COLOUR IMAGING


‘If you have good sensitivity of a vision system, you can produce better colour’


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cameras that provide high-end colour reproduction. It recently launched 4k and 8k prism-based line scan cameras with 10GE and SFP interfaces. Product manager Paritosh Prayagi said:


‘Trilinear sensors are structured with individual lines – that’s R,G and B when you’re talking about colour imaging. You have the sensor on the same physical plane, but the point that it looks on the object is very different: an object moving at an unknown speed, or something that is rolling or in freefall, for example. ‘Many fruits and vegetables roll on a belt,


or are dropped from a specific height with rotation, which can cause problems when using trilinear cameras. Tis is because, if they are not stable, the trilinear camera cannot compensate for the movement in high-speed.’ Tis is an area where a prism-based


camera comes into its own. Te prism separates the light onto three sensors to get the RGB image. ‘Basically, there is a single optical access that is reflected from the object, and then we split the light inside the camera,’ Prayagi explained.


Te right path Tis method is beneficial because it means that there is no need for optical compensation, as there is only one path to the object, as opposed to trilinear’s three for RGB. ‘It actually results in much better colour information,’ said Prayagi, as there is no blur, and no need to compensate for the positioning mismatch from the object’s surface, so you get a better colour.’ Another advantage of the prism approach,


said Prayagi, that also results in better sensitivity, which influences the colour, is that there are three individual sensors in the prism approach. ‘Tis means that we can control the analogue gain, which is the gain on the sensor level. It’s basically the signal amplification and we can control the exposure time for individual sensors.’ Prayagi went on to explain that CMOS


sensors are more sensitive on the green wavelengths, so a CMOS sensor projected onto a surface for white balancing will produce a very green-dominant object. ‘Te red and the blue channels then need to be balanced, in order to see a white


Objects with anomalies, such as the wrong colour or size, can be identified for rejection


background,’ he said. ‘In trilinear cameras, you pull up the signal of the blue and the red channel to match with the green, but in the process of doing that, you also amplify the noise, which is not ideal.’ Te prism approach, however, provides the opportunity to amplify the gain, but also change the exposure time per channel, thanks to the three individual sensors. Tis way the noise does not have to be amplified. ‘Tis results in a better colour reproduction,’ said Prayagi. ‘If you have good sensitivity of a vision system, it means that you can produce better colour.’ In terms of the evolving applications for


colour for which the prism-based line scan camera allows, one area where it is proving particularly effective is web inspection. Prayagi said: ‘In this type of application, the web is moving at very high speeds and


20 IMAGING AND MACHINE VISION EUROPE OCTOBER/NOVEMBER 2020


it can migrate, to an extent. Tere are many mechanisms to reduce the vibration of the webs. It’s a 200- to 300-year-old discussion, how to remove the vibration of moving objects while printing, but still there is a vibration, and this vibration can be visible in the image.’


Mind the gap Trilinear technology is forced to compensate for the line gap caused by the vibration, between the R, G and B lines, which is achievable, said Prayagi, if the vibration is constant. ‘Of course, in reality it is not. So you start to see these colour fringes.’


Another option is to use dual line


technology, which is a bayer-pattern line scan. But, Prayagi added, there still needs to be compensation for one line, because,


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