Pachometer

[Little_Girl]

An instrument for measuring thickness, as of the glass
of a mirror, or of paper; a pachymeter.

[Marquetta]

Any of various instruments used to measure thickness, originally of the glass of mirrors, but later also of paper, concrete, etc.; (Ophthalmology) an instrument for measuring the thickness of the cornea of the eye, typically using either light from a slit lamp or ultrasound.