Opthalmic Frames
Measurements & Markings

Summary of Frame Measurements (Box System)
A	Horizontal measurement of box enclosing lens area
B	Vertical measurement of box enclosing lens area
C	Width of the lens along the central datum line
Effective Diameter	Twice the distance from the geometrical center of the lens to the bevel farthest away from it.
Frame Difference	The difference between the horizontal and vertical measurements.
Bridge Size (DBL)	The least distance between the two lenses.
GCD	The geometric center distance also known as the DBC, distance between centers, and sometimes referred to as the FPD or Frame PD, is the distance from the geometric center of one eyewire to the geometric center of the other.
Overall Temple Length	Distance from the center of the center barrell screw hole to the end of the temple tip.Length to bend. Measurement from the center of the barrel to the middle of the bend.
LTB	Length to bend. Measurement from the center of the barrel to the middle of the bend.
FTB	Front to Bend. Distance between the plane of the frame front and the bend of the temple. Applies to frames where the endpieces wrap around and there is some distance between the frame front and the beginning of the temple.

Theoretically, the GCD may be calculated by adding the A measurement to the DBL. In practice, however, there are often differences in the actual physical measurements and those marked on the frame. A competant optician is capable of taking accurate measurements, and this skill plays an especially important roll as it relates to frame dimensions.