One of the things you may be doing on the job is interpreting the prescription order. This includes the selection of the lenses and frame from which the patient's new eyeglasses will be made. In order to do this, you must understand the method of measurement used by the industry.

For many years, the measurement of eyeglass frames was a mixture of methods and numbers. There was no one standard by which all frames were measured. Since 1962, however, all frames made by the members of the Optical Manufacturers Association have been measured and marked using one system, called the BOXING SYSTEM of frame measurement. It is now the official standard for the industry.

The Boxing System uses a constant reference point for all measurements, the bevel apex of the edged lens. This reduces the chance for error in interpreting the prescription. All dimensions are expressed in millimeters. (See American National Standard Requirements for Dress Ophthalmic Frames, Z80.5, for a detailed explanation of how the Boxing System is applied to dress ophthalmic frames).

Figure 5-1 The Boxed Lens

The A-dimension is the distance between vertical tangents; the B-dimension is the distance between horizontal tangents; the intersection of the Box Diagonals defines the center of the lens shape.

Consider the lens shape shown in Figure 5-1. If we draw a square that completely encloses the lens, then the lens is called a boxed lens. For reference, we say that the box sides are tangent (they touch but do not intersect) to the bevel apex of the lens. This is an important concept in the standardization process. It allows us to compare the size of both frame and lens at the same time. So when we say a frame has certain dimensions, we really mean that a lens of that dimension will fit the frame.

The vertical measurement of a frame is known as the B-dimension. It is sometimes referred to as the frame depth. The horizontal measurement of a frame is called the A-Dimension. It is also called the eyesize.

The intersection of the two box diagonals defines the box center. The box center is the geometric center (GC) of the frame opening or aperture. It is also called the geometric center of a lens edged for a given frame.

Note the "N" printed on the lens shape. This indicates the location of the shape relative to the nose. "N" stands for the nasal direction. The opposite or temporal side of the frame shape is in the direction of the patient's temples.

Figure 5-2 Boxed Frame Design

In Figure 5-2, the letters stand for the major dimensions of a frame front. Suppose you have boxed both the right and left lenses (one pair) as if they were inserted into their frame. If you did, the boxed pair would look like those in Figure 5-2. As you can see by looking at the diagram, two new dimensions have been added. One is called the DBL, or the Distance Between Lenses. The DBL is equal to the minimum horizontal distance between two lenses that are mounted in a frame. The measurement is taken from the bevel apex of one lens to the bevel apex of the other. In the Boxing Sys-tem, the DBL is referred to as the "bridge size" of the frame.

By looking at Figure 5-2, you can also see that a name has been given to the distance between the geometric centers of the lenses. It is the DBC. In the jargon of our industry, the DBC is often referred to as the "frame PD." PD stands for "interpupillary distance" and will be discussed in more detail a little later. The "frame PD" (DBC) is computed according to the following formula: 

DBC = A-dimension + DBL

As you look again at Figure 5-2, you can see that the lenses are drawn as though the patient were looking at you through the frames. This is the way the lens containers of a frame are specified in frame catalogs and on packaging. It is important to remember that the right and left lenses of a pair of eyeglasses, and their angular measurements, are specified as they would be oriented on the patient, as a matter of standardization.

Look at Figure 5-2 again. Note the angular marking around each eye shape. Another standard of the Boxing System requires that the angular measurements of a patient's prescription be specified as shown facing the patient. Zero degrees is always at the right box extremity with angles increasing in a counterclockwise fashion.

Standardized frame markings look like this: 52 .20. That is, there are two numbers separated by the box symbol. Anytime you see a frame that is marked like this, you will know that it was made according to the Boxing System of measurement. In such a frame, the first number will be the A-dimension. The second, on the other side of the box, will be the DBL.

You already know that the DBC (frame PD) is the sum of the A-dimension and the DBL. Therefore, by adding the two numbers in the frame marking you can quickly figure out the DBC. Figure 5-3 illustrates the variety of accepted locations for frame dimension markings.

Figure 5-3 Locations for Frame Size Markings

Interpupillary Distance

The distance between the patient's pupils is called the interpupillary distance and is sometimes abbreviated PD. The PD is measured with a millimeter rule. The resulting measurement is called the binocular PD. The binocular PD is shown in Figure 5-4.

Figure 5-4 Interpupillary Distance

A more accurate method of measurement recommended today is corneal reflection. It involves measuring the distance from the pupil, using light reflected from the cornea, to the center of the nose where the center of the frame bridge rests. This measurement is called the monocular PD. If the monocular PD has been given on an order, then the laboratory will be given a value for each eye. The instrument used to measure the corneal reflection is called the corneal reflection pupillometer (also known as CRP).

Minimum lens size

Frame catalogs, as well as most lens manufacturers, list a quantity called the effective diameter, or ED. The ED defines the minimum diameter lens that will fit a frame when the geometric center of the lens is exactly centered in the frame. The ED is twice the longest radius from the geometric center of a lens to the apex of the edge. Its angular location is specified in degrees from the horizontal lens centerline for the right eye, measured counterclockwise from the zero degree position as viewed by the observer. See Figure 5-5. Note that there are two EDs defined for this example lens shape. The ED shown at approximately 40 degrees conforms to the definition above. That is, the lens optical center will coincide with the geometric center of the frame shape. However, if the lens is decentered-in (right) toward the nasal direction to match the interpupillary distance requirements of the patient (you will learn more about this in the Finish Room Training Course), the radius located at approximately 150 degrees will be the primary effective diameter.

In Figure 5-5, the effective diameter defines the minimum size lens that will fit in the frame shape shown in the outline. The effective diameter and its angular location in a frame are also used in lens layout. This information helps the layout person to compute the thinnest possible plus lens for a given frame.

Figure 5-5 Effective Diameter

The Effective Diameter (ED) of a lens shape is the minimum size lens that will fit in the frame shown in the outline.