digital stabilisation as there is physical movement involved. Digital stabilisation, it says, takes the image or video and crops the shaky parts, resulting in lower resolution images.

The OIS component in the new HTC One can counter motion on two axes, detecting and correcting pitch and yaw movements at up to 2kHz. The system is designed so the lens can tilt by an average of 1° in all directions from its centre point, dealing with various movements (see fig 1).



It adds that lab tests have shown its OIS approach produces blur free images at shutter speeds as slow as 1/7.5s, although a new sensor means faster shutter speeds can be used.

The answer to the question posed above, according to Mehran Ayat, senior director at InvenSense's Imaging Business Unit, is that OIS requires several challenges and features to come together. "While the basic concept seems simple enough, achieving the required performance and cost effectiveness, bearing in mind the need to meet really challenging space constraints, is no simple task."

Almost all mobile phone makers rely on camera module integrators to supply a tested plug-and-play solution. And these companies have to guarantee volume manufacturing capability to bring all this off and support the many skews with varying feature levels demanded by a fast moving and ultra-competitive market.
"It is a really complex and interdependent ecosystem – OIS gyroscope, image sensor, lens and actuator with a companion control and driver chip, all in the most compact module housing achievable," Ayat told New Electronics.

"We pretty much own the two axis OIS MEMS gyroscopes market," he claimed. "Every smartphone except one uses our parts and, over the past year, we have seen enormous traction, with demand doubling."

The only real competition comes from ST Microelectronics, which supplies the OIS gyro used in early models in Nokia's highly regarded Lumia range, a fact confirmed by Dr Eric Mounier, senior analyst MEMS Devices and Technologies at French group Yole Development. Most in the mobile phone sector agree that Nokia (now part of Microsoft, of course) has done more in pioneering and deploying OIS technology than any other in the sector. However, Ayat notes the Lumia 920 wasn't the first OIS enabled model in the market. "We sold our dedicated OIS gyros to Panasonic, which brought a model using the technology to market ahead of Nokia."

All the other OIS enabled models currently available are Android based, although Ayat stresses the gyroscopes and other components are OS agnostic. The list includes the popular HTC One, the LG G2, the Nexus 5 and the Zubia Z5S. There are constant rumours that Samsung's next model will be OIS enabled, but none have so far proven to be true, despite one of the South Korean group's operating companies being a major player in the module integration business. Others include Taiwanese giant Foxconn, LG, Sharp, Mitsumi, Sanyko and Sony.

The main players in the OIS controller sector include Rohm, Sanyo, Renesas, ON Semiconductor and Maxim, all of whom have been busy shrinking their devices to meet the demands of module integrators and incorporating dual digital interfaces to work with the gyros. The major suppliers of image sensors include Sony, Samsung, Sharp and Aptina.

Where the UK expects to make an important difference in the 'ecosystem' is with the OIS camera lens. Currently, this is predominantly a voice coil motor (VCM) based device that implements OIS by moving the lens backwards and forwards using magnetic field coils wound around the lens within the camera module.

Another 'interesting' option, according to Ayat, is the technology being developed by UK based Cambridge Mechatronics (CML). "Its approach is slightly different in that it deploys smart metal alloy technology in the actuator."

Although sensitive about discussing its technology, CML noted in a press announcement in November 2014, that 'CML and its new manufacturing partner have successfully completed its OIS lens motor reliabity test programme. The suite of test specifications stipulated by by our module integrator and handset customers include several individual and combinations tests in line with industry standards, including drop testing, extended operation cycles and environments at the extremes of high and low temperature.

The company added this is a 'key milestone in the market readiness of CML's latest product offering, allowing customers to confidently design our actuator into their products'. The release also noted plans for its OIS lens motors to be shipping inside smartphone handsets in 'early 2014'.

None of the companies involved in the development of OIS technology wished to comment on the likelihood that next generation iPhones could deploy OIS, rather than an electronics based approach. Ayat merely suggested that, as far as gyroscopes are concerned, 'every phone vendor in the world is our target'.

The Lumia 920, launched in 2012, was the first Nokia phone to offer OIS. Since then, it has deployed more advanced versions of the technology, developed, according to Juha Alakarhu, head of imaging technologies, in close collaboration with lens supplier Zeiss. "Our approach actually shifts the entire assembly of optical components, so the camera can compensate for a much greater amount and form of movement," he told New Electronics.

Asked what differentiates Nokia's approach to that of other smartphone makers, Alakarhu confirmed the company works with several unnamed manufacturing partners. "Unlike most phone companies, which settle for off the shelf solutions, Nokia drives the development of its smartphone cameras. This includes the hardware design and software development, including the image processing algorithms that work within OIS systems."

Nokia suggests its so called 'floating optics' approach picks up and cancels about 50% more movements per second than conventional OIS set ups. Because the impacts of camera shake are reduced, users of the 920 can have the shutter wide open in low light for as long as 0.25s before shake becomes noticeable.

The Lumia 1020, launched last year and boasting a 41Mpixel sensor, features a 'second generation' OIS. "Because the optics are much bigger, the whole lens system rests on ball bearings and is actively moved with very small motors to counteract the unwanted camera movements detected by a gyroscope", said Nokia.

The 'really clever engineering' in the 1020 means 'the camera can tolerate up to five times longer exposure times compared to traditional mobile cameras – a significant benefit in low light situations'.

Not surprisingly, in view of the huge potential, there are several alternative approaches. French company Varioptic proposed in 2010 'liquid' lenses capable of altering their optical properties to address autofocus, but has to date found no takers. Norwegian start up poLight is developing TLens (or Tuneable Lens). Here, a piezoelectric crystal acts as the actuator. Embedded on a thin glass membrane and with a polymer layer between this membrane and the lens support underneath, when a voltage is applied to the piezo element, it instantly forces the glass membrane to bend, generating an optical power variation that allows focusing (see fig 2). The company has contracted TI to manufacture the optical MEMS part.



Meanwhile, French group Wavelens, spun out of CEA-LETI, is working on optical MEMS adapted to autofocus and zoom applications.

Ayat and Dr Mounier both stress these approaches will not only have to offer something special in terms of performance and compactness, but also cost effective and volume manufacturing partners to succeed. They will need time, enlightened backers and even some luck to make inroads into the VCM autofocus business.

Implementing optical image stabilisation
There are different approaches to implementing OIS in a camera phone, the most traditional being one based on module position. Here, the OIS system is a control loop that starts from the X and Y axis gyros sensing the pitch and yaw of hand jitter. The gyros measure angular velocity that, when sampled and integrated over time (via the OIS controller) for both X and Y axes, can provide the direction and displacement of the lens.

Once the displacement and angle of the hand jitter has been determined, it drives the actuators – mostly VCMs at the moment – to cancel jitter by moving the camera module in the opposite direction. In this typical set up, sensors are also deployed to feed back the position of the module being moved as a reference to close the loop. The loop runs in real time at hundreds of times a second, so it can precisely track and compensate for hand jitter, stabilising the image optically. The aim is to have minimal impact on the module height.

Another method is to use the gyro output to close the loop by mounting it directly on to the camera module. Here, the gyro performs two tasks: it detects hand jitter and the camera movement itself.

In both instances, the challenge for the OIS vendor is to 'tune' the final module for optimal performance, mainly due to variation in the mechanical parts and actuators, along with the gyro's orientation offset relative to the module.

Author
John Walko

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