the box measurment

The Box Measurement System

Please read to increase your understanding of frame measurements:

a.    System for Ophthalmic Dispensing, 4th Edition: Chapter 2 - Frame Measurements and Markings

b.    The following text:

The Box Measurement System

Figure 1112-7

Finding the geometric centre of any lens in relation to the major reference point (MRP) ensures the lens is properly decentered (when mounted in the ophthalmic frame, the optical centre of the lens is placed directly before the pupil) prior to edging. This geometric centre point is also known as the boxing centre of the lens.

Figure 1112-8

The Boxing System uses the smallest rectangle that encloses a lens shape with the horizontal and vertical measurements. The horizontal lens measurement is the A dimension, and the vertical lens measurement is the B dimension. The horizontal line at the midpoint of the B dimension is known as the datum line or the 180-degree line.

Figure 1112-9

The effective diameter is known as the ED. The ED is found by determining the longest distance from the geometric center to the edge of the lens shape and doubling that measurement. The ED is not the greatest distance from edge to edge of the lens shape though this technique is often used. A simple oval shape such as the one above demonstrates that the eye size (A) and effective diameter (ED) are the same.

Figure 1112-10

The ED line crosses the datum line at an angle (X). The angle of the effective diameter is determined by measuring the right lens from the right side of the datum line to the ED line. The ED is used to work out the minimum blank size (MBS), and knowing the angle of the effective diameter allows the surfacing lab to make the lens as thin as possible.

Figure 1112-11

Every ophthalmic frame has a measurable distance between lenses (DBL). The DBL is the smallest distance between the two lenses for that frame and is often known as the bridge size. Once again, the box system of measurement is used to determine the DBL, so the distance between the two boxed lenses becomes the DBL. This measurement may not match the bridge size indicated on the frame. The A measurement and the DBL are taken from the deepest edge of the groove of the frame.

Figure 1112-12

The distance between the geometrical centers (boxing centers) may be known as the geometric center distance (GCD), the distance between centers (DBC), or the frame PD. The frame PD is determined by adding the A measurement to the DBL