Scanners have long been the standard tool in the office and in private use to digitize printed material, nearly every printer provides this function these days. The method behind it’s operation is known: traditional scanners capture the images line by line and combine these individual "image strips" in order to produce an overall image.
In industrial applications, the principle of line-by-line imaging has also been in use for a number of years with line scan cameras. This technology is perfectly suited for the inspection of flat continuous materials or web material in order to detect defects or flaws. Such line scan camera systems usually require an additional lighting unit, which illuminates the object with intense light on a narrow strip captured by the line scan camera. These cameras have one or more lines of light-sensitive pixels that continuously scan moving objects with a high line frequency.
Line scan sensors typically operate with resolutions between 512 and 16,384 pixels and edge lengths of 7 μm, 10 μm and 14 μm. In the case of very high resolutions, smaller pixels are used in the design of the sensor in order to take the optics into consideration. Since a sensor with 8,000 pixels and an edge length of 10µm, for instance, could only be "exposed" without distortion by a lens with an image circle diameter of at least 8cm.
In industrial production processes, the objects under inspection are often transported on in-line conveyor belts. Line scan cameras are mounted above or below these belts to capture the moving objects line by line. The lines are then “stiched“ together to produce a continuous image for evaluation. In practice, incremental encoders coupled to the drive unit are typically used to compensate for differences in the transportation speed of the test objects, which sometimes occurs, for instance, due to variations in the conveyor belt drive. The optical characteristics of line scan cameras require a certain minimum distance between camera and test object, which is sometimes around 2 metres. This can cause problems in applications with limited space or potential vibrations.
The technology used in office scanners was neither robust nor fast enough for industrial applications. A few years ago, with the development of the Line Scan Bars, Mitsubishi Electric Europe created a feasible and cost-effective alternative to line scan cameras, which is exclusively distributed by STEMMER IMAGING.
The Line Scan Bars use the same concept as used in desktop scanners. However, image formation in the Line Scan Bars is achieved via a double-row lens array, using gradient index rod lenses, matched to a colour tri-linear CMOS sensor. Each individual rod lens captures an image of a very small region of the target. Thanks to some overlap in the individual images, a clear, sharp, combined image is produced along the narrow line of the sensor at a resolution of 600 dpi. Rod lenses are designed for a working distance of 12mm and allow a depth of field of approx. +/- 0.5mm which makes this technology only suitable for objects with a height deviation of less than 1mm.
Compared with line scan cameras, the Contact Image Sensors are easy to install and held in place directly above the surface of interest at a fixed height of 12mm, with all LED illumination and optics built-in. The CIS can be installed in just a few minutes since it is easier carry out the adjustment directly above the object. Furthermore, they enable the fast acquisition of almost distortion-free images, as the long sensor line maps the object on a 1:1 scale.
One of the major benefits of CIS is their compact and easy-to-install format, which was slightly modified in the new CIS generation and adapted to the requirements of the printing industry. For seamless scanning of larger material widths several sensors can be arranged next to one another.
Mitsubishi Electric’s Line Scan Bars are currently available in two series: the AX models feature different widths of 309mm, 617mm and 926mm with up to 21,888 pixels. Equipped with CameraLink interfaces they are perfectly suited for a variety of imaging tasks.
The more powerful CX models of Line Scan Bars are especially designed for very high inspection speeds. These high-speed models are available in different widths of 367mm, 587mm and 807mm, with up to 19,008 pixels, allowing for extremely fast data output via one or two CoaxPress interfaces depending on the size of the sensor. Reliable surface inspection can be performed at enormous speeds delivering high-quality, distortion-free images up to the edges of the scanned material. They allow the scanning of surfaces at a speed of 138 metres per minute with a maximum resolution of 600 dpi. The commonly used resolution of 300 dpi enables reliable inspection of up to 450 metres per minute, whereas the resolution of 150 dpi even allows inspection speeds of 1,200 metres per minute.
Typical application areas for the Line Scan Bars include the print and thin film production industries, where they are used to detect surface defects on paper, plastics, fabrics, banknotes and even holograms. The Swiss company Hunkeler AG, for example, one of the world market leaders in the paper processing industry, has been successfully using the Mitsubishi Electric Line Scan Bars for its products for years.
In addition, the Mitsubishi Electric products can also be used for the surface inspection of wood, glass and other materials, they can even form the heart of an automated optical inspection system for circuit boards.
Other interesting application areas include PCB inspection in the electronics manufacture or the inspection of solar cells or wavers in the solar industry.
The packaging industry is an up-and-coming market for the Line Scan Bars: The basic materials used for packaging are often web-shaped materials, such as paper sheets, films or aluminium strips, which are ideally suited for these surface inspection systems.
The Mitsubishi Electric Line Scan Bars are among the most important and technically interesting products distributed by STEMMER IMAGING. They are, however, just a small selection of STEMMER IMAGING’s comprehensive product range. Europe’s largest imaging technology provider supplies its customers with all components and services required to create reliable machine vision systems in every performance category.