Large field of view (FOV) measurement systems typically provide images that appear clear, but does that guarantee measurement accuracy? This depends on the specific optical design employed.

Large FOV measurement systems now offer an alternative to traditional video measuring microscopes and comparators, overcoming the limitations of small fields of view and shallow depths of focus. Large FOV systems can typically provide a field of view of up to 4.0 inches (100 mm) and a depth of focus exceeding 1.0 inch (25 mm).

One of the primary reasons to consider a large FOV system is its ability to measure features anywhere within the field of view. Since large FOV systems allow viewing more part features simultaneously, stage movement is reduced, leading to high-speed measurement. However, like traditional measurement systems, large FOV systems also have their limitations. In large FOV systems, optical characteristics such as telecentricity and distortion can significantly impact measurement accuracy. When combined with a large field of view, telecentricity is one of the most important features for any large FOV system on the market.

As mentioned, telecentricity is an optical property that ensures the size and shape of an object remain constant regardless of its distance from the lens.

To maintain measurement accuracy with a large field of view and a large depth of focus, an optical system must be telecentric and have very low distortion. A critical consideration for large FOV systems is that a large field of view and a large depth of focus alone do not guarantee that the optical system will produce telecentric images. For example, the human eye has a very large depth of focus; however, placing a finger at arm's length and then moving it closer to the eye will make the finger appear larger. Distortion, often manifesting as a spherical "warping" of the image, is difficult for the human eye to detect but can significantly affect measurement accuracy.

The lenses required for an optical system to produce a truly telecentric image are complex and vary in the degree of telecentricity they achieve.

In large FOV systems, achieving a large depth of focus while maintaining true telecentricity is challenging due to the difficulty in accurately positioning lenses within the focal plane. Therefore, assuming that an image is an accurate representation of the part being measured simply because it appears in focus is risky. Selecting a large FOV measurement instrument that ensures measurement accuracy is crucial.