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The Metric System
The metric system, which is used in Europe, is now much more common in the United States. In fact, most rulers now have a
metric scale on one side and the more well-known scale of inches
on the other.
The metric system is important to you because you must able to use the metric system to place the optical characteristics of the
lenses correctly. The ruler used in the optical laboratory is called
the PD ruler. It is used by the customers of the laboratory to
determine the Interpupillary Distance (PD). The lab worker uses
the PD ruler to measure the frame and lens optical characteristics.
The PD ruler uses the metric scale almost exclusively, so you must
know the rudiments of the metric system and be able to apply
them. Figure 4-2 shows a drawing of the PD rule.
Figure 4-2 The PD Ruler
The unit of measure in the metric system is the METER, which is
about 39 inches. The measuring stick is called a METER STICK
and is divided into CENTIMETERS. One hundred centimeters
equals one meter. One inch on a regular ruler contains
approximately 2.54 centimeters.
Dioptric Power
A lens is a combination of two curves which have been ground on
some type of transparent refractive material, usually plastic, glass,
or polycarbonate.
The curves are usually placed on the lens so that the front of the lens is convex (bowed out), or plus, and the back is concave, or
minus. Look at Figure 4-3.
Figure 4-3
Lens Surfaces
The power of a lens is expressed in diopters (di OP ter). A diopter
is a unit of measure that represents the amount of bending that
takes place as the light passes through the lens.
You recall from an earlier lesson that this bending, or refraction, is
what allows the light to focus on the retina and to produce a clear
image, rather than a blurred image.
The diopter power of a simple lens is represented as a combination of the two powers the convex and concave sides of the
lens. For example, if the front of the lens was a +8.00 D (diopters), and
the back of the lens was a -6.00 D, then the combined diopter
power of the lens would be (+8) + (-6) or +2.00 D. Figure 4-4 shows
a picture of this lens.
Figure 4-4 Plus Lens
The sphero-cylinder lens prescription is written with a spherical
correction, cylinder amount and axis. An example would be +2.00
-1.00 x 180. The +2.00 is the spherical power, the -1.00 is the
cylinder amount and 180 is the axis in degrees. Figure 4.5 shows a
picture of this lens.
Figure 4-5 Front and Back Curves
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