QMT Features: August 2014
Ultra-high resolution imaging
Pixel shifting techniques can achieve very high resolutions in industrial imaging says Stemmer’s Mark Williamson


Many industrial inspection applications such as flat panel display inspection, PCB inspection and document scanning require ultra high resolution imaging to allow the required level of detail to be produced in the image.

Flat panel displays (FPDs) cover a growing number of technologies including OLEDs (organic light emitting diodes) and LCDs (liquid crystal displays). Each new generation of FPDs has increasing pixel densities requiring even higher resolution and sensitivity for quality inspection combined with pressure not to increase inspection times. 

Document scanning applications include continuous verification and/or quality inspection of numbered print and inspection of symbols and labels on web, sheet or single documents, as well as inspection of security features by checking the presence, position and integrity of applied features such as such as foil and hologram devices and base paper inserts like security threads. 

High resolution imagingThe majority of high resolution imaging applications utilise area scan cameras and recent advances in CMOS and CCD technology have resulted in the availability of camera sensors with more and more pixels, providing ever increasing spatial resolution while maintaining or improving the camera frame rate. However, the number of available pixels is not the sole consideration for industrial imaging cameras. Other requirements include pixel size & shape, sensitivity, frame rate, full image exposure control and triggering characteristics, dynamic range, spectral response, image pre-processing, partial scanning, advanced multiple region readout and sequence control among others. In addition cameras are characterised by the interfacing standard that they employ to transfer data to the host computer for measurement and analysis (e.g. CameraLink, GigE Vision, USB3 Vision, CoaXPress, Camera- Link HS etc). 
How many pixels are enough?Some recently announced high resolution area scan cameras include the 29 Megapixel AVT Prosilica GT6600 and the Thermoelectric Peltier cooled Vieworks VP-29MC-M/C 5, both of which have sensors with 6576 x 4384 pixels while the new JAI Spark SP-20000C-PMCL is a 20 Megapixel camera providing 5120 x 3840 pixels. But what if the application requires more pixels? One solution would be to use multiple cameras to produce a number of images that are stitched together, or to keep to a single camera and move the sample or the camera to produce a number of images that can be stitched together. Another option is to use pixel shift technology. A different approach is the use of line scan cameras, which in their simplest form use a sensor with a single line of pixels. Typical resolutions vary from 512 x 1 pixels up to 16384 x 1 pixels. Line scan cameras build up an ‘area image’ (typically in a frame grabber) line by line by moving either the sample or the camera.
The resulting image has a fixed width, but variable length which can be adjusted to suit the sample unlike the fixed aspect ratios from area scan cameras (typically 4:3, 16:9 or 1:1). Even with a 4:3 ratio a 16k line scan camera could produce a 358 Mpixel image compared to around 20 - 30 Mpixels from an area scan camera.

However, relative movement at an appropriate rate between the sample and the camera is essential to allow the image to be produced and must be uniform otherwise distortion will appear in the image. In addition, if the line scan camera’s line sensor is not perpendicular to the direction of movement, the resulting image will be skewed.
Pixel shift technologyPixel shift technology significantly extends the resolution capability of area scan sensors. The Vieworks VNP- 29MC CameraLink monochrome and colour cameras feature a 29 Megapixel resolution (6576 x 4384 pixels) CCD sensor as standard, but also have pixel shift technology which can provide an extended resolution up to 260 million pixels (19728 × 13152) for ultra high resolution applications. The sensor is mounted on a precise piezoelectric crystal stage which allows the CCD to be nano-shifted by 1/3 or 1/2 of a pixel. This enables the standard resolution to be extended by 4 times to 115 million pixels or by 9 times to 260 million pixels. 
The sensor can be shifted precisely by 1/2 pixel in the X and Y directions, with the resulting image being a combination of the 4 individual images giving improved resolution (4 shot result image) in comparison with the standard output image (1 shot result image). Image combination is carried out in software on the processing PC. (See Fig 1)
The pixel shifting process has additional benefits for colour imaging, since unwanted artifacts such as colour moiré or false colour pixels often associated with Bayer colour CCD cameras are eliminated. In addition, the colour resolution is optimized (See Fig 2). 
The VNP-29MC sensors can be thermo-electric Peltier cooled to as much as 15 degrees below, reducing noise and enabling exposure for a long period of time or at higher gain levels to increase camera sensitivity. This makes the cameras ideally suited to low light or low contrast level or non-uniform brightness industrial applications. The improvement in resolution is illustrated on a section of banknote shown on the first page of this feature. The smaller inset picture of the banknote is recorded at the standard resolution settings, but the pattern is pixelated and unclear. The larger picture shows the same region using pixel shift technology and the pattern is clearly resolved. In addition to the obvious benefits of improved resolution, at any given resolution setting, the pixel shift gives a larger field of view if fitted with the appropriate lens, which means it could avoid the need to stitch multiple images together or remove the need for multiple cameras, saving both time and money.www.stemmer-imaging.co.uk
  
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Rob Tremain Photographer
www.4exposure.co.uk
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