Recent advancements in Contact Image Sensor (CIS) technology are transforming machine vision systems. With high speed, high resolution, high dynamic range, and true metrology, CIS is ideal for inspection tasks in spatially restricted environments, including battery, print, and PCB production lines.
One of the CIS advantages is its small form factor compared with conventional line scan cameras. The working distance for a CIS is typically 10~20mm from the object being inspected, as compared with 250~500mm using a conventional line scan camera-based inspection system. Inline automatic optical inspection (AOI) is now becoming an important part of a modern production line to enable real-time, high-throughput quality control. For production equipment with compact designs, only CIS can fit. CIS also offers a higher degree of integration that combines camera, lens, and lighting components and makes system designs much quicker and easier. Because of these advantages, more and more machine vision applications are adopting CIS technology.
High speed and resolution: boost system throughput
Speeds and resolutions are key technical specifications in vision systems and are currently the limitations of many CIS products. That is why the next generation of CIS technology needs to overcome these limitations and boost imaging performance. For example, AOI requires high speeds and high resolutions to meet the throughput requirements of battery production lines, which play a key role in today’s clean energy strategy of transitioning to electric vehicles (EVs). The combination of smaller pixel sizes with higher line rates ensures that the required detectability is met while the system throughput is not compromised.
High resolution not only improves detectability but also increases measurement accuracy. Figure 2 shows the difference in the measured Modular Transfer Function (MTF), mean value, and standard deviation of black and white line pair targets with 45 µm/line and 500 µm/line using 900dpi and 600dpi. The latter is the common resolution most CIS modules offer today in the marketplace. However, accuracy is significantly improved—by about 50%—by using 900dpi.
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28µm (900dpi) |
42 µm (600dpi) |
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MTF (target 45 µm/line) |
40.9% |
27.6% |
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Measurement (target 500 µm/line) |
Mean value |
501.2 µm |
504.8 µm |
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standard deviation |
5.928 µm |
13.33 µm |
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High-speed data transmission from an AxCIS module to the frame grabber is supported using Camera Link HS (CLHS) interface with fiber optic cables. Each fiber optic cable can handle 2.4Gigapixel per second of data with a long cable length of 300 meters. A frame grabber can connect to multiple CIS modules, depending on speed requirements, to reduce the system-level cost. In addition, fiber optic cable has complete immunity to electromagnetic radiation, and this significantly improves data integrity in high-speed transmission in a harsh industrial environment.
High dynamic range: detect dark and reflective materials
In monochrome mode, advanced CIS technology uses a dual-line sensor with independent exposure control. This unique sensor architecture enables high dynamic range imaging with different exposures for each line to capture bright and dark scenes respectively, in a single scan. The two images captured by each line can be output and processed separately or be fused into one high dynamic range image before processing. The technology has been proven to be effective, for example with imaging relatively dark electrodes coated on highly reflective metal rolls, significantly improving the detectability under various lighting conditions.
Metrology: high accuracy with no missing pixels
Another reason to use CIS is the telecentric characteristics of a self-focus lens that results in undistorted images with 1:1 optical magnification for metrology, in addition to detecting defects. Currently, most CIS modules in the marketplace use a butting process to butt the silicon dies together within a CIS. This results in uncontrollable gaps in between the individual die with missing pixels. Data interpolation is therefore needed for those missing pixels. This approach not only misses true data but also results in uncontrollable sensor width when using a poor manufacturing process.
We’ve used a different design—by staggering CMOS sensor architecture—that eliminates any gaps in between the silicon. All dies are staggered to cover the entire field of view (FOV) without any missing pixels. The module’s image processing aligns the images from each die and removes the overlapped pixels. The unique design enables true metrology applications and provides accurate measurement in the horizontal direction. After calibration, about 50um accuracy can be achieved at 20°C ambient temperature over 800mm field of view.
Unlike conventional line scan camera applications, a CIS is placed very close to the samples being inspected. Therefore, it needs to be protected from dust. Therefore, when selecting a CIS, it is important to ensure there is an IP60 rating on its optical path, so it won’t be contaminated by a dusty environment.
Trends in Contact Image Sensors
Originally developed for consumer electronics such as printers and scanners, Contact Image Sensor technology has found important applications in machine vision. Markets will continue to drive the technology to higher speeds and resolution to meet the increasing demands. A larger working distance and depth of field are also required to accommodate multifield lighting configurations. For certain materials, multispectral capability is also needed. The integration of all these advanced features will help the industry move forward to increasingly higher-value solutions to accomplish complicated tasks.
Conclusion
The continued development of Contact Image Sensor technology is opening new possibilities for machine vision systems in industries where precision and space efficiency are critical. The ability to deliver high speed, resolution, and dynamic range, coupled with true metrology in a compact form, makes CIS a key component in industries like battery production, printing, and PCB inspection. By addressing challenges such as pixel gaps, high-speed data transmission, and dynamic range, CIS technology is well-positioned to meet the increasing demands of modern production lines. Companies that leverage these advancements will be well-positioned to meet the growing demands of modern production environments.
