In addition to correcting vision, eyewear can help protect the eyes. Here are some general guidelines to consider when selecting safety eyewear frames:

• Consider frames with side shields, which protect against objects coming at the eyes from an angle, as well as from the front.
• The frames should be made of impact-resistant plastic or polycarbonate.
• For cold weather wear, look for frames made of nylon, rubber or propionate, which do not become brittle in the cold, hold their shape, and are less likely to injure the face in a fall or when something strikes them.
• Titanium is an unusually tough metal used in some sports eyeglasses.
• Wrap-around temples keep the frame more firmly in place.
• Spring hinges allow the frame to flex without breaking.

As for the lenses, safety eyeglasses can be prescription or nonprescription. They can be made of extra-thick glass or plastic. A commonly used plastic is polycarbonate, which offers the same kind of impact resistance in lenses as it does in frames.

One aspect of the eye protection that glasses can offer, and that often is overlooked, is protection against ultraviolet light, which can cause eye damage as well as sunburn. Plastic lenses can be treated with a special dye that will prevent ultraviolet light from reaching the eye. Lenses that screen out both the ultraviolet-A and ultraviolet-B wavelengths in sunlight offer the best protection. The label will indicate that the sunglasses screen both ultraviolet-A and ultraviolet-B, or the label may say that sunglasses screen light “below 400 nm,” or nanometers, the wavelengths of ultraviolet light. For night driving, lenses can be treated with antireflective coatings that reduce glare — something that can be of major importance with a person who has early cataracts or another condition that makes glare a problem.

Eyewear can be customized for protection against eyestrain. An eyeglass wearer who spends hours in front of a computer screen can choose special bifocal lenses for looking down at the keyboard or up at the monitor. A person who looks up at work constantly can get bifocals with near-distance segments at both the top and bottom of the lenses. Although a good fit is important for all eyewear, it is especially recommended for safety eyeglasses.

A prescription for eyeglasses contains the powers for the lenses of each eye (including a reading addition if necessary), information on the separation of the lenses, and sometimes special requirements such as types of tinting, coatings, and so on.

Although each eye doctor writes a prescription in a somewhat different way, it usually contains the same elements. The lens correction for nearsightedness is indicated by a minus sign (-), while a plus sign (+) indicates farsightedness. Cylindrical lenses for astigmatism are designated by cyls., cx, or (x).

Let’s analyze the following prescription:

O.D. -3.00 D c/w -2.00 D cx 180
Add: +1.50
PD: 62

• The “O.D.” indicates the prescription for the right eye. O.D. stands for the Latin phrase “oculus dexter” and indicates the right eye. “O.S.” stands for the Latin phrase “oculus sinister” and indicates the left eye.
• The “-3.00 D” means that the patient has -3.00 diopters of nearsightedness.
• The “c/w” means “combined with.”
• The “-2.00 D cx 180″ means that the patient also has -2.00 diopters of astigmatism at axis 180 degrees.
• The “Add: +1.50″ indicates the power of a bifocal addition.
• The “PD: 62″ is a measurement of the distance between the pupils, a necessary measurement for ensuring a proper fit for the glasses.

Lenses of any material can be treated with coatings to give the lenses desired properties. Scratch-resistant coatings applied to the front and back of a lens can reduce the risk of sight-marring scratches for glass and plastic lenses. Ultraviolet-resistant coatings can be applied to all lenses; they are not needed for polycarbonate lenses, which have built-in ultraviolet resistance. Antireflective coatings, similar to those on microscope and camera lenses, reduce glare by blocking reflected light, a benefit that is especially important for night driving. Antireflective lenses also make lenses appear thinner, a cosmetic benefit. A wide variety of tints and coatings are available for modern lenses:

Glass lenses are tinted by adding oxides to the glass during the manufacturing process. Plastic lenses can be tinted in a wide variety of shades and colors by dipping the lenses in special dyes. Tints increase comfort in bright light situations, reduce glare from computer screens or fluorescent light, and make a fashion statement. Many eye care offices and optical shops custom-tint lenses in-house in a matter of minutes. If the tint is on the surface of a lens, the color and/or the shade can be changed.

Photochromic lenses change in color and light transmission when exposed to different light intensities. The rate of darkening depends on the ambient temperature. These lenses do not become as dark when driving because the car’s windshield absorbs some of the ultraviolet light that causes the lenses to darken. Photochromic lenses are now available in plastic as well as glass, and they may be either brown or gray in color.

Polarizing lenses are very useful in protecting eyes from reflections when skiing or participating in water sports. They are available in plastic, glass, and high-index materials, but they are relatively expensive compared to other lenses.

Ultraviolet protective lenses protect the eyes from ultraviolet sun rays that have been shown to cause cataracts and certain diseases of the eye. If you spend a lot of time outdoors, have your lenses anti-UV coated. The best eye protection from UV radiation is a good pair of sunglasses that is rated to block 99 percent to 100 percent of the full UV spectrum. The lenses should filter wavelengths up to 400 nanometers to cut out all harmful rays. UV coating can be applied to all glass and plastic lenses, except polycarbonate, which doesn’t need coating because it absorbs virtually all UV rays.

Mirror-coated lenses are coated with a layer of reflective material that greatly reduces the amount of heat entering the eyes. However, these lenses reduce visual acuity and are somewhat like looking through a two-way mirror. This kind of coating is purely cosmetic and frequently seen on glasses worn by state troopers. Mirror coatings come in a number of colors and are highly reflective. They usually are applied on sunglass-dark lenses. The persons looking at the wearer do not see the eyes of the wearer.

Anti-reflective coatings (A-R coatings) are metallic oxide coatings that are vacuum-applied to the lens surface to reduce reflections from the front lens surface and eliminate reflections from the back. This reduced amount of reflection enhances the appearance of the glasses and also allows more light to pass through the lenses, which can improve vision in low-light situations. Anti-reflective coatings also provide some protection from scratching.

Scratch-resistant coatings (also known as “hard coat”) are applied to the front and back surfaces of lenses to protect against accidental scratching and to improve durability. Some high-quality lenses come from the factory with scratch coatings already applied. No coating can make lenses completely scratch proof.

A large variety of prescription eyeglass lens material exists. The classic glass lens is scratch-resistant, comes in many colors, absorbs ultraviolet light, and causes the least amount of distortion. But glass lenses are heavier than newer polycarbonate plastics, and are breakable, so that they must be treated with heat or chemicals for sports or other endeavors that are breakage-prone.

The best ultraviolet resistance is offered by high-index plastic lenses, which are also thin and light. But they are the most expensive of all lenses, are easily scratched, and can shatter on impact. High-index lenses are made of either glass or plastic that is compressed, giving them a high index of refraction (IOR). These lenses bend light more than other lenses, so a person who requires a great deal of correction does not have to wear extremely thick lenses. Many different kinds of high-index lenses are available from American manufacturers.

Allyl resin plastic lenses are the lightest of all lenses. But they are thicker than other plastic lenses, are less scratch-resistant, and do not block as much ultraviolet light as polycarbonate and glass lenses.

Aspheric lenses also aid in cutting down on thickness and weight, especially when combined with high-index materials. Aspherics have curves that are nonspherical and change gradually from the center to the outside of the lens. Aspherics are now available in single-vision, bifocal, and progressive-addition designs.

Some multifocal lenses are designed for special functions rather than everyday use. A double-D multifocal lens, with a half-moon bifocal at the bottom and an upside-down flat region at the top, is for jobs that require good vision at the near point and at a greater distance. Golfers may wear flattop bifocals that provide enough near-vision correction for reading a scorecard. More than 100 lens designs now are available in the United States. Here are some of the specialized designs:

Computer lenses are designed so the monitor and papers next to it are in focus when the head is held in a natural position.

Double bifocals are sometimes prescribed for people who need close vision overhead. These lenses have a reading area at both the top and the bottom of the lens and are often helpful to pilots, electricians, and others who need clear near vision when looking up as well as down.

Eyeglass lenses are usually classified into one of four categories: single vision, bifocals, trifocals, or multifocals.

Single vision lenses have the same focal power throughout (top to bottom) and can be used to correct nearsightedness, farsightedness, astigmatism, or a combination of these disorders. Most people who wear glasses before the age of 40 have single vision lenses.

Bifocal lenses have two parts: the upper part normally used for distance vision and the lower part used for near-vision tasks such as reading. Many people after reaching age 45 develop a condition called presbyopia, which is a deterioration in the ability of the eye’s natural lens to expand or contract in order to focus on close objects. People with presbyopia need a special lens for reading and may need a different lens for seeing in the distance. Benjamin Franklin invented bifocals so he wouldn’t need to switch glasses when reading. They serve that same purpose today. Until recently, bifocal eyeglasses had a line going across the entire length of the lens. Now there are a number of variations on the theme. All work the same way, by reserving part of the lens for near-vision correction. The rest of the lens can be used for distance vision correction or can have no correction at all. The segment for near-vision correction can have one of several shapes:

• Half-moon, or flat-top segment
• Round segment
• Narrow rectangular area called a ribbon segment
• The entire bottom half of the lens, in what is called variously the Franklin, Executive, or Ribbon style

Trifocal lenses have three different focus areas: the top for distance vision, the center for intermediate vision, and the bottom for near vision. Although bifocal lenses usually are designed so that the line of division is at the level of the lower eyelid, trifocals are fitted higher, with the top line of the middle region at the level of the pupil.

Multifocal literally means “having more than one focus,” so bifocal and trifocal lenses are actually types of multifocal lenses. In addition to bifocal and trifocal lenses, the other multifocal is a progressive addition (no-line bifocal) lens that increases in power from top to bottom. Progressive lenses have no clear dividing lines as the focus changes from bottom to top. They have become popular in recent years because they look like single vision glasses.

The focusing ability of an eyeglass lens is determined by the difference in curvature on its front and back surface, its thickness, and its index of refraction. This index varies according to the density of the material from which the lens is made. The degree to which a given lens material bends light is its index of refraction. The higher the IOR, the greater the light-bending ability — and usually, the greater the cost of a lens.

An eyeglass lens surface is either spherical or cylindrical. A spherical surface is similar to the shape of a ball in that it has the same degree of roundness throughout its surface. It can be either convex, which means it curves outward, or concave, which means it curves inward. A concave surface is always on the back side of a lens. A cylindrical surface is not evenly curved, but more like the shape of an egg or a football. Spherical lens surfaces are sometimes called toric surfaces. Convex lenses are used to correct hyperopia or farsightedness, while concave lenses help correct myopia or nearsightedness, and cylindrical lenses correct astigmatism.

The first high-quality optical glass was developed in Germany toward the end of the 19th century. Prior to that time, eyeglass lenses were made from Brazilian quartz crystal and were poor in quality. Until 19l4, most glass used in optical lenses was imported from Germany. At that time, John Bausch and Henry Lomb developed a high-quality glass in their Rochester, N.Y., factory. Glass remained the primary material for eyewear lenses until the 1940s when the Pittsburgh Plate Glass Company developed a plastic lens material named CR-39. This material remains the choice for most lenses today because it weighs about half as much as glass and is more impact-resistant. Like glass, it can also be ground, polished, and tinted.

The other plastic used in lenses is polycarbonate (poly), a lightweight, almost unbreakable material developed by NASA as part of the space program. This is the material commonly used in children’s, sports, and safety glasses. A major advantage of poly is that it can be ground much thinner than either glass or CR-39. It is, however, more expensive and usually needs an anti-scratch coating because it is softer.

No one knows for certain when eyeglasses were invented, although documents from the 13th century prove the existence of eyeglasses at that time. Several sources quote a manuscript written in Rome in 1289 by a member of the Popozo family that says, “I am so debilitated by age that without the glasses known as spectacles, I would no longer be able to read or write.” A painting done by Tommaso da Modena in 1352 includes the first known artistic representation of eyeglasses. Historians credit the Chinese with carving the first frames more than 2,000 years ago, but apparently those frames did not contain lenses and were used to protect their eyes from “evil forces.” The frames were carved from tortoiseshell, a sacred material. The use of a magnifying glass was first recorded in about 1000 A.D. It was called a reading stone and was placed on top of reading material to magnify letters. Monks used it to copy manuscripts. Later, Venetian glassblowers constructed lenses that could be held in a frame in front of the eyes. Glasses for distance vision first appeared around the middle of the 15th century, and there are various references in literature of that time to spectacles for “distant vision.”

In the 15th century, the printing press was invented, making reading materials more available to the public and increasing the need for glasses. Early eyeglasses were held by hand in front of the eyes or designed to “perch” on the nose. It wasn’t until the 17th century that a London optician perfected the use of side pieces that rested on the ears. In 1784, Benjamin Franklin invented a bifocal lens with the top half for viewing at distance and the bottom half for reading.

In the 19th century, a method was found for examining eyes and prescribing eyeglasses. Prior to that time, those who needed glasses simply tried on various pairs until they found one that worked. Dr. F. C. Donders of Holland wrote the first textbook on examining and prescribing eyeglasses. This book stimulated an interest by eye physicians in prescribing glasses, while opticians set up their shops to fill prescriptions. Some of these opticians elected to also examine eyes and dispense their own prescriptions. This group evolved into the modern profession of optometry.