Eye practitioners can test you if you suspect you are color deficient. Telltale signs include a lack of interest in colors, choosing color combinations that don’t appeal to most people, a preference toward blues and yellows and indifference toward reds and greens, unawareness of fall foliage, difficulty in seeing veins and freckles, and the preference of black-andwhite TV over color.

In the past there was nothing available to improve color blindness. But several years ago Dr. Harry Zeltzer developed a special, red-tinted contact lens that improves the color perception of those who suffer the most common type of color blindness (red-green). (A red-tinted lens in eyeglasses doesn’t work because the lens is too far away from the eye.) The X-Chrom lens has been a godsend to people whose jobs and interests require the ability to identify colors. Telephone repairmen, hunters, drivers, home handymen, painters, cosmeticians, printers, textile workers, chemists, photographers, artists, decorators, and anyone who selects clothes (even if it’s just their own)—all can benefit from this type of lens. It has even helped color-deficient children who were wrongly labeled as learning deficient.

The X-Chrom lens is deep red in color. Only one tinted lens is prescribed, usually for the nondominant eye. It works by changing the relative intensities of red and green objects. The nondominant (corrected) eye supplies the brain with information about colors that are normally hard for it to identify; the dominant (uncorrected) eye continues to supply the colors the eye is capable of seeing correctly. As if by magic the wearer sees colors he never even knew existed. At first colors may seem to glow or be more vibrant, and objects that used to blend into the background may seem to jump out. But the eye and brain soon adjust to the unfamiliar information: the newly perceived colors are identified and learned.

If necessary the X-Chrom lens can be manufactured to incorporate a prescription to correct refractive errors. In that case the wearer keeps his regular clear lens in the dominant eye and the X-Chrom corrective lens in the other. Usually a third, clear contact lens incorporating the necessary prescription is kept on hand for use in the nondominant eye when the X-Chrom is not worn. Those who do not wish to wear a contact lens in both eyes can wear glasses over the X-Chrom lens.

The fitting, adjustment period, wearing, care, and handling of the lens are the same as for conventional hard lenses, but the cost is higher. The X-Chrom may be worn during both daytime and nighttime, but many elect to remove the lens at night because of the reduced illumination that will be exaggerated with this lens. Some wearers experience a slight reduction in visual acuity, or their depth perception is affected because the two eyes receive unequal amounts of illumination. Cosmetically the red color of the X-Chrom lens is very obvious. All these disadvantages must be weighed against enhanced color perception. The color-blind patient, therefore, has to be well motivated in order to wear this lens.

Clearly the X-Chrom offers no guarantees and is not for every color-deficient patient. In the words of its developer it’s “an aid, not a cure.” But for most color-blind people it means improved color perception and opens up new vistas in everyday pleasures and work.

As shown in the smaller drawings for each type of refractive error, corrective lenses work by compensating for the eye’s deficiencies. In the case of myopia, where the eyeball is too long, the total refractive power of the eye is greater than is required for an eye of normal length. The rays of light come into focus before they reach the retina. A concave lens (thicker at the edges, thinner in the center) is used to focus the image farther back, on the retina. This is also called a minus lens because it causes the light rays to diverge or bend outward, in effect weakening the total refraction. A hyperopic eye, where the eyeball is too short, has a refractive power that is less than that of a normal eye. The light rays don’t focus until they’ve fallen behind the retina. This condition requires a convex lens (thicker at the center, thinner at the edges) to bring the focus point forward, so it lands on the retina. This is also called a plus lens because it converges the rays, making the total refractive power of the eye stronger. A presbyopic eye also requires a convex lens, which is often prescribed in the form of bifocals. (Yes, they can make bifocal contact lenses.) Astigmatism requires special curves in the lenses that focus the rays of lightat a specific axis.

A contact lens, with the layer of tears trapped beneath it, works by canceling out the refractive error. In effect it becomes another layer of the eye. It may be considered a prosthesis, or artificial body part, which actually becomes an integral part of the wearer’s optical system. Since it is placed directly on the living eye, it must go far beyond having the correct prescription; it must fit the cornea well enough not to interfere with the normal metabolism of the eye.

Of primary concern is the amount of oxygen that reaches the cornea, which is normally supplied by the air and the tears in exchange for waste carbon dioxide. Since every lens in use acts as a barrier against both the tears and the air reaching the cornea, contact lenses are designed and fitted to overcome this problem. Though the eye can adapt somewhat, a too-diminished gas exchange radically alters the metabolism of the cornea and its health will suffer. The primary effect of such suffocation is edema, a swelling of the cornea that can be likened to a blister on your skin. Edema may lead to more serious injuries to the delicate cornea, and is the most prevalent of all contact lens side effects. The race to design a better contact lens is now largely a battle against the oxygen barrier.

In designing a contact lens, there are two ways to increase the gas exchange and prevent or reduce edema:

THROUGH THE TEARS

With each blink the eye trades new tears for old. By improving the tear-pump mechanism, more spent, carbon dioxide-rich tears are alternately forced out from under the lens and more fresh, oxygen-rich tears are injected.

In order for the tear pump to work, contact lenses must be able to move slightly with each blink. Both soft lenses and hard lenses float on a thin layer of tears.

THROUGH THE LENS ITSELF

In gas-permeable lenses special materials (silicone and cellulose acetate butyrate) allow gases to pass through the lens material. In soft contact lenses the lenses themselves absorb the tears in which the oxygen is dissolved and transmitted to the cornea.

Ideally, both mechanisms should operate because even in gas-permeable lenses the oxygen transmission is not 100 percent. In addition to supplying oxygen, tears also meet other requirements of the cornea, such as lubricating the corneal surface, flushing away foreign bodies, and preventing infection because of its lysozyme content.

In conventional hard lenses oxygen is supplied mainly through the tear pump. In some cases fenestrations, or tiny holes drilled into the lenses, can make them somewhat gas permeable because air can pass through the holes. Making lenses smaller and thinner increases the tear supply; so can the use of drops of artificial tears. As frivolous as this sounds, proper blinking is crucial for proper tear flow and contact lens comfort.