What is a hand-held width measuring device called that allows for precise measurements?

The hand-held width measuring device that allows for precise measurements is known as a Vernier Caliper. This instrument consists of a main scale and a sliding vernier scale, which enables the user to measure both internal and external dimensions with high accuracy. It is especially useful in optical settings where exact measurements are critical, such as when measuring the thickness of lenses or the dimensions of frames. The dual scale system allows for readings to be taken to a much finer measurement than a standard ruler, which is essential for precision in optometry and optics.

While other tools listed, such as the Centration Tool, are also important in optometry, they serve different specific functions, typically related to aligning and centering lenses rather than measuring dimensions. Understanding the distinctions between these tools is crucial for their effective application in various optical practices.