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.
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 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
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.
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
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
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
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.
Frame Marking Conventions
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
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.
5-4 Interpupillary Distance
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).
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.
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.