One of the most common sights almost anywhere in the world
is -- eyeglasses! Since we depend so much on the lenses inside
those frames to improve our view of the world, you might
wonder just what goes into creating them.
In this edition of HowStuffWorks,
we will talk about how the eye focuses, how a lens works, how
to read a prescription, and finally, how the lens is made,
including the steps involved in grinding and shaping plastic
lens blanks to fit an individual's prescription and frame.
Let's start with some vision basics.
How Your Eye Focuses
On the back of your eye
is a complex layer of cells known
as the retina. The retina reacts to light and
conveys that information to the brain. The
brain, in turn, translates all that activity into an image.
Because the eye is a sphere, the surface of the retina is
look at something, three things must happen:
Aberrations: ghost images, halos, waves or
rainbows caused by imperfections in the curve or lens
Index of refraction: a ratio used to compare
Plus lens (+): a lens that is thickest at the
center; moves the focal point forward
Minus lens (-): a lens that is thinnest at
the center; moves the focal point backward
Focal point: a spot in space where refracted
light meets; may be actual (plus lens) or assumed (minus
Pupillary center: the point on a lens
directly in front of the pupil
Astigmatism: a condition caused by a
distortion in the cornea that creates an additional lens
To do all that, the eye has a lens between
the retina and the pupil (the "peep hole" in the center
of your eye that allows light into the back of the eye) and a
transparent covering, or cornea (the front window). The
lens, which would be classified a "plus" lens because it is
thickest toward the center, and the cornea work together to
focus the image onto the retina. (For more information on how
the eye functions, see How Vision
- The image must be reduced in size to fit onto the
- The scattered light must come together -- that is, it
must focus -- at the surface of the retina.
- The image must be curved to match the curve of the
Sometimes, for different reasons, the eye doesn't focus
Most vision problems occur when the
eye cannot focus the image onto the retina. Here are a few of
the most common problems:
- The surfaces of the lens or cornea may not be smooth,
causing an aberration that results in a streak of
distortion called astigmatism.
- The lens may not be able to change its curve to properly
match the image (called accommodation).
- The cornea may not be shaped properly, resulting in
In addition, lenses can be made to
correct for double vision when the eyes do not work together
("crossed eyes"). The lenses do this by moving the image to
match the wayward eye.
- Myopia (nearsightedness) occurs when a distant
object looks blurred because the image comes into focus
before it reaches the retina. Myopia can be corrected with a
minus lens, which moves the focus farther back.
- Hyperopia (farsightedness) occurs when a close
object looks blurred because the image doesn't come into
focus before it gets to the retina. Hyperopia, which can
also occur as we age, can be corrected with a plus lens.
Bifocal lenses, which have a small plus segment, can
help a farsighted person read or do close work, such as
- Astigmatism is caused by a distortion that
results in a second focal point. It can be corrected with a
Corrective lenses, then, are prescribed to correct for
aberrations, to adjust the focal point onto the retina or to
compensate for other abnormalities. You can read more about
vision problems in How
Refractive Vision Problems Work.
How a Lens Works
The best way to understand
the behavior of light through a curved lens is to relate it to
a prism. A prism is thicker at one end, and light
passing through it is bent (refracted) toward the
thickest portion. See the diagram below.
A lens can be thought of as two rounded prisms
joined together. Light passing through the lens is always bent
toward the thickest part of the prisms. To make a minus lens
(above on the left), the thickest part, the base, of
the prisms is on the outer edges and the thinnest part, the
apex, is in the middle. This spreads the light away
from the center of the lens and moves the focal point forward.
The stronger the lens, the farther the focal point is from the
To make a
plus lens (above on the right), the thickest part of the lens
is in the middle and the thinnest part on the outer edges. The
light is bent toward the center and the focal point moves
back. The stronger the lens, the closer the focal point is to
Compound lens: a lens having both a spherical
and a cylindrical component
Cylindrical curve: a curve that radiates
along a straight line, like a pipe cut lengthwise
Diopter (D): the refractive power of a lens;
the higher the number, the stronger the lens
Refraction: the bending of light
Spherical curve: a curve that is the same in
all directions, like a basketball cut in half
Placing the correct type and power of lens in front of the
eye will adjust the focal point to compensate for the eye's
inability to focus the image on the retina.
The strength of a lens is determined by the
lens material and the angle of the curve that is ground into
the lens. Lens strength is expressed as diopters (D),
which indicates how much the light is bent. The higher the
diopter, the stronger the lens. Also, a plus (+) or minus (-)
sign before the diopter strength indicates the type of lens.
Plus and minus lenses can be combined, with the total lens
type being the algebraic sum of the two. For example, a +2.00D
lens added to a -5.00D lens yields:
(+2.00) + (-5.00) = -3.00 or a 3.00D minus
lens shapes are commonly used in optometry: spherical and
- A spherical lens looks like a basketball cut in
half. The curve is the same all over the surface of the
- A cylindrical lens looks like a pipe cut
lengthwise. The direction of a cylinder curve's spine (axis)
defines its orientation. It will only bend light along that
axis. Cylinder curves are commonly used to correct
astigmatism, as the axis can be made to match the axis of
the aberration on the cornea.
Lens and Prescription
To make a
lens, the first thing you need is a lens blank. Blanks
are made in factories and shipped to individual labs to be
made into eyeglasses. The raw lens material is poured into
molds that form discs about 4 inches in diameter and between 1
and 1 1/2 inches thick. The bottom of the mold forms a
spherical curve on the front face. A small segment with
a stronger curve may be placed in the mold to form the segment
for bifocals or progressive lenses.
Base curve: a simple spherical curve; the
primary lens curve
Lens blank: basic spherical lens; the lab
grinds the back of the blank to match the prescription
Optical center: a spot on a spherical lens
where light enters at a 90-degree angle to the lens
Segment: the portion of a lens added for
reading (bifocal or trifocal); it may be added
separately to the lens blank or formed as a blended
curve onto the base
How to Read the
Most prescriptions have four parts:
A short form prescription from
the optometrist or ophthalmologist might read:
- The base (spherical) strength and type (plus or minus)
- The cylinder strength and type
- The cylinder axis orientation (in degrees with 90 degree
vertical; an "x" means "at")
- The strength of bifocal segment ("plus" indicating "in
addition") and type
2.25 -1.50 x 127 plus +2.00
power of the lens with the cylinder is +2.25 + (-1.50) =
+0.75D. At the segment, the power is (+0.75) + (+2.00) =
+2.75D. And in case you've ever wondered, OD means right eye
and OS, left eye.
- A +2.25D spherical base curve (plus lens)
- A -1.50D cylinder at 127 degrees (a minus cylinder lens
is added to the base curve)
- An additional bifocal segment of +2.00D
Overview: How the Lens is
In the lab the patient's full prescription
gives these exact details:
technician selects a lens blank that has the correct segment
(called an add) and a base curve that is close to the
prescribed power. Then to make the power match the
prescription exactly, another curve is ground on the back of
the lens blank.
- The total power (in diopters) the finished lens must
- The strength and size of the segment (if needed).
- The power and orientation of any cylinder curves.
- Details such as the location of the optical center and
any induced prism that may be needed.
For example, a very common lens
blank is +6.00 diopters. If the prescription calls for a total
of +2.00 diopters, a -4.00 diopter curve is ground on the
back: (+6.00D) + (-4.00D) = +2.00D. (See the illustration
below.) If it is needed, the cylinder curve is also ground at
the same time.
- In most labs the equipment is designed to grind minus
curves, so a strong, plus lens blank is usually selected.
- If the base curve is too strong, then a minus curve is
ground in the back of the lens, which reduces the total
power of the lens.
If the prescription calls for a minus lens, the +6.00
diopter lens blank can still be used. To create a lens with
the strength of -2.00 diopters, a -8.00 diopter curve is
ground on the back: (+6.00D) + (-8.00D) = -2.00D.
Steps: Making a Lens
lenses can be made with glass or plastic, but nowadays,
plastic is the most common. While several different types of
plastic are used in making lenses, all of them follow the same
general manufacturing procedures. Most of the steps outlined
also apply to glass, although a few important differences are
noted at the end.
Generator: a compound surface grinder used to
grind curves in the surface of the lens
Induced prism: a technique that moves the
optical center away from the pupillary center
A lab, even an automated one, follows 12 steps to make
Step 1: The technician chooses a lens blank of the
desired material with the proper base curve and, if needed,
A lens blank will be ground
match the patient's
Step 2: If the prescription calls for a cylinder, a
line is marked on the front of the lens to define 180 degrees,
and then another line is drawn that matches the axis of the
second curve. If there is a segment, the segment edge is used
as the 180 degree line. Often the optical center of the lens
is made slightly above the segment edge, and the line is
marked the appropriate distance. (Note: When there is no
segment or induced prism, the lens may be left unmarked and
the cylinder axis determined after the lens is ground.)
A lens blank is marked to show where the
cylinder axis will
Step 3: Since the front of the lens will be left as
is, it is covered by a special tape to protect it.
The technician puts a protective covering
over the front of the lens blank to keep it from being
Step 4: Depending on the type of equipment, the lens
must be prepared to fit onto the generator, which is
commonly a compound surface grinder capable of grinding two
curves at once.
A compound grinder, called a
generator, grinds the required curves into the back of
the lens blank. The two large dials on the console set
the spherical and cylindrical curves that will be ground
into the lens.
A chuck receiver (called a block) is placed on the
front of the lens over the protective tape. If there is a
cylinder curve, the lens is oriented so the cylinder axis
matches the cylinder sweep axis of the generator.
A chuck receiver, called a lens block, must
be attached to the front of the lens so it can be
mounted in the
The center of the block will become the optical center of
the lens. Depending on the equipment, the lens may be held in
place by special adhesive pads, with a special alloy that
"glues" the lens to the block or with plastic.
Step 5: The lens is inserted in the generator.
The lens blank, attached to the lens block,
is inserted in the generator. The generator has pins
that align the
The lens might need other processing besides the compound
curves produced by the generator, so the lens may also be
tilted in the chuck. This tilt will offset the optical center
(called induced prism) often used to allow thinner
lenses or to accommodate special requirements of the
The lens is ground within a
rubber-lined grinding chamber. The cone-shaped quill, or
grinding wheel, is at the center. The quill has a
diamond cutting surface along its outer edge and is
angled so only the outside edge touches the lens.
Step 6: The curves are set on the machine and the
lens is generated (ground). This step may either be fully
automated or operated by hand, where the operator manually
sweeps the quill (grinding wheel) across the lens, gradually
advancing the lens until the desired lens thickness is
achieved. Lens thickness is determined by curve type (plus or
minus), lens material (some plastics are tougher and may be
ground thinner), or other considerations (safety glasses, for
instance, are made thicker than lenses for everyday use). If
the lens gets too hot during the operation it may warp or
tear, so it is cooled by water, which also washes away the cut
material (called scarf).
Step 7: The lens is taken off the generator and
placed in a special sanding machine (called a cylinder
machine) to remove any marks left by the generator. To do
this, sandpaper is glued to a block with reversed, matching
curves (a +2.00 base/+2.50 cylinder, for example, to match
-2.00/-2.50 generated curves), and the lens and block are
rubbed together. Meanwhile the lenses are kept cool and
cleaned with water.
A cylinder machine can sand two lenses at the
same time. Air pressure holds the lens and the sanding
block together, and a timer switches the machine off at
a preselected time.
Following the sanding operation, the lenses are polished on
an identical machine, except that felt polishing pads washed
with polishing compound are used instead of sandpaper and
water. When this step is completed, the lens is optically
clear without visible scratches.
After sanding, the lenses are polished so
they are perfectly clear without any scratches. Liquid
polish flows over the lenses and into a reservoir to be
Step 8: The block is removed from the lens, and the
lens is washed and inspected. Sometimes special coatings may
be applied to the lens. At this point the lens blank has had
additional curves ground in the back of the lens and it has
been polished. However, the large diameter blank still has to
be sized and shaped to fit into the frame selected by the
patient. Several methods are used, depending on the equipment,
but they are all based on the following description.
Step 9: The lens blank is shaped on a linear lathe
(called an edger) using either a ceramic or diamond
grinding wheel or stainless steel blades. The lens must again
be prepared to accept a chuck, but since only the edge is
being cut, a much gentler system is used. A small chuck
receiver is placed where the geometrical center of the
finished lens will be, and the lens is then oriented on the
180 axis. Usually, only an adhesive pad is needed to hold the
receiver on the lens. The lens is chucked in the edger and
held in place by a pressure pad that presses on the opposite
side of the lens (like holding a very large coin between your
thumb and forefinger at its center).
The lens is mounted into an edger. The
edger's chuck turns slowly as the lens is cut to
Step 10: A pattern in the shape of the frame is
inserted in the edger. Patterns are commonly plastic and may
be supplied by the frame manufacturer or made in the lab.
A red pattern is used in the edger to
the final shape of the
Newer edgers do not use patterns; instead, the shape is
determined by a probe that measures the frame and stores the
information in a computer, which in turn controls the edging
operation. As it operates, the slowly turning lens is brought
into the fast turning cutting surface, which is either a
grinding wheel or steel blades, until a guide contacts the
pattern, which is rotating to match the lens. If the frame has
a complete rim surrounding the lens, a bevel, or ridge, is cut
along the edge of the lens that will fit into a groove in the
frame; otherwise, the edge is left flat.
Step 11: The lenses, now cut to fit the frame, are
prepared for inserting into the frame.
- If the lenses are to be tinted, the dyeing is done at
this point. Special dyes are kept in heated containers and
the lenses are immersed. The density of the tint is
determined by how long the lenses are left in the dye.
Lenses may be only partially tinted (fade), tinted different
colors at top and bottom, or tinted a custom color by
combining different colors. Also, special UV blocking dyes
may be applied in the same way.
Lenses that need to be tinted are dipped in
- If the frame is rimless, a groove is cut along the edge
of the lens to receive the string that holds the lens to the
frame. Any sharp edges are trimmed and smoothed and, if
desired, the edge is polished on a buffing wheel.
Step 12: The lens is inserted into the frame. Fit
and orientation is double checked, any worn screws or hinges
are replaced as needed, and the frame is made square. The
finished eyeglasses are then thoroughly cleaned and packaged
for delivery to the patient.
A technician checks the finished lenses
for scratches and
Glass lenses are ground and polished much the same way as
plastic except that diamond cutting surfaces are used, and
some details may vary. The blanks are made of relatively soft
glass and must be tempered, either by chemicals or heat, to
strengthen them before inserting into the frame.
Advances in automation are rapidly changing how lenses are
made. For example, the vast majority of labs now use computers
to determine curve parameters and lens choice, and equipment
is available that will combine several steps or even do the
entire operation automatically.
For more information, check out the links on the next page.
Lots More Information!
More Great Links
About the Author
Broten is an American Board of Opticianry-certified optician
and certified laboratory technician at Lenscrafters Inc. in
Portland, Oregon. He holds a bachelor's degree in biology and
did extensive research in fish vision while pursuing his
Author's note: I am indebted to Erik
Schopp, A.B.O-certified optician and general manager of
Lenscrafters #671, and Dr. Dawne R. Griffith, O.D. with Dr.
Robert D. Forbes & Associates, for their invaluable
assistance in reviewing this article. Optics and optometry are
complex subjects beyond the scope of this article. In
presenting the basic principles of these two disciplines, I've
oversimplified somewhat for the sake of brevity. For this I
apologize. Any errors in fact or theory are entirely mine. I
encourage interested readers to seek professional advice, as
this article is a brief overview and not intended as a guide
to diagnoses. Also, I am grateful to Lenscrafters store #671
in Portland and to Joshua Boyd, lens technician, for help in
taking the photos used with this article.