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Repeatability and reproducibility for precision machine vision, medical, and global security systems

Posted by Gretchen Alper on Thu, Jun 20, 2013

For OEMs building systems in series production, whether in machine vision equipment, medical devices, or global security products; repeatability and reproducibility of the cameras can be just as important as raw performance.  Image uniformity can influence the accuracy of the system or the degree of closeness of measurements to the actual (true) value.  But the precision of a measurement system (or the degree to which repeated measurements under unchanged conditions show the same results) depends on repeatability and reproducibility.  (Click here for a nice comparison of accuracy versus precision.)

A general definition of repeatability is the variation in measurements taken by a single person or instrument on the same item and under the same conditions.  With respect to cameras, this refers to the consistency of a single camera to generate the same results despite changing working conditions, such as temperature, vibration and time.  To achieve this consistency, camera design considerations and specialized functionality are necessary to control and to maintain the overall camera performance.

The stability of a camera reduces the down time of the overall system or additional software adjustments for the OEM.  It avoids unnecessary customer support and maintenance and enables them to implement more complex measurement algorithms, such as for 3D measurements involving multiple images. 

In general reproducibility is defined as the ability of an experiment or study to be accurately reproduced, or replicated, by someone else working independently.  In the context of cameras, it is referred to as camera-to-camera consistency.  Procedures during incoming inspection and throughout manufacturing such as alignment and final testing, confirm the reproducibility.

For instance, at Adimec we start with an incoming test of every image sensor to verify compliance to our own specifications.  This provides us with a dependable starting point and allows us to reproduce cameras with tight tolerances. For our customers we have tightly controlled outgoing qualification and camera inspections that comply with their incoming inspections.

Besides working with well-established camera design rules one aspect that contributes to highly reproducible cameras is very precise image-sensor mounting and alignment to the optics. This allows our customers to reduce the mechanical and optical alignment time during manufacturing of their systems. In our factory the image sensor is positioned in six degrees of freedom (3 translational + 3 rotational) to guarantee that the active pixels are always in the same place (within certain well-defined tolerance ranges) with respect to the mechanics of the camera or even the supplied optics.  A key to the accuracy and quality of results achieved in Adimec cameras is the fully automated sensor mounting and alignment processes equipment and procedures we use. 

In addition, each camera undergoes a rigorous, individualized assembly and test procedure process during which mechanical and electrical adjustments are made to ensure all cameras are consistent and documented.

See a video of extended shock testing:

Thorough final testing makes sure every camera (or electro-optical module) in a series is as close to identical as possible.  Reproducible cameras allow for simpler tool matching, drop-in replacements, and automated set-up rather than complicated, costly and time-consuming alignments each time. 

For OEMs where the camera is a critical component of the system, repeatable and reproducible cameras support the precision requirements while the image quality and other performance specifications contribute to the accuracy.

 

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Topics: Vision System Optimization

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