Optimizing visual measurement

While many consider multisensor measurement a mature market, there are still details and capabilities that can significantly impact its effectiveness, its suitability for any particular application, and the user’s return on investment. For example, it is widely believed that ‘if you can't touch it you can’t really trust the data.’ While this has some basis in fact, there are visual technologies that are every bit as accurate as contact measurement (in addition to delivering visual’s particular advantages in certain applications). For example, magnification is a key factor contributing to the accuracy of visual measurements and can greatly improve an optical system’s accuracy. Unless you know exactly what you are going to measure, both now and in the future, you will want a wide range of readily adjustable magnifications. A wider field of view lets you quickly cover a larger area; while a higher magnification lets you address small parts or finer features with greater accuracy. Higher magnification can also narrow depth of field to eliminate irrelevant details.

Another key factor in the accuracy of visual measurement is the angle at which the system views the feature being measured. Ordinary lenses collect images from a conical area in front of the lens. This creates a true image of only what is in the center of the field of view; anything not in the center is viewed at an angle and, therefore, distorted. Telecentric lenses view everything in their field of view ‘head on’ and without distortion, greatly improving accuracy.

Features can tell you a lot about a system’s capabilities, but the best way to determine the real accuracy of a visual technology is GR&R (gage repeatability and reproducibility), the comparison, preferably in a live demonstration, of repeated measurements. That will tell you how accurate the system is at the demo site, but realizing that same accuracy in your own installation depends on one more step: the qualification of a system and certification of equipment during your installation. For maximum accuracy, a system should be qualified in true 3D using a step gauge. And at installation, the system must be calibrated to ensure that it is performing to its full potential. In this process, the accuracy of visual sensing can be adjusted to match that of the system’s contact sensing.