What instrument is best for measuring the thickness of a lens?

The caliper is the most suitable instrument for measuring the thickness of a lens due to its design and functionality. Calipers can provide precise measurements of the lens's thickness at various points, which is essential for ensuring that the lens meets specific optical and manufacturing standards. They are equipped with adjustable jaws that allow the user to measure both the outer and inner thickness of the lens accurately.

While the microscope is capable of providing detailed views of the lens surface, it is not intended for direct measurement of thickness. A lensometer is primarily used to measure the power of the lenses and to verify the prescription, rather than measuring physical dimensions. A refractometer is used to measure the refractive index of the lens material, which is unrelated to the actual thickness measurement of the lens itself. Therefore, the caliper's ability to accurately measure dimensional features establishes it as the optimal tool for this task.