The Back Vertex Distance (BVD) Gauge is an accurate device for measuring the distance between the back of a lens to the cornea in mm. The set comes with a pouch and calculator disc. The instructions for plus and minus powers are printed on the reverse side of the calculator. The advantage of this gauge is once the measurement is made, the distance set on the scale does not move when the measurement is completed.  It also compensates for 1 mm to account for the eyelid thickness.