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Greg Blackman on the latest developments in line scan camera technology

Certain inspection tasks lend themselves particularly well to line scan imaging. Webs of paper or textiles that flow through machinery in one continuous reel, for example, are best inspected using line scan cameras, building up an image line by line. Line scan imaging is also the method of choice for inspecting electronics like PCBs and flat panel displays, but here it’s resolution that’s the key consideration.

High-end line scan cameras provide up to 16,000 pixels per line and, as Marc Damhaut, CEO of Euresys, explains, a 16k camera scanning a square object will result in a 16 x 16k image. ‘That’s a lot of pixels,’ he says, ‘around 200 million pixels in total. There’s no 200 Megapixel camera and so this level of resolution is something that can only be achieved with a line scan system.’ Euresys’ provides its Grablink Camera Link frame grabber series for line scan applications.

With cameras generating these large amounts of data, high-speed vision interfaces have to be used to offload the images. Teledyne Dalsa developed HS Link for its high-resolution, high-speed line scan cameras, which has subsequently been adopted by the Automated Imaging Association (AIA) as a global standard in the form of Camera Link HS. A draft version of the standard, which offers 33.6Gb/s bandwidth, was released at the Vision trade fair in Stuttgart at the beginning of November.

CoaXPress, a global standard developed by a consortium of companies including Active Silicon, is another high-speed interface suitable for line scan applications. Active Silicon’s CEO Colin Pearce comments: ‘The key thing is speed of inspection. If you can inspect flat panels faster, then the cheaper they become to manufacture. Also, with higher resolution you can inspect smaller features – for PCB inspection the tracks are getting finer and finer so you need a higher resolution. To analyse images at the same speed with higher resolution you need much faster bit rates.’

Active Silicon’s FireBird CoaXPress frame grabber card features four inputs, each providing 600MB/s data rate and a maximum speed of 2.4GB/s. The frame grabber can be used with e2v’s ELiiXA+ line scan camera, a 16k camera delivering 100,000 lines per second and using the CoaXPress interface.

‘The maturity and bandwidth of CoaXPress was key for us when choosing the interface for the ELiiXA+ camera,’ comments Sébastien Teysseyre, marketing manager, industrial cameras at e2v. ‘Being able to transmit both data and power over up to 40m of simple and cheap coaxial cables was an added benefit.’

Lighting the scene

One of the challenges with line scan applications is that the faster the line speed, the less light is available. e2v has addressed this with its multi-line sensor technology. Chris Beynon, technical director at Active Silicon, explains: ‘Unless you’re doing something clever with the sensor, as e2v has done, you’ll start getting worse signal-to-noise ratio as the speed increases. With any camera, the faster you run and the more pixels you have for a given size of sensor, the signal-to-noise ratio can become a problem. e2v has developed its multi-line sensor technology to solve that.’

The ELiiXA+ camera uses a four-line CMOS solution, in which the same image is sequentially acquired four times and then summed to increase the amount of light captured. This gives a good signal-to-noise ratio while maintaining high spatial resolution without any particular mechanical constraints.

Teledyne Dalsa uses similar tricks in the form of its time delay and integration (TDI) technology, which accumulates multiple exposures of the same object to increase the integration time.

Markus Schnitzlein, CEO of Chromasens, notes that one disadvantage with line scan applications is that the intensity of the light source required has to be high, as you can only integrate over the time period of one line – a typical integration time is 20 microseconds. He adds that, on the other hand, the light can be focused to one narrow line with high intensity (up to five million lux), illuminating only that particular section of the object under inspection. Chromasens has recently introduced the allPIXA colour line scan camera, with a line frequency of 110kHz.

‘Homogenous illumination is often required for line scan applications,’ comments Chris Varney, managing director at Laser Components, which provides laser line illumination suitable for line scan systems. ‘Cameras are very sensitive to changes in light intensity and therefore it’s important that the light is homogenous along its length.’ He adds that power consumption is much reduced with a laser line compared to illuminating the whole part. ‘With a line laser, all the power from the laser is used without wasting energy.’

Basler’s product manager, Marc Nehmke, adds that sufficient illumination for high-speed imaging is expensive and therefore camera models with high sensitivity are valuable. Basler has launched its Racer series of line scan cameras, which offers high sensitivity and 2k and 4k resolutions, with 6k, 8k, and 12k resolutions planned for release in 2012.

CMOS sensors

In areas like flat panel display inspection, banks of cameras that can number 30 or more are arranged to cover the surface of the panels. Here, high resolution is particularly beneficial, as it allows manufacturers to reduce the number of cameras required. It’s also important for the cameras to be small so that they can be integrated easily. Basler’s Racer series has a small housing with a width of 56mm, which can incorporate 2k to 12k resolution sensors. Nehmke notes that keeping the sensor housing small is easier with CMOS technology due to the low power consumption and excellent heat dissipation.

He also adds that pixel sizes are decreasing from 14μm to 7μm or even 3.5μm, which means the sensor can be housed in more compact enclosures. ‘A 12k sensor isn’t as large anymore using a smaller pixel size.

‘Smaller pixel sizes and CMOS technology mean smaller camera housings can be made and, in particular, the width of the enclosures can be kept small,’ Nehmke continues. ‘This allows a cascade of cameras to be installed in a multi-camera system for surface inspection.’

Nehmke feels CMOS sensors have gained ground on CCD over the last few years in terms of image quality. ‘There is a general trend towards using CMOS sensors over CCD in both line scan and area scan cameras,’ he says. Basler’s Sprint line scan series is based on CMOS sensors, while its Runner line scan series uses CCD.

Active Silicon’s FireBird CoaXPress frame grabber provides a maximum speed of 2.4GB/s, essential for high-end line scan inspection tasks

CMOS sensors are faster than CCDs, as every single pixel will have its own processing path, its own A/D converted and amplifier, and every pixel is read out separately. On the other hand, as Schnitzlein of Chromasens points out, CMOS sensors are not as uniform as CCDs, because a CCD has only one or two electronic processing pathways.

‘The big advantage of a CMOS sensor is that it’s fast and you can read out individual regions of interest,’ he says, but adds that CMOS sensors capture less homogeneous images than CCDs.

‘Chromasens will continue to incorporate CCD sensors into its line scan cameras as long as they are fast enough. Most of our applications are related to high-quality imaging and making measurements from those images, which requires the homogeneity provided by CCD sensors,’ Schnitzlein states.

‘To make measurements, you need the same properties in every part of the image,’ he continues. ‘For example, we are doing a lot of colour print inspection where colour intensity is measured on every part of the document. The same accuracy of measurement is required over the entire document. We can’t live with large deviations in noise in different parts.’

Agreeing with Nehmke, Schnitzlein notes that one trend for colour line scan imaging is to reduce the pixel size and incorporate more pixels onto one line. This, he notes, is not good for image quality as you cannot gather more photons in a smaller pixel. Also, fabricating optics for these smaller pixels becomes difficult.

‘The largest line scan sensors available are around 10cm in length,’ Schnitzlein says. ‘To fabricate larger and larger sensors becomes very expensive and 10cm is really the limit. Most technological advances are aimed towards reducing the size of sensors while increasing the pixel counts, which means shrinking the size of the pixels. Some area scan sensors have pixel sizes of 2μm, which is close to the wavelength of light. It’s really complicated to get the best trade off between sensor size and number of pixels.’

Motion capture

In line scan applications, the camera scans the object under inspection, exactly in the same way as a photocopier. In order to capture a high resolution image without distortion, it’s important to know the speed of the object moving relative to the camera. Euresys’ Grablink Camera Link frame grabber series provides control, not only between the frame grabber and the camera, but also between the frame grabber and the motion control system.

‘We need to know where the object is in relation to the camera at all times so that each line can be acquired in the correct position,’ explains Damhaut of Euresys. ‘Therefore, we need a tight control between the camera and the frame grabber – that’s fairly easy – but also between the motion control unit of the linear stage that moves the object.’

The Grablink frame grabber series has driver interfaces for the motion encoder, allowing the movement of the linear stage to be known. This means the camera can be synchronised with the movement of the object. Image acquisition is thus controlled and any distortion avoided.

‘Traditionally, encoders have had set resolutions – they will trigger the camera every 10μm, for instance,’ Damhaut says. ‘The Grablink frame grabber allows the user to choose the resolution they want to acquire the image at, with great flexibility, without changing the encoder. When initially debugging the system you might not know the resolution, or you might want to change the resolution depending on the part. This frame grabber allows you to do this.’

The Grablink board uses dividers and multipliers to acquire lines from the camera at a variable resolution. New Grablink cards also have the ability to detect which direction the part is moving. ‘A standard encoder card won’t be able to tell you this,’ explains Damhaut. ‘Occasionally there can be vibration in the machine causing the object to move backwards for very short time periods. The Grablink cards can analyse the information from the encoder, which includes directional data, to ensure image acquisition ceases when the object is moving backwards and is started again at the right location when it reverts to forward motion.’

Most line scan applications don’t put excessive demands on the speed or resolution, with the majority of inspection systems operating at 20 to 40kHz. Of course, there are areas like flat panel display inspection requiring very high speed imaging and line scan sensors will continue to get faster with higher resolution to cater for these applications. CoaXPress and Camera Link HS are also important advances providing the bandwidth to cope with the data rates involved.