Defective pixels, defect pixel correction feature
Defective pixels, also known as dead pixels, show incorrect light levels. Most defective pixels either show a too bright or a too dark light level. The neighboring pixels show correct light levels. Artifacts such as smear, blooming, or Bayer artifacts have other causes.
All CCD and CMOS sensors from every sensor manufacturer typically have some defective pixels, which are the result of inevitable minor impurities or manufacturing process issues. Sensor manufacturers perform quality control inspections and sensor grading to ensure that sensors meet acceptable defective pixel counts. Nevertheless, aging through heat, intense illumination, and exposure to gamma rays may induce additional defective pixels over time. This incremental number is considered normal and is neither warranted by the sensor manufacturer nor by camera manufacturers. You can find warranty information at: www.alliedvision.com/en/support/warranty.html
The count of defective pixels typically scales with sensor resolution.
Example: 4 to 5 defective pixels can usually be located in the center area of sensors < 1 Megapixel, up to 10 or more in a multi-megapixel sensor. The precise counts are often confidential between the sensor manufacturer and the camera manufacturer.
Consumer cameras have defective pixels too, but the camera firmware typically hides them through nearest-neighbor interpolation algorithms. At the slow frame rates of consumer cameras and for subjective viewing of tourist snapshots, that is generally what the customer wants. But for machine vision applications and many scientific applications, the user wants only 'real data' without the camera doing any interpolation. If any interpolation needs to be done, the user wants to choose the algorithm and know which pixels were computed, rather than have it done magically in the background.
Some industrial camera manufacturers are starting to offer user-selectable correction features, typically on more advanced CCD cameras and on CMOS cameras (since CMOS traditionally had more defective pixels than CCD). Currently, Allied Vision Technologies offers this on Pike and Stingray CCD cameras and on selected CMOS cameras in other families. The "defect pixel correction" feature relies on lighting, exposure, and gain setting. Higher gain and longer exposure times tend to show more bright defective pixels in the dark field.
When selecting a camera for your application, please ask your Allied Vision sales representative whether defective pixels might be a concern, whether your software could make the adaptation, or whether you should choose a camera with the defect pixel correction feature.
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