SUPPLEMEN
SUPPLEMENT FEAT RE FEA
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COLOUCOLOUR LINE SC
SCANNIN G
NG COLOUR LIN SCAN TECH rk llia so latest colo lin sca
alternative approach are three or four-chip cameras, where a prismis used to separate the component wavelengths which are simultaneously detected by the sensors. Perhaps not surprisingly, each of these methods has its own particular
advantages. The infographic shown in Figure 2 not only shows a schematic for the different approaches, but highlights their performance
ine scan imaging is a well-established technique used inmany different application areas. These include the inspection of continuousmaterial and imaging obj
L 1). It is particularly useful if the obj
bjects on a conveyor belt (Figure bjects
under inspection vary in length and a high resolution image is required. Line scan is also a very convenient way of imaging cylindrical components. Trying to do this with an area scan camera would be difficult and would involve correcting the distortion caused by imaging the curved surface, plus having to takemultiple images to capture the en tire surfac e. However, by rotating the cylinder in front of a line scan camera an ‘unwrapped’ image of the entire surfa ce is generated .
THE NEED FOR COLOUR HE NEED FOR COLOUR
As in traditional area scan imaging, monochrome line scan sensors are
perfectly adequate for many applications. However, when colour is an essential element of the inspection, then a colour solution is required. Applications for colour line-scan imaging include recycling (e.g multi-coloured bottle tops), postal sorting, web inspection, food grading,
label inspection material
and assembly inspection of wire harnesses. Monochrome line scan
cameras utilise a sensor with a single row of pixels, which are scanned at a high line frequency to build up an image of the object moving underneath. However for colour segmentation into the three basic colours red green and blue (an d sometimes a fourth, IR) more complex sensor technology is required.
,
COLOUR LINE SCAN METHODS Colour line scan cameras fall into two categories: single chip cameras, where the output fromtwo, three or even four lines is
COLOUR INE SCAN ME HODS S8 S8 MAR MARCH 201 2017 | MACH MACHINE VISI VISION SYSTEMS EMS
Figure 2. Colour line scan infographic
Figure 2. Colour line scan infographic
Figure 1. Line scan image acquisition on a conveyor
Figure 1. Line scan image acquisition on a conveyor
accuracy, speed and cost helps indicate the most appropriate choice for a particular application.
S NGLE CH P CAMERASAMERAS SINGLI CHIPI
The simplest approachmakes use of a dual-line sensor. For each horizontal pixel, real data exists for two out of the three colours. Using onboard preprocessing, the third colour is
interpolated fromthe adjacent pixels, so that image data for red, green and blue can be transmitted. There are a number of formats in which the coloured pixels are positioned. One uses an alternating pattern similar to the area scan Baye r format. As an alternative method, the first row is covered with green filters, whereas the pixels of the second line use alternated red and blue filter layouts. One advantage of this second format is that we get full resolution in one wavelength. For many applications this is a good compromise in a cost-effective package. For tri-linear sensors, three lines are positioned adjacent to each other, spaced by an even number of pixels . The spatia l differences between the lines are compensated for either in the camera, frame grabber or software. This
architecture has good sensitivity, but is limited to imaging flat, 2D obj
bjects. A key
in terms of also
COLOUR LINE SCAN TECHNO LOGYLOG Y
and sol tions avai ble tom nufacturers combined to generate the output. An
Mark Williamson of Stemmer Imaging looks at the latest colour line scan developments, technollogy and solutions available to manufacturers
mme ma lo
ts, t ech
ooks at the o
consideration when comparing different tri-linear cameras is the interline spacing, which can vary betweenmanufacturers. Cameras with closer line spacing are less prone to vibration and webmovement errors and the camera’s performance will be closer to that of a prismbased camera .
THREE CHIP AMERAS HREE CHIP CAMERAS
In this colour line scan configuration, a prism splits the light into red, green and blue components, each of which is detected by its own sensor within the camera. This arrangement can cope with 3D obj
bjects without suffering spatial
displacement problems, as the R, G and B pixels are coincident for any given object position. This means that there are no halo effects. Prism-based systems are well suited to complex, high speed continuous applications .
COLOUR AND INFRARED INE SCAN AMERAS
COLOUR AND INFRARED LINE SCAN CAMERAS
For some print applications or for the detection of security features an infrared channel may also be required The two options for this are either a quad linear line scan system, or a prism based
.
system. These cameras consist of either: • A sensor with three RGB-lines and an additional line with a dichroic filter instead of a colour filter to block either the visible or the IR range.
• A prism with four sensors. Three of these sensors cover the RGB wavelengths and fourth is sensitive in the NIR .
CHOOSING HE BES CHOOSING THE BEST CAMER AMER A
Understanding the basic differences in line scan technology is clearly important in choosing a colour line scan camera, but there are many other factors to consider, such as speed of inspection needed which determines the camera’s line rate requirements, and the type of data transfer interface to be used (GigE Vision, CameraLink HS, CoaXpress etc). With a huge portfolio of line scan cameras and a wealth of experience in line scan applications, Stemmer Imaging can advise end users, OEMs and systems integrators as to the best solution for a particular application.
Stemmer Imaging
www.stem er-i
T: 01252 780000T: 01252 780000 ww temmer-imaging.
o.uk aging.co.uk /AUTOMATION AT /AUTOMATION
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