Precision Measurement Depends Upon Resolution
A determining factor for delivering high precision and low uncertainty in machine vision metrology is the resolution of the acquired image. In this context, the term resolution (or image resolution) means the size of an individual pixel in real-world units. Simply put, if a camera sensor contained 1000 pixels in the horizontal direction, and optics were incorporated that acquired an image that covered an area in the real-world scene that were 1 inch in width, a single pixel would represent 0.001”. Note that this is a fundamental metric that does not change with camera manufacturer or analysis software.
How many pixels are enough for a specific application?
As a gauge, the smallest unit of measurement (some exceptions noted later) in a machine vision system is the single pixel. As with any measurement system, in order to make a repeatable and reliable measurement one must use a gauge where the smallest measurement unit (as a general rule of thumb) is one tenth of the required measurement tolerance band. In the example just described, the system could be estimated to provide a precision measurement to approximately +/- 0.005” (a tolerance band of 0.01”, ten times the gauge unit).
Engineers first using machine vision for measurement often seriously underestimate the number of pixels required to achieve a desired level of measurement precision uncertainty. In fact, it may require multiple cameras, specialty cameras such as line scan imagers, or multiple views of a single part to achieve the required resolution for the specified inspection tolerance.
Expand The Resolution If Needed
Sometimes we can squeeze out additional resolution in an imaging system mathematically using algorithms that report features to sub-pixel repeatability. Some examples would be gray-scale edge analysis, geometric or correlation searching, regressions such as circle or line fitting, and connectivity in some cases. If one can take into account sub-pixel results through the use of these tools, then the smallest unit of measurement can be less than a single pixel, described earlier. Note though, that estimates provided by vendors for sub-pixel capability are only that, and usually are made for best-case imaging, optics, and part presentation. Take care in using arbitrary sub-pixel expectations as a determining factor for specifying system measurement capability. Test the system with actual parts and images to empirically determine the sub-pixel capability.
Use High-resolution Optics
Imaging is a function of optics and lighting (and as we will discuss later, part presentation). For most applications, the only optics used will be a lens assembly, but the selection of that lens is critical to the metrology application. Beyond delivering an image of the proper real-world size to the sensor, for metrology the lens must reproduce the image as accurately as possible without distortion. Furthermore, lenses have a resolution metric as well, which often is specified as line pairs per mm or inch (lp/mm, lp/in), and by extension may have a specification for MTF (modulation transfer function) or more simply the ability of the lens to produce high contrast at high lp/mm. The higher the pixel count, the more important these lens metrics become. Ensure that the specified optics are high-quality, high-resolution products designed for machine vision applications.
Telecentric lenses are extremely useful for measurement applications in many cases. A telecentric lens uses a combination of optics to virtually eliminate all distortion caused by parallax in an image. The result is an image that is parallel to the sensor for mostly all of the image. Planar geometric relationships (in the image plane) are completely preserved making measurements more direct and straightforward. As always, test the imaging before specification.
For applications that require a very small field of view (for example less than a few millimeters), consider the use of microscope optics and/or high-magnification optics specially made for machine vision. These are available from a number of vendors. It is not recommended that standard optics be pushed to higher magnification using extenders or add-on magnification.