What instrument is used to measure vertex distance?

The instrument specifically designed to measure vertex distance is the distometer. Vertex distance is the space between the back surface of a spectacle lens and the front surface of the cornea. Accurately measuring this distance is crucial in the process of fitting glasses as it affects the effective power of the lenses and the visual experience of the wearer.

A distometer is equipped to provide precise measurements of this distance, making it a valuable tool in optometry and ophthalmic dispensing practices. The accuracy of the measurement can significantly influence the comfort and functionality of the prescription eyewear.

While other instruments like a vertometer are used for assessing lens power, they do not focus specifically on vertex distance. Exophthalmometers measure the projection of the eye from the orbit, and calipers are used for other forms of measurement but lack the specificity and design for checking vertex distances. Thus, the distometer stands out as the correct choice for measuring vertex distance due to its dedicated function and design for this specific task.