About 14 million Americans, two-thirds of them women, wear contact lenses. Since the original hard lenses were invented in the 50s, contact lenses have been steadily improved to correct a greater array of problems and fit more and more people. Last year contact lens sales increased 30 percent over the previous year, which lead to an ever greater increase in the number of suppliers offering cheap and discounted content lenses.

Hard contact lenses arc made of a rigid plastic and allow no oxygen to pass through them to the eye. They are smaller than soft lenses so air can get around them. They generally are not as comfortable as soft lenses and require several weeks to adjust to them. Most wearers cannot keep them in for more than 12 to 18 hours. Many eye specialists believe that wearing hard lenses slows the deterioration of vision.

Soft contact lenses are made of very pliable water-absorbent plastic and provide more comfort and longer wearing time than most hard lenses. They need very little break-in time. But soft lenses do have drawbacks. They require more care than hard lenses. Because they absorb water, they also absorb bacteria and other substances. They arc less durable. While a hard lens can last as long as 10 years, a soft lens wears out, sometimes within a year, and can tear. Apparently the pros out-weigh the cons; soft lenses now account for more than 60 percent of contact lens use.

Though soft lenses were welcomed by people who couldn't tolerate hard lenses, they were off limits to others, particularly those with astigmatism. Then came gas-permeable hard lenses offering the advantages of hard lenses - durability, little care and can be worn by astigmatics - as well as allowing oxygen to the eye and providing more comfort and longer wearing time. Besides the three basic types of lenses, new lenses designed to correct specific problems are being developed at a rapid pace. They include:

Soft lenses for astigmatism. Until recently people with astigmatism could not wear soft lenses because the material fits unevenly over the rough surface of the cornea. A soft lens called a toric lens, with two prescriptions ground into it, is now available. Several area optometrists say they have been prescribing it with great success, while others say they are encountering difficulties in obtaining and/or fitting it. U is available for $200 to $400'.

Bifocal lenses and monovision. Contact lens wearers who developed the need for bifocals until recently had the choice of wearing reading glasses with their contact lenses, or giving up their contacts entirely and switching to bifocal glasses. A few years ago a was developed and has had varying degrees of success. Another method of correcting the combination of myopia and farsightedness is monovision, a system of placing a lens to correct nearsightedness on the dominant eye and another lens to correct farsightedness on the other eye.

Contact lenses to correct color blindness. One red lens, called an X-Chrom lens is placed on the non-dominant eye and corrects the distortion of colors. Though color blindness, which occurs mostly in men, is not usually a serious handicap, patients who need to distinguish colors in their job are greatly aided by this concept

Orthokeratology. This is a somewhat controversial procedure that can be compared to orthodonture for eyes. are worn to shorten the distortion in the cornea, actually correcting myopia and eventually making glasses or contact lenses unnecessary. The procedure takes from a few months to a few years and ranges in price from about $500 to $2,000. It is strongly advocated by some optometrists, while others believe it is only somewhat successful.