How Corrective Lenses Work

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 curved.

When you look at something, three things must happen:

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 Works.)

Sometimes, for different reasons, the eye doesn't focus quite right:

Most vision problems occur when the eye cannot focus the image onto the retina. Here are a few of the most common problems:

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.

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.

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 lens.

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 the lens.

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:

Lens Shapes

Two basic lens shapes are commonly used in optometry: spherical and cylindrical.

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.

How to Read the Prescription

Most prescriptions have four parts:

A short form prescription from the optometrist or ophthalmologist might read:

2.25 -1.50 x 127 plus +2.00

This means:

Total 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.

In the lab the patient's full prescription gives these exact details:

The lab 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.

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.

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 1 through 3

Corrective 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.

A lab, even an automated one, follows 12 steps to make prescription lenses:

Step 1: The technician chooses a lens blank of the desired material with the proper base curve and, if needed, add power.

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.)

Step 3: Since the front of the lens will be left as is, it is covered by a special tape to protect it.

Steps 4 through 6

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 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.

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 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 prescription.

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).

Steps 7 through 9

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.

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.

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).

Steps 10 through 12

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.

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.

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.

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 about corrective lenses and related topics, check out the links on the next page.